The human body [Enhanced Credo edition] 9782764408926, 9781784021146, 1784021148, 2764408927

Table of contents :
THE BODY'S BUILDING BLOCKS. The human cell --
Chromosomes and DNA --
Cellular activity --
Body tissues. THE ARCHITECTURE OF THE BODY. The skin --
Bone structure --
Bone growth --
The skeleton --
The head --
The spine --
The hand and the foot --
The joints --
The muscles --
Muscle tissue. THE NERVOUS SYSTEM. The nerves --
The central nervous system --
The cerebrum --
The neurons --
The motor function of the nervous system. THE FIVE SENSES. Touch --
Sight --
Hearing --
Balance --
Taste --
Smell. BLOOD CIRCULATION. Blood --
The cardiovascular system --
The blood vessels --
The heart --
The lymphatic system --
Immunity --
The endocrine system --
The urinary system. RESPIRATION AND NUTRITION. Respiration --
Speech --
The digestive system --
The teeth --
The stomach --
The intestines --
The liver, pancreas, and gallbladder. REPRODUCTION. The male genital organs --
The female genital organs --
Fertilization --
The life of the embryo --
Maternity.

Citation preview

THE VISUAL GUIDES

Understanding The

Human Body

QA INTERNATIONAL

The human

body

Publisher

Jacques Fortin

Editorial Director

François Fortin

Executive Directors

Stéphane Batigne Serge D’Amico

Illustrations Editor

Marc Lalumière

Art Director

Rielle Lévesque

Graphic Designer

Anne Tremblay

Writers

Computer Graphic Artists

Reviewers

Stéphane Batigne Josée Bourbonnière Nathalie Fredette Jean-Yves Ahern Pierre Beauchemin Maxime Bigras Yan Bohler Mélanie Boivin Jocelyn Gardner Danièle Lemay Alain Lemire Raymond Martin Annie Maurice Anouk Noël Carl Pelletier Simon Pelletier Claude Thivierge Michel Rouleau Frédérick Simard

Dr Alain Beaudet Department of Neurology and Neurosurgery McGill University

Dr Amanda Black Department of Obstetrics and Gynaecology Queen’s University

Dr Richard Cloutier Département de dermatologie Centre hospitalier universitaire de Québec

Dr Luisa Deutsch KGK Synergize

Dr René Dinh Dr Annie Goyette Département d’ophtalmologie Centre hospitalier universitaire de Québec

Dr Pierre Duguay Dr Vincent Gracco School of Communication Sciences and Disorders Faculty of Medicine McGill University

Dr Pierre Guy Orthopedic Trauma Service McGill University Health Centre

Dr Michael Hawke Department of Otolaryngology Faculty of Medicine University of Toronto

Page Layout

Véronique Boisvert Geneviève Théroux Béliveau

Dr Patrice Hugo Dr Ann-Muriel Steff

Researchers

Kathleen Wynd Jessie Daigle Anne-Marie Villeneuve

Dr Roman Jednak

Copy Editor

Jane Broderick

Translation

Käthe Roth

Production

Mac Thien Nguyen Hoang

Prepress

Kien Tang Karine Lévesque

Procrea BioSciences Inc. Division of Urology The Montreal Children’s Hospital

Dr Michael S. Kramer Departments of Pediatrics and of Epidemiology and Biostatistics Faculty of Medicine McGill University

Dr Pierre Lachapelle Department of Ophthalmology McGill University

Dr Denis Laflamme Dr Maria Do Carmo MD Multimedia Inc.

Dr Claude Lamarche Faculté de médecine dentaire Université de Montréal

Dr Sheldon Magder Faculty of Medicine McGill University

The human body was created and produced by QA International 329, rue de la Commune Ouest, 3e étage Montréal (Québec) H2Y 2E1 Canada T 514.499.3000 F 514.499.3010 ©2007 QA International. All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying and recording, or by any information storage and retrieval system, without written permission from the Publisher. ISBN 978-2-7644-0892-6

Printed and bound in Slovakia. 10 9 8 7 6 5 4 3 2 1 04 03 02 01

www.qa-international.com

Dr Nelson Nadeau Dr Louis Z. G. Touyz Faculty of Dentistry McGill University

Dr Teresa Trippenbach Department of Physiology McGill University

Dr Martine Turcotte Dr Michael Wiseman Faculty of Dentistry McGill University

The human

body

QA INTERNATIONAL

Table of

41 40 38 36 34 32 30 28 27 26 24 22 20 18

6 | The body’s building blocks 8 10 12 14

The human cell Chromosomes and DNA Cellular activity Body tissues

The movements of the hand The action of the skeletal muscles The muscles of the head Muscle tissue The skeletal muscles The joints The hand and the foot The spine The head Types of bones The human skeleton Bone growth Bone structure The skin

16 | The architecture of the body

42 | The nervous system 44 46 48 50 52 54

4

72 70 68 67 66 64 62 60 58

Neurons The central nervous system The brain The cerebrum The peripheral nervous system The motor functions of the nervous system

Smell Taste receptors Taste Balance Perception of sound The organ of hearing Sight The eye Touch

56 | The five senses

contents 110 The liver, pancreas, and gallbladder 109 The intestines 108 The stomach 106 The teeth 104 The digestive system 102 Speech 100 Respiration 98 The respiratory system

74 | Blood circulation 76 78 80 82 84 86 88 90 92

Blood The cardiovascular system Arteries and veins The heart The cardiac cycle The lymphatic system Immunity The endocrine system The hypothalamus and the pituitary gland 94 The urinary system

96 |Respiration and nutrition

112 | Reproduction 114 116 118 120 122

The male genital organs The female genital organs Fertilization The life of the embryo Maternity

124 | Glossary 126 | Index

5

What is the human body made of? Although our bodies are very complex, they are composed of fundamental units that are very similar to each other. These

microscopic basic components are

assembled to form the different tissues that form all the body’s organs. Cells are also the sites of intense and constant activity: they themselves.

manufacture living matter, consume energy, and continually reproduce

The body’s building blocks 8

The human cell The body’s basic component

10

Chromosomes and DNA The code of life deep within cells

12

Cellular activity Cell division and protein synthesis

14

Body tissues Groupings of cells

The human cell The body’s building blocks

The body’s basic component The human body contains about 60 billion human cells. These cells, the basic components of the human body, are invisible to the naked eye, as their diameter generally is less than a few hundredths of a millimeter. Although they take many forms, depending on their location and their function, they always have a welldefined structure: an exterior membrane, a nucleus, and a number of internal elements floating in a gelatinous medium, the cytoplasm. DIFFERENT TYPES OF CELLS The human body contains a great many types of cells, which are differentiated according to their function. Despite their different sizes and shapes, all have the same general structure. Cytoplasm, which fills the intracellular space, is a jellylike substance composed of water, proteins, lipids, ions, and glucose.

The rods of the retina contain light-sensitive pigments.

Lysosomes contain enzymes that perform intracellular digestion.

The nucleus of the neutrophil has several lobes.

Microtubules, which form the skeleton of the cell, make it easier for organelles to move within the cytoplasm.

Erythrocytes (red blood cells) color the blood red.

Made mainly of lipid molecules, the cell membrane forms a selective water-insoluble barrier. The ovum is the largest cell in the human body.

Spermatozoids have a long flagellum.

Neurons (neural cells) can be up to 1 meter in length.

The irregular shape of osteocytes (bone cells) enables them to lodge in very narrow cavities of bony tissue. 8

Enveloped in a double membrane, mitochondria produce and store energy.

Enzymes enclosed in peroxisomes perform oxidization.

Cilia, formed of a group of microtubules covered by the cellular membrane, can propel the cell or move a substance outside the cell. Large cilia are called flagella.

THE STRUCTURE OF HUMAN CELLS

The body’s building blocks

Human cells (like those of all higher orders of life) are called eukaryotes – that is, their genetic material is enclosed in a nucleus defined by a nuclear membrane. The rest of the cell is composed of cytoplasm, a semiliquid medium structured by a network of microtubules and microfilaments. The organelles that float in the cytoplasm (mitochondrion, Golgi apparatus, endoplasmic reticulum, lysosome) perform different cellular functions, such as storing energy, synthesis and transportation of proteins, and digestion of foreign bodies.

Chromatin, the main component of the nucleus, is a filament formed of DNA and proteins. The nuclear membrane has a large number of pores.

Ribosomes are made in the nucleolus, in the center of the nucleus.

free ribosome The endoplasmic reticulum (ER), located near the nucleus, consists of a network of membranous pockets and canals. The rough ER is covered with ribosomes that synthesize proteins, while the smooth ER does not have ribosomes and produces other types of substances.

The Golgi apparatus resembles a series of membranous sacs attached to the rough ER. It collects the proteins synthesized by the ribosomes, sometimes changes them by adding carbohydrates, then releases them into vacuoles.

Microfilaments are made of a protein, actin. With the microtubules, they form the cytoskeleton, which gives the cell its shape.

Vacuoles, small liquid-filled vesicles, move from the Golgi apparatus to the cellular membrane, where they release the proteins that they contain.

TRANSPORT OF PROTEINS IN THE CELL

Each cell has two centrioles, formed of bundles of microtubules placed at a right angle to each other. They play a role in cell division.

Protein synthesis, one of the main functions of the cells, is performed in small particles called ribosomes. There are two types of ribosomes: free ribosomes, which secrete their products directly into the cytoplasm, and ribosomes attached to the endoplasmic reticulum, which release their proteins outside the cell. Proteins move through the network of membranous sacs in the endoplasmic reticulum, are processed by the Golgi apparatus, and then migrate toward the cellular membrane inside a vacuole.

9

Chromosomes and DNA The body’s building blocks

The code of life deep within cells Each cell in our body has a nucleus. Although nuclei are only a few microns in diameter, they are the site of fundamental mechanisms, such as cell division and protein synthesis. The substance responsible for these phenomena, deoxyribonucleic acid (DNA), is in the form of very long helicoidal molecules in constant motion. During the process of cell division, these filaments twist around on themselves to form chromosomes. DNA molecules are unique in that they are formed of two strands linked by several billion successive bonds. The sequence of these components constitutes a code that is capable of commanding the production of a large number of specific proteins and also replicating itself.

nucleolus

The nucleus is separated from the cytoplasm by a porous nuclear membrane.

sister chromatids

centromere

The chromosomes float in a gelatinous substance, the nucleoplasm.

Human cells have 46 chromosomes, except for sexual cells, which have only half this number. Chromosomes cannot be observed except during cell division. At that time, they divide into two sister chromatids that remain attached to each other for a short time by a central zone, the centromere.

INSIDE THE NUCLEUS With the exception of red blood cells, all cells in the body contain a nucleus. Some, like the muscle cells, even have several. The nucleus of a cell contains one or several nucleoli and filaments of chromatin floating in the nucleoplasm. Chromatin, which generally looks like a string of beads, is composed of long DNA molecules wound around proteins called histones. When cells divide, this filament rolls up into a spiral, becomes condensed, and is organized to form characteristic small rods, the chromosomes.

10

THE MOLECULAR STRUCTURE OF DNA

The nucleotide is the basic component of the DNA molecule. It is composed of a phosphate group and a sugar, deoxyribose, to which one of the four bases attaches.

The body’s building blocks

DNA is a polymer – that is, its molecule is formed by the grouping together of a large number of simpler molecules. It can be visualized as a very long, twisted ladder whose two uprights are linked by billions of rungs, each of which is composed of two smaller molecules, nitrogenous bases. There are only four different nitrogenous bases in DNA: adenine, thymine, cytosine, and guanine. These molecules are linked up not at random but according to a strict rule resulting from their molecular structure: adenine can link only with thymine, and cytosine only with guanine. These bases are called complementary.

Adenine can link up only with thymine.

deoxyribose phosphate group

thymine

The nitrogenous base, linked to deoxyribose, links up with its complementary base to form a rung in the DNA molecule.

guanine Cytosine is the complementary base for guanine.

chromatin Each chromosome has a single DNA molecule, 2 millionths of a millimeter wide but several centimeters long.

THE GENETIC HERITAGE AND HEREDITY All of the cells in an individual’s body have resulted from the division of a single initial cell, and so they all contain absolutely identical DNA filaments. The sequence of nitrogenous bases is always different from one human being to another; the DNA composition of each human being is therefore unique. When the DNA molecule wraps around eight histone molecules, it forms a mass, the nucleosome, which supports it.

Much of our genetic heritage is linked to our belonging to the human race: all humans, for instance, have the same organs. However, other, more specific, genetic characteristics (physical features, predisposition to certain diseases) are transmitted from one generation to the next at the time the sexual cells merge. This mode of transmission is called heredity. 11

Cellular activity The body’s building blocks

Cell division and protein synthesis Like more complex living organisms, the cells in our bodies are born and die. Different cells have very different life spans: a few hours for white blood cells, four months for red blood cells. When they die, most cells are replaced by identical cells. Their life can thus be described as a cycle during which they prepare for and complete their reproduction by cellular division.

phase M

phase G2

phase S

The cell cycle comprises four successive stages: the three phases of the interphase (phases G1, S, and G2) and phase M. Phases G1 and G2 are phases of growth and intense metabolic activity. G1 is the longest and most variable phase (from 10 hours to several months, depending on the cell; even an entire life for neurons). Phase G2 lasts one to two hours. Phase S, which can last from four to eight hours, is the period during which replication of DNA takes place. Phase M corresponds to cell division itself and lasts only a few minutes.

phase G1

REPLICATION OF DNA

nucleotide

DNA molecule

matrix

An essential step in cell division consists of copying the cell’s genetic material, its DNA. To do this, the two strands of the double helix separate and become matrices for the synthesis of two new strands according to the principle of base pairing. When the DNA molecule has completely replicated, the cell has two absolutely identical molecules.

newly synthesized strand

cytoplasm chromosome

Q

pair of centrioles nucleus

CELL DIVISION Cell division, or mitosis, comprises several distinct steps. The DNA molecules, deployed as chromatin during the interphase, coil and thicken during the prophase Q, which makes the chromosomes visible. The nucleolus disappears and the two pairs of centrioles move apart and migrate to opposite ends of the cell, while a system of microfilaments, the mitotic spindle, forms between these two poles. Gradually, the nuclear membrane disintegrates and the chromosomes move along the filaments of the mitotic spindle. During the metaphase W, the chromosomes line up at the center of the cell. When their centromeres divide, the anaphase begins E: the chromatids, which have become complete chromosomes, are drawn to one or the other end of the cell. In the telophase R a new nucleus forms at each end of the cell. The chromosomes uncoil to become chromatin once more, while a new nuclear membrane is formed. The mitotic spindle disappears and the cytoplasm begins to separate during a phase called cytocinesis T. At the end of the process, the original cell is replaced by two new identical cells Y. 12

W

mitotic spindle

E

R

new nucleus T

Y

SYNTHESIS OF PROTEINS Proteins are large molecules formed by the grouping together of several amino acids. Some proteins play specific roles in the body’s functioning (hormones, antibodies, enzymes), while others constitute its living material. The synthesis of proteins, which is one of the cell’s main functions, is performed according to instructions coded in genes, segments of various lengths of the DNA molecule. Each gene is distinguished by a particular sequence of nitrogenous bases. The synthesis of a protein consists of transcribing this sequence onto a messenger molecule, then translating it into the sequence of amino acids that form the protein. The bases of the messenger RNA molecule are complementary to those of the gene that produces it.

nucleotide

The messenger RNA molecule is composed of the same bases as DNA, except that uracil replaces thymine.

DNA molecule

W 0

E 0

Q 0

pore

codon matrix

ribosome

R 0

nuclear membrane

T 0

amino acid

Y 0

TRANSCRIPTION AND TRANSLATION The first phase in the process of synthesis of proteins, transcription, takes place in the cell nucleus. When a gene is activated, its two strands separate, and one of them serves as the matrix Q for a molecule of messenger ribonucleic acid (RNA-m) W. Once formed, this molecule leaves the nucleus through one of its pores E and attaches itself to a ribosome R, where it is translated. Translation consists of converting the molecule of RNA-m into a sequence of amino acids. The bases of the RNA-m are processed not one by one, but in groups of three, called codons T, which serve as matrices for specific amino acids. As the codons are processed, the amino acids Y are assembled in the order defined by the sequences of the gene’s bases. When the RNA-m molecule has been completely translated, the sequence of amino acids forms a protein U.

RNA-m

U 0

protein

13

Body tissues The body’s building blocks

Groupings of cells In the human body, cells do not function separately. They are grouped together in different tissues that compose the organism’s organs. There are four types of tissues in the human body: epithelial tissues, which form the covering of many parts of the body; connective tissues, which play a support role; muscle tissues; and nerve tissues. Aside from cells, the tissues contain extracellular liquid, in which substances needed by the body to function (such as hormones, proteins, and vitamins) circulate and dissolve.

microvilli

basement membrane

nucleus of an epithelial cell

EPITHELIAL TISSUE The epithelium (or epithelial tissue) covers most of the internal and external surfaces of the body, including skin, mucus, blood vessels, glands, and cavities of the digestive system. The epithelial cells are cubical, columnar, or squamous (flat) and are tightly packed against each other to form coverings that can include one or more layers. They sit on a basement membrane that connects them to the underlying vascularized tissues. On the outside of the body they are impermeable, but on the inside they play a role of absorption and secretion within the organism, due to the microvilli that cover certain epithelial cells.

CONNECTIVE TISSUE Unlike the epithelium, connective tissue has relatively few cells, floating in a very abundant intercellular matrix composed mainly of fibers and a semi-liquid substance. Connective-tissue cells fall into two primary categories: fibroblasts and macrophages. The intercellular matrix of connective tissue involves mainly three types of fibers formed of proteins: collagen fibers, elastic fibers, and reticular fibers. The density and positioning of these fibers, as well as the presence of other, more specific cells, gives connective tissue very different aspects. Cartilage, bone tissue, blood, and most of the tissues that make up the organs are connective tissues.

Reticular fibers form solid branched networks.

Elastic fibers are able to return to their original length after being stretched.

Collagen fibers, made of bundles of fibrils, are very strong. They make the matrix flexible and rubbery.

Fibroblasts make tissue fibers. 14

Macrophages destroy undesirable elements (foreign bodies, debris, dead cells).

Muscle cells are called fibers, but they should not be confused with the protein fibers present in connective tissue.

The tissues that form muscles are distinct because of the way their cells are bundled. There are three types of muscle tissue: skeletal muscle, cardiac muscle, and smooth visceral muscle.

Skeletal muscle tissue is formed of very elongated multinuclear fibers. These cells look striated due to the alternation of the two types of filaments that compose them. cell nucleus

The body’s building blocks

MUSCLE TISSUE

The fibers of cardiac muscle tissue are also striated, but they are differently organized, with numerous, tight ramifications.

Smooth muscle tissue includes shorter, spindle-shaped cells. These fibers have only one nucleus and are not striated.

NERVE TISSUE The brain, the spinal cord, and nerves are formed of nerve tissue, which consists of a dense tangle of cells. There are two categories of cells in nerve tissue: neurons, which are the true nervous cells, and glial cells (astrocytes, oligodentrocytes, microgliocytes, Schwann cells, etc.). Glial cells are ten times more numerous and generally smaller than neurons. They do not play a direct role in nerve functions but support, protect, and nourish the neurons. They are also capable of dividing by mitosis, which neurons cannot do.

Neurons are highly specialized cells that transport and transmit nerve impulses by establishing innumerable connections between each other. Tiny microgliocytes rid nerve tissue of foreign bodies and dead cells. neuron The axon is the main extension of the neuron. Oligodendrocytes are the most common glial cells. They have extensions that coil around the axons of the neurons of the central nervous system.

The many extensions of the astrocytes finish in “feet” that form barriers, called hemato-encephalic barriers, between the neurons and blood capillaries. 15

From the phalanges to the bones of the skull, the 206 bones that make up the human skeleton play an essential supportive and protective role. But the architecture of the human body is not determined solely by its skeleton: our organism also has more than 600 muscles with which we control our limbs and move around. This

strong,

efficient basic structure could not function without the protective envelope that covers it. The skin, with 1.5 m2 of total surface area, is the largest organ of the human body.

The architecture of the body 18

The skin The body’s protective envelope

20

Bone structure Flexible yet strong tissues

22

Bone growth From cartilage to bone tissue

24

The human skeleton The bony structure of the body

26

Types of bones Form determined by function

27

The head A grouping of flat and irregular bones

28

The spine The central axis of the body

30

The hand and the foot The extremities of the limbs

32

The joints The junctions between the bones

34

The skeletal muscles Motion generators

36

Muscle tissue Bundles of contractile cells

38

The muscles of the head An infinite variety of movements

40

The action of the skeletal muscles From contraction to movement

41

The movements of the hand Incredible dexterity

The skin The architecture of the body

The body’s protective envelope We may not think of it this way, but the skin is the largest organ in the human body: an adult’s skin covers an area of 1.75 m2 and represents 7% of total body mass. This envelope is composed of a superficial layer, the epidermis, and a deeper layer, the dermis. With the different types of cells that it contains (keratinocytes, melanocytes, sensory receptors), the skin fulfills a number of important functions that protect us against the external environment. THE LAYERS OF THE EPIDERMIS The epidermis is an epithelial tissue composed essentially of keratinocytes. These cells are formed in the deepest layer of the epidermis (the basal layer) and then are pushed toward the spinous layer by younger cells. As they migrate, the keratinocytes become impregnated with a fibrous protein, keratin, which gradually replaces their cytoplasm. By the time the cells reach the outer layer (the horny layer), their nuclei have completely disintegrated. These dead, flattened keratinous cells make the skin impermeable. The dead cells that make up the horny layer are constantly sloughed off to make room for new cells.

spinous layer

pore

Although it is very thin (0.1 mm), the epidermis plays a major role in body defense, forming a physical barrier. The cells of the basal layer are constantly multiplying through mitosis. Different types of tactile receptors detect the stimuli of touch, pressure, and temperature. The dermis is composed of connective tissue rich in blood vessels and nerves. blood vessel nerve The hypodermis, located under the dermis, contains mainly adipose (fatty) tissue.

The perspiration produced by the sweat glands exits the skin via tiny orifices, the pores. adipose tissue

THE SKIN’S DEFENSES Human skin has many means of defending itself against various assaults. The epidermis contains two proteins: keratin, which makes it impermeable, and melanin, which blocks ultraviolet rays. Perspiration protects against certain bacteria, cools the skin, and evacuates certain substances. Sebum, released by sebaceous glands attached to hair follicles, is a fatty substance that keeps the skin from drying out and protects it from bacteria. When sensory receptors detect injuries, the central nervous system is able to react.

18

medulla cuticle

Hairs, made by the hair follicles in the dermis, grow over most of our skin. They have sebaceous glands, which coat them with sebum; arrector muscles, which pull them upright when necessary (cold or fright); and nerve receptors, which detect the lightest touch.

PIGMENTS FOR SUN PROTECTION The deepest layer of the epidermis contains specialized cells called melanocytes. Activated by melanocyte-stimulating hormone produced by the pituitary, melanocytes produce melanin, a dark-brown pigment. Melanin molecules released by the cellular extensions of melanocytes enter the keratinocytes and settle over cell nuclei to protect them from potentially carcinogenic ultraviolet rays.

The architecture of the body

cortex

melanin keratinocyte Melanocytes comprise 8% of the epidermic cells. The color of the skin depends not on the number of melanocytes, but on their size and degree of activity.

Sebaceous glands produce sebum, a substance that coats the hairs and skin with oil. arrector muscle of hair hair follicle

HOW THE SKIN FORMS SCARS When the skin sustains a deep injury Q, down to the dermis or even the hypodermis, a substance generated by blood coagulation, fibrin W, rapidly forms a clot at the bottom of the wound. When the clot dries up, it creates a crust E, which has to be eliminated so that the cells of the epidermis can migrate to form a new epidermis. At the same time, fibroblasts (young cells) R and capillaries (small blood vessels) of the dermis multiply to reconstruct the tissues T. As tissues grow, they push the crust toward the normal surface of the epidermis, where a small swelling, or scar, may form Y.

epidermis

dermis

crust

fibrin

scar Y 0

E 0 T 0 0 W 0 Q R 0

deep wound

fibroblasts

reconstructed tissue 19

Bone structure The architecture of the body

Flexible yet strong tissues A bone is six times as strong as a bar of steel of the same weight. This remarkable strength comes from the nature of the bone tissues. All bones are composed of groupings of compact and spongy (or cancellous) tissues in different proportions and positions depending on the type of bone. These tissues contain collagen, a protein that gives bones their flexibility, and mineral salts (calcium, phosphorus), which are responsible for their solidity. Long bones, such as the femur, are composed of a hollow cylindrical central portion, the shaft, and two bulges at the ends, the epiphyses. Between the shaft and the epiphyses are the metaphyses. proximal epiphysis The epiphyses are composed mainly of spongy tissue covered with articular cartilage. They contain red marrow, a tissue that produces several types of blood cells.

metaphysis

shaft

metaphysis distal epiphysis

SPONGY BONE TISSUE In adults, the interior of the epiphyses and metaphyses is formed of spongy bone in an irregular honeycomb formation. This structure gives the bone its lightness.

trabecula

20

The cavities between the trabeculae are filled with marrow, blood vessels, and nerves.

The shaft and metaphyses are completely covered by a fine vascularized membrane, the periosteum.

The outer layer of bones is formed of compact bone tissue, which is very dense and remarkably resistant to pressure and shocks. Compact tissue is composed mainly of osteons, small cylinders made of a number of concentric layers of hard matrix. Osteons are packed tightly together and are connected by longitudinal canals (haversian canals) and transversal canals (Volkmann’s canals), which contain lymphatic and blood vessels. In spite of its density and hardness, compact bone tissue is alive. Tiny cavities (lacunae) and canals (canaliculi) between the lamellae are filled with osteocytes, mature bone cells responsible for providing nutrition to the bone tissue.

lamella The lacunae of the osteon are connected by tiny canals, canaliculi, where extensions of the osteocytes are lodged.

The lacunae are filled with osteocytes. periosteum

osteon

Volkmann’s canals connect the haversian canals to the periosteum and medullary canal.

An osteocyte is a bone cell completely surrounded by matrix. Its many extensions carry nutrient elements.

At the core of every osteon is a haversian canal, through which circulate blood vessels, lymphatic vessels, and nerves.

The concentric lamellae of an osteon are composed of collagen fibers.

The shafts of long bones are frequently subjected to great pressure. They are made mainly of compact bone tissue.

The medullary canal, in the center of the shaft, contains rich in fat yellow bone marrow.

The architecture of the body

COMPACT BONE TISSUE

Bone growth The architecture of the body

From cartilage to bone tissue Bone formation starts during the embryonic stage, but many parts of the skeleton are still made of cartilage at birth. Bones do not reach their final size until adulthood. This growth takes place through a process called ossification: cartilaginous cells multiply, die, and are replaced by bone cells. ENDOCHONDRAL OSSIFICATION The embryo’s skeleton is formed of cartilage models that approximate the shape of the bone. Starting in the sixth week of pregnancy, cartilage cells in the center of the model grow, explode, and die, which causes calcification. At the same time, osteoblasts (cells that produce bone tissue) multiply on the perichondrium.

When the fetus is about three months old, blood vessels begin to penetrate the calcified model and a primary ossification center appears. Osteoblasts deposit bone tissue on the calcified cartilage and form bony trabeculae. As the process extends toward the epiphyses, the trabeculae at the center of the shaft are gradually destroyed by other cells, which enables the bone to remain lightweight.

The model is formed of hyaline cartilage.

The epiphysis remains completely cartilaginous up to birth.

The cartilage is covered by a membrane, the perichondrium.

Calcified cartilage is transformed into bone tissue by osteoblasts.

When the osteoblasts begin to make bone tissue, the perichondrium is transformed into periosteum.

bony trabecula Osteoblasts located under the periosteum produce compact bone tissue.

calcified cartilage

penetrating artery

GROWTH OF THE HAND BONES At birth Q, the wrist is made of cartilage. The bones of the fingers (phalanges) and the palm (metacarpal bones) are still incomplete. At around four years of age W, the carpal cartilage begins to ossify to form the wrist, while the metacarpal bones and phalanges develop. By puberty E, most of the bones in the wrist are formed. The bones in the palm and fingers continue to lengthen. By adulthood R, all the bones in the hand and wrist have finished growing. phalanx metacarpal bone

Q 22

wrist

W

carpal bone

E

R

The destruction of cartilage and its replacement with bone tissue leave a thin cartilaginous layer, articular cartilage, on the surface of the epiphysis. Meanwhile, the epiphysis and the shaft continue to be separated by growth plates, which allows ossification to continue and the bone to grow longer. In adulthood, this band of cartilage finally ossifies, but it remains visible as an epiphyseal line.

spongy bone tissue articular cartilage

A secondary ossification center allows for bony development of the epiphysis.

The architecture of the body

At birth, the shaft has a central cavity (the medullary canal) surrounded by a cylinder of compact bone tissue. Arteries penetrate the epiphyses, which causes secondary ossification centers to appear. The process of bone formation is similar to that for the diaphysis, except that the bony trabeculae are not destroyed. Thus, the shafts do not contain a medullary canal but are filled with spongy bone tissue rich in red bone marrow.

hyaline cartilage

growth plate epiphyseal artery spongy bone tissue

Blood vessels are essential to ossification, since bone tissue, unlike cartilage, is vascularized. compact bone tissue

compact bone tissue

During childhood, the medullary canal contains red bone marrow.

REPAIR OF A BROKEN BONE When a bone is fractured, the blood vessels that it contains are broken. Blood flows into the injury, and after a few hours it forms a plug called a hematoma Q. In a few weeks, a soft tissue made of specialized cells (fibroblasts and chondroblasts), a fibrocartilaginous callus W, replaces the plug and fuses the two parts of the bone. The fibrocartilaginous callus is gradually invaded by osteoblasts, which convert it into bony callus E. After several months, the compact bone tissue is totally reconstructed and only a thickening R of the bone remains at the site of the fracture. fibrocartilaginous callus

hematoma

Q 0

bony callus

W 0

thickening

E 0

0 R

blood vessel 23

The human skeleton The architecture of the body

The bony structure of the body Like other vertebrates, human beings have an internal skeleton that supports the different muscles in the body and protects the vital organs. The positioning and articulation of the bones of the skeleton also determine the nature of the body’s movements. The adult human skeleton contains about 206 bones, but this number can vary slightly from individual to individual: some people, for example, have an extra pair of ribs. The bones of the human body are part of the axial skeleton (the bones of the skull and the face, the vertebrae, the ribs, and the sternum) or the appendicular skeleton, formed of the upper and lower limbs and the limb girdles (the bones of the shoulders and the hips) that attach them to the axial skeleton.

THE PELVIC GIRDLE OF MEN AND WOMEN Although the woman’s skeleton is generally smaller than the man’s, they are fundamentally the same; only the pelvis is noticeably different. Seen from the front, the woman’s pelvis appears wider, though less massive, than a man’s. The woman’s ischia are also more spread out, leading to the pelvic outlet, the opening formed by the bones of the pelvis and the sacrum. This anatomical arrangement facilitates the passage of the baby at childbirth. It also changes the orientation of the acetabulum, which has an effect on how men and women walk.

man’s pelvis

woman’s pelvis FRONT VIEW ilium sacrum pubis

hip bone

ischium obturator foramen BOTTOM VIEW

sacrum ilium coccyx

ischium

pelvic outlet

SIDE VIEW ilium

pubis 24

sacrum The head of the femur is articulated in the acetabulum. ischium

skull

mandible

The upper limbs are attached to the axial skeleton by the pectoral girdle, which comprises the shoulder blades (scapulae) and the clavicles (collarbones). The humerus is the bone of the upper arm. It articulates with the shoulder blade at the shoulder and with the bones of the forearm, the radius and ulna, to form the elbow joint. The hand is formed of carpal bones, which articulate with the radius at the wrist, the metacarpal bones, and the phalanges of the fingers. clavicle shoulder blade sternum

THE AXIAL SKELETON

humerus

The axial skeleton is formed of the 80 bones of the skull, the spine, and the thorax. Aside from their role in protecting the vital organs (cerebrum, heart, lungs, spinal cord), these bones provide the body with structure, and they support the bones of the limbs.

ribcage spine

The architecture of the body

THE UPPER LIMBS

hip bone sacrum ulna radius carpal bones metacarpal bones phalanges

femur

THE LOWER LIMBS

kneecap

The pelvis, composed of two hip bones and the sacrum, attaches the lower limbs to the axial skeleton. A hip bone results from the fusion of three bones: the ilium, the pubis, and the ischium. The pelvis also protects the organs of the pelvic cavity (rectum, bladder, internal genital organs).

tibia fibula

tarsal bones

The femur, which articulates with the pelvis, is the longest bone in the human body. At its lower end, it and the tibia form the knee joint, which is protected by the kneecap (patella). The tibia and fibula are bound together by short, dense ligaments. The foot is composed of 26 bones. The tarsal bones structure the ankle and heel, the metatarsal bones form the sole of the foot, and the phalanges are the bones of the toes.

metatarsal bones phalanges 25

Types of bones The architecture of the body

Form determined by function The some 200 bones that form the human skeleton have a variety of shapes. There are generally four types of bones, classified by their appearance: long, flat, irregular, and short. This classification highlights the match between a bone’s shape and its function.

THE FLAT BONES The flat bones, which are thin and flattened, play two essential roles. Some, such as the pair of parietal bones, which are part of the skull, protect fragile organs. Others, such as the shoulder blade, have a large surface area to which tendons can attach.

parietal bones

shoulder blade

clavicle

humerus

THE LONG BONES

thoracic vertebra

The long bones, such as the humerus and clavicle, are, as their name implies, long and thin. Some are quite small, such as the phalanges of the fingers. The four limbs of the human body contain mainly long bones, to which the motor muscles attach.

IRREGULAR BONES hip bone

26

Many of the irregular bones are complex, and they have a wide variety of shapes and sizes depending on their function. The vertebrae are stacked on top of each other to form a protective channel through which the fragile spinal cord passes. The pair of hip bones form the bony pelvis to which the lower limbs are attached.

kneecap

THE SHORT BONES

talus

The short bones are small and more or less cubical in shape, providing the joints with flexibility. This is the case for the talus, which enables the ankle to turn. The kneecap, which is enveloped in a ligament, is a particular type of short bone called sesamoid, because of its resemblance to a sesame seed.

The head If you look closely at a skull, you will notice that it has fine, irregular lines. These are sutures, rigid joints at the borders of the different cranial bones. The skull is not a single bone, but is formed of eight different bones that gradually fuse together during growth. The more numerous bones of the face are irregular in shape and define the cavities of the mouth, the nasal cavities, the eye sockets (orbits), and the sinuses. sphenoid bone The two parietal bones form most of the skull.

The frontal bone forms the front and top of the eye sockets. It contains air-filled cavities, the frontal sinuses.

The architecture of the body

A grouping of flat and irregular bones

lacrimal bone

suture

The two nasal bones meet along the front edge of the nose.

occipital bone The temporal bones are pierced by the auditory meatuses, which link the middle ear to the outside.

The two zygomatic bones are more commonly called cheekbones.

auditory meatus maxilla

The lower jaw (mandible) is the only jointed bone in the head. INTERIOR OF THE HEAD

BOTTOM OF THE HEAD The palatal bone is the back part of the upper jaw (maxilla).

The sphenoid bone is attached to all the other bones in the skull.

sphenoid bone

frontal sinus The ethmoid bone, a light bone with complex shapes, has several holes through which the olfactory nerves pass. foramen magnum

The vomer forms the back of the nasal partition.

The carotid artery passes through the carotid canal to join the heart to the cerebrum. The brain stem passes through the foramen magnum to connect the skull to the spine. parietal bone

THE SKULL OF A NEWBORN At birth, the bones of the skull are not completely fused together. They are linked by membranes with wide areas called fontanels. The skull bones therefore have a degree of mobility, which enables the head to deform during childbirth and then for the skull to adapt to growth of the cerebrum during the child’s early years.

fontanels frontal bone

27

The spine The architecture of the body

The central axis of the body The spine, also called the vertebral column, is the central axis of the human body. It extends from the back of the skull to the pelvis and is made of a chain of small bones, the vertebrae, which house the spinal cord and serve as points of attachment for the ribs and muscles. THE VERTEBRAE Human beings have 33 vertebrae, which anatomists divide into five categories: cervical, thoracic, lumbar, sacral, and caudal. Although they have slightly different proportions, all vertebrae have a similar structure: a body to which bony prominences, the processes (or apophyses), are attached. The column contains a central channel, the spinal foramen, through which the spinal cord passes.

The first vertebra, the atlas, is articulated with the occipital bone.

CERVICAL VERTEBRAE spinous process

The seven cervical vertebrae are the most mobile in the spine.

The vertebral arterial foramen provides a passageway for blood vessels and nerves.

vertebral body THORACIC VERTEBRAE The 12 thoracic vertebrae, which are larger than the cervical vertebrae, also have longer apophyses, which attach to the ribs.

transverse process

The spinal foramen houses the spinal cord. LUMBAR VERTEBRAE The spinous process provides a point of attachment for the muscles of the back.

The five lumbar vertebrae form a massive body capable of supporting the weight of the abdomen.

vertebral body

SACRUM AND COCCYX The five sacral vertebrae fuse together in late adolescence to form a single bone, the sacrum, which is joined to the bones of the pelvis. The coccyx is formed when the four atrophied caudal vertebrae fuse at between 20 and 30 years of age. 28

The sacral wing is formed by the fusion of the transverse processes of the sacral vertebrae. The sacral foramens are passageways for sacral nerves.

coccyx

ARTICULATION OF THE VERTEBRAE

transverse process

The architecture of the body

Except for those that form the sacrum and the coccyx, all vertebrae are mobile. They are articulated with each other via small prominences, the inferior and superior articular processes. The body of each vertebra rests on an intervertebral disk, a gelatinous mass that serves as a shock absorber. This unique structure makes the spine both strong and very flexible.

superior articular process

spinous process inferior articular process The spinal nerves run through the vertebral foramens.

body

intervertebral disk

THE RIBCAGE The thorax, which is the upper part of the human trunk, contains the lungs and the heart. These vital organs are protected by the ribcage, a bony cage formed by 12 pairs of ribs articulated with the thoracic vertebrae and the sternum. The 10 top pairs of ribs are attached to the sternum by costal cartilage, which is flexible enough to allow the ribcage to change shape during respiration. The two lowest pairs of ribs, which are not attached to the sternum, are called floating ribs. clavicle shoulder blade thoracic vertebra

The head of the rib articulates with the vertebra through its two facets. rib costal cartilage

head of the rib rib

The sternum is a long, flat bone, rich in red marrow.

sternum thoracic vertebra The costal cartilage links the rib to the sternum.

floating ribs

29

The hand and the foot The architecture of the body

The extremities of the limbs As the human species has evolved, the function of the hands and feet has become very differentiated: the hands are used to grasp, while the feet provide stability and mobility for the body. In spite of these functional differences, the hand and foot have very similar skeletons. In both, there are five digits formed of phalanges, a central part composed of five long bones, and a back part composed of short bones that join them to the limb. Our two hands and two feet contain a total of 106 bones, more than half of all the bones in the human skeleton.

THE BONES OF THE HAND The palm of the hand is formed of the five metacarpal bones, each of which is extended by phalanges that form the bones of the fingers. Each finger has three phalanges (proximal, middle, and distal), except for the thumb, which has only two (proximal and distal). A complex grouping of eight carpal bones makes up the wrist. Two of these, the scaphoid and the lunate, articulate with the radius.

finger

distal phalanx middle phalanx

palm of the hand proximal phalanx

wrist

phalanges (fingers) little finger ring finger middle finger metacarpals (palm) index finger trapezoid

carpals (wrist)

trapezium capitate scaphoid radius

hamate triquetral pisiform lunate ulna

30

thumb

back of the hand

horny layer of the epidermis

Each finger and toe has a nail at the end. This small protective plate consists of horny epidermal cells produced by a matrix located over the distal phalanx. Nails are hard because of their very high concentration of the protein keratin.

matrix

The base of the nail is protected by a fold in the skin, the cuticle. The fingernail grows an average of a tenth of a millimeter every day. Therefore, a completely new fingernail grows in about every six months. distal phalanx

The architecture of the body

THE NAILS

tibia

THE BONES OF THE FOOT The skeleton of the foot has a structure similar to that of the hand. A group of seven bones composes the tarsus, which forms the ankle and articulates with the tibia and fibula. It is followed by the five bones of the metatarsus, which form the foot itself, then the phalanges. Like the fingers of the hand, each toe has three phalanges (proximal, middle, and distal), except for the big toe, which has only two.

ankle

medial malleolus

fibula

lateral malleolus

The epiphysis of the tibia forms a bony projection called the medial malleolus.

tarsus The lateral malleolus is formed by the end of the fibula. metatarsus

toe big toe phalanges

The talus is the central bone of the ankle. Tucked behind the ends of the tibia and fibula, it distributes the body’s weight between the calcaneus and the scaphoid bone.

scaphoid bone

The calcaneus, the heel bone, supports much of the body’s weight. It is also where the Achilles’ tendon attaches.

31

The joints The architecture of the body

The junctions between the bones The points of contact between the bones are essential for the mobility and solidity of the skeleton. The nature of the tissue that forms the joint between two or more bones determines, in large part, the amplitude of the movement associated with that joint. Fibrous and cartilaginous joints have very little mobility, while synovial joints allow a wide variety of movements. However, the nature of the movement also depends very much on the shape of the bones. first rib synchondrosis

FIBROUS AND CARTILAGINOUS JOINTS Certain bones, like those of the skull, are connected by very dense fibrous tissue. These fibrous joints, also called sutures, render bones immobile so that they perform a protective function. When two bones are linked by cartilaginous tissue, the joint permits very limited movement. This is the case for the joint between the first rib and the sternum, called the synchondrosis, and the joints between the bones of the pubis, known as the symphysis.

sternum synovial membrane

fibrous capsule

middle phalanx

SYNOVIAL JOINTS Most joints are mobile – they allow bones to move in relation to others, in some cases with great amplitude. These joints are contained in a fibrous capsule solidly attached to the periosteum. The membrane that lines the interior of the capsule produces synovial fluid, which fills the synovial cavity; it lubricates the joint and nourishes the cartilage that covers the end of the bones. synovial cavity

articular cartilage

distal phalanx

LIGAMENTS Most bones are connected to each other by ligaments, fibrous tissues that stabilize and reinforce the synovial joints. The knee joint has several types of ligaments. On either side of the leg, the collateral ligaments join the femur to the tibia and fibula and prevent the knee from moving from side to side. The patellar ligament strengthens the joint in the front, while the cruciate ligaments limit the knee from moving front to back. KNEE JOINT (FRONT VIEW)

collagen bundle

KNEE JOINT (BACK VIEW)

femur kneecap lateral collateral ligament The ligaments are formed of connective tissue with a uniform patellar ligament structure: several layers of collagen bundles overlap to make the tissue fibula elastic and strong. tibia 32

cruciate ligaments

medial collateral ligament

DIFFERENT TYPES OF SYNOVIAL JOINTS

Gliding joints permit only small lateral movements. They are found between the vertebrae and the ribs, in the carpus and tarsus, and between the navicular bone and the cuneiform bones.

navicular bone first cuneiform bone

The elbow is a hinge joint (or trochlear joint) that allows flexion and extension along a single axis. The convex projection of the humerus turns within the hollow of the ulna. humerus

The architecture of the body

The synovial joints are divided into six categories according to the nature of the movements they permit: gliding, hinge, pivot, ball, ellipsoidal, and saddle.

second cuneiform bone tibia fibula ulna A pivot joint (or trochoidal joint) allows a bone whose end is inserted in a bony or ligament ring to pivot on its longitudinal axis. This is the case for the fibula, whose head articulates with the tibia. lunate bone

A ellipsoidal joint (or condyloid joint) is bi-axial, as it allows movements on two different axes. The wrist joint, in which the scaphoid and lunate bones turn in the cavity of the radius, belongs in this category.

radius bone scaphoid bone

humerus

The hip and shoulder joints are ball joints, which allow movements along three axes. By turning in the glenoid cavity of the shoulder blade, the humerus can also make a circumduction movement – a complete circle.

omoplate

A saddle joint resembles an ellipsoidal joint, but it allows movements of greater amplitude because the two bony ends have convex and concave surfaces. The joint between the metacarpal bone of the thumb and the trapezium is a good example.

trapezium

first metacarpal bone 33

The skeletal muscles The architecture of the body

Motion generators There are muscles in every part of the human body – more than 600 in all, everywhere from the face to the limbs to the viscera. Together, they represent almost half of our body mass. Most of our muscles are attached to the bones of the skeleton; these are called skeletal muscles. They contract when they receive messages via nerve impulses, bringing their ends closer together, which causes bones to pivot in their joints and generates movements that can be very complex. They are also responsible for maintaining body tonus and posture.

zygomatic muscle

ilium

The sartorius is the longest muscle in the body (50 cm). It is attached to the ilium and inserts into the tibia after spanning two joints (the hip and the knee). It causes the thigh to flex and rotate. femur

frontal muscle

masseter sternocleidomastoid trapezius deltoid

Contraction of the greater pectoral enables a number of arm movements. anterior serratus

biceps of arm

abdominal rectus tibia external oblique

tensor of fascia lata

brachioradial radial flexor of wrist

aponeurosis internal abdominal long oblique muscle adductor rectus of thigh

sartorius vastus lateralis vastus medialis

long peroneal muscle The abdomen is protected by several layers of muscles whose fibers are oriented in different directions. The external oblique muscle, which is the external layer, covers the internal abdominal oblique muscle, which in turn rests on the transverse abdominal muscle. These three muscles have a membranous part (an aponeurosis) at the center of the abdomen, where they join to the abdominal rectus. 34

anterior tibial muscle extensor muscle of toes

origins of the humerus

bellies of the triceps of arm

BETWEEN MUSCLE AND BONE: THE TENDON A skeletal muscle spans one or several joints and is attached to the bone by whitish fibrous bundles called tendons. Contraction of a muscle generally makes only one bone move, while the other stays immobile. The point of attachment on the immobile bone is called the origin of the muscle; the one on the mobile bone is called the insertion. The central fleshy part of the muscle is called the belly. Some muscles have several origins and, therefore, several bellies. Depending on the number of their tendons, they are called biceps, triceps, or quadriceps.

tendon insertion on the ulna hip bone

occipital sternocleidomastoid

Movements of the shoulder blade are controlled by the trapezius.

The architecture of the body

origin of the shoulder blade

sacrum

The largest muscle in the body is the greatest gluteal: it can weigh up to 1 kg. It is responsible for extension of the hip and stabilization of the body in an upright position.

deltoid infraspinous

broadest of back

femur The triceps of arm extends the forearm.

The extensor muscle of fingers stretches the fingers (except for the thumb). greatest gluteal

semimembranous semitendinous

gastrocnemii

great adductor biceps of thigh

ischium femur

The biceps of thigh, which is on the back of the thigh, links the ischium to the femur at the head of the fibula and to the tibia. It controls flexion of the leg. The Achilles’ tendon, capable of supporting a weight of 450 kg, is the strongest tendon in the human body.

tibia fibula 35

Muscle tissue The architecture of the body

Bundles of contractile cells When the fibers that compose the skeletal muscles are examined under a microscope, long filaments can be seen within the cells. These myofibrils have very specific colored striations that are intimately connected to the mechanism of contraction of the fibers. THE ANATOMY OF SKELETAL MUSCLES The skeletal muscles are composed mainly of filiform muscle fibers with an average length of 3 cm and up to 50 cm. Grouped in abundantly vascularized bundles, these cells have long threads, the myofibrils.

Z line A band

I band

The sarcomere, bordered by two Z lines, is the structural unit of a myofibril. It is composed of an A band surrounded by two I half-bands. Myofibrils extend the entire length of the muscle fiber, but they are no more than 1 to 2 microns in diameter. A muscle fiber has a number of nuclei. Blood capillaries supply the fiber with oxygen and glucose. motor neuron A fiber bundle includes 1 to 100 muscle cells.

The tendon, made of the same material as the epimysium, is an extension of the muscle that attaches it to the bone.

The epimysium is an envelope of connective tissue that keeps several muscle-fiber bundles together. The deep fascia covers the epimysium and separates the muscles from each other.

muscle fiber

Each bundle of fibers is covered with a layer of connective tissue, the perimysium. 36

THICK AND THIN FILAMENTS

Myosin is the main component in thick filaments. The molecules of this protein, arranged in bundles, face outward. Thin filaments are composed of three proteins: actin, tropomyosin, and troponin.

The filaments are regularly distributed within the myofibrils: a thick filament is surrounded by six thin filaments.

The architecture of the body

The characteristic bands that appear along myofibrils are caused by two types of filaments: thick and thin. The dark-colored A bands are composed of both types of filaments, while the light-colored I bands contain only thin filaments.

thin filament

troponin molecule actin molecule thick filament

tropomyosin molecule

head of a myosin molecule

CONTRACTION OF SKELETAL MUSCLES In a muscle at rest Q, the thick and thin filaments of myofibrils are loosely interlaced in such a way that the existing spaces between two consecutive thick filaments form I bands. When a neuron transmits a nerve impulse to the muscle fiber W, the heads of the myosin molecules are energized. They connect with the actin molecules of the thin filaments, where they discharge their energy. This reaction makes the thin filament slide toward the center of the A band, which shortens the sarcomeres: the muscle fiber contracts. When the nerve impulse stops, a chemical reaction blocks the interaction between the myosin and the actin, which returns the thin filaments to their initial position: the muscle relaxes.

Z line

thin filament

head of a myosin molecule

thick filament

Q

I band

A band

I band

W 37

The muscles of the head The architecture of the body

An infinite variety of movements Smiling, blinking, chewing, frowning, and pouting: the movements of the human face are innumerable and extremely varied. No fewer than 50 muscles, some of them very small, are always at work under the skin, enabling us to eat, speak, see, move the head, and express emotions. Facial expressions are a mode of communication in themselves.

Linked to the occipital muscle by the epicranial aponeurosis, which covers the top of the skull, the frontal muscle wrinkles the skin of the forehead, raises the eyebrows, and pulls the scalp forward.

The orbicular of eye controls the eyelids and the perimeter of the eye socket.

The dilator of nostril controls the opening of the nostril.

nasolabial levator

The origin of the zygomatic muscles is the cheekbone.

The orbicular of mouth closes the mouth and is the insertion for a number of other facial muscles at the commissures.

The levator of lower lip pushes out the lower lip and wrinkles the skin of the chin.

The sternocleidomastoid muscle is responsible for rotation, forward flexion, and lateral tilt of the head.

38

Although they are not very powerful, the facial muscles are capable of controlling small movements of the skin that change the aspect of the human face, resulting in a wide variety of expressions. Some expressions have a universally recognized and understood meaning, such as joy, anger, and surprise, while others are more subtle and personal.

zygomatics

triangular of lip

corrugator

frontal

The architecture of the body

FACIAL EXPRESSIONS

orbicular of the mouth

MUSCLES UNDER THE SKIN Most muscles in the head are unusual in that they do not control the movement of a bone but act on the skin of the face. This is why they are called skin muscles, or mimic muscles. The orbicular muscles of eye and mouth are particularly important: they are sphincters (ring-shaped muscles) that cause orifices to open or close. On the other hand, the masseter and temporal muscles are not skin muscles but mastication muscles. Inserted on the mandible, they control the closing of the jaw.

The top of the skull does not have muscles, but it is covered by the epicranial aponeurosis, a large tendon that links the frontal muscle to the occipital muscle. anterior auricular

superior auricular

occipital muscle

frontal muscle The temporal muscle, which has its origin in the parietal bone, is involved in chewing by raising and retracting the mandible. pyramidal muscle nasal

posterior auricular The masseter is the main muscle responsible for chewing.

sternocleidomastoid muscle

The buccinator is the main muscle in the cheek. The risorius pulls the commissure of the lips backward. triangular of lips The platysma pulls the skin of the chin downward, lowers the commissure of the lips, and stretches the skin of the neck. 39

The architecture of the body

The action of the skeletal muscles From contraction to movement Unlike the actions of the smooth and cardiac muscles, which work involuntarily, the movements controlled by the skeletal muscles are voluntary: we decide to walk, talk, or pick up an object. Most movements that we make, however, involve a number of muscles acting together without us being completely aware of them. In fact, a single muscle cannot function in isolation, since it is capable of only one action: contraction.

AGONIST AND ANTAGONIST MUSCLES Most movements of the skeletal bones are caused by pairs of muscles located on either side of a joint. The muscle responsible for a movement is called agonist, while the opposed muscle, which resists the movement, is called antagonist. For the movement to be reversed, the muscles must exchange roles. This is what happens in the upper arm, which has two main muscles: the biceps, located at the front, and the triceps, at the back. triceps of arm When a nerve impulse is sent to the biceps Q, it contracts, thus bending the forearm at the elbow joint, which serves as a pivot. The triceps, which is relaxed, is stretched by the movement of the forearm. Q 0

biceps of arm For the forearm to straighten to its initial position, the triceps W must in turn contract, while the biceps automatically relaxes.

THE EYE MUSCLES Humans can orient their eyeballs very quickly and accurately toward the objects that they want to look at. This ability is provided by six skeletal muscles that connect each eye to the eye socket. By coordinating the actions of these muscles, we can turn our eyes along three axes. The trochlea is a fibrocartilaginous pulley through which the tendon of the superior oblique muscle passes.

medial rectus The superior rectus muscle contracts to direct the eye upward.

superior oblique

eyeball

inferior rectus

inferior oblique lateral rectus

40

W 0

Incredible dexterity Human beings have an aptitude unique in the animal kingdom: we can grasp and manipulate objects with great precision. This dexterity is due to the skeletal structure of the human hand and the complex group of muscles in the forearm, which enable us to make movements as varied as playing a piano, turning a tap, and writing. THE ANTERIOR MUSCLES OF THE HAND AND FOREARM

The architecture of the body

The movements of the hand

The muscles responsible for flexion of the wrist, hand, and fingers are located on the anterior face of the forearm. Most of them originate at the end of the humerus just above the elbow and extend to the metacarpal bones and phalanges via long tendons. A number of ligaments and a membrane called the palmar aponeurosis protect these tendons. The hand also contains a number of intrinsic muscles, including the one that provides the thumb with mobility.

Around the fingers, the tendons are wrapped in protective tendon sheaths. The superficial flexor of the fingers is extended to the phalanges by tendons.

transverse metacarpal ligament adductor of thumb

palmar aponeurosis abductor of the little finger human hand

The palmar carpal ligaments hold in the tendons of the forearm muscles.

short abductor of thumb short flexor of thumb long flexor of thumb

The ulnal flexor of wrist bends the wrist.

superficial flexor of fingers

long palmar chimpanzee hand radial flexor of wrist

THE OPPOSABLE THUMB Although there are many similarities, the human hand is differentiated from the monkey hand by a fundamental trait: mobility of the thumb. Humans can touch their thumb to any of the other fingers on the hand. This ability enables them to be very accurate and effective pincers.

The brachioradial flexes the forearm. pronator teres

41

The cerebrum, a complex and not fully understood organ, is the site of consciousness, intellectual activity, and emotions. It is also where the various functions of the body are regulated and controlled, physical stimuli are felt, and voluntary movements are triggered. This role of

centralization and

coordination of the entire body is made possible by a vast network of nerves, which fulfill both motor and sensory functions. Through them, the nerve centers can receive messages from all parts of the organism and order the needed actions.

The nervous system 44

Neurons Cells that transmit nerve impulses

46

The central nervous system The control center for the nerve network

48

The brain The core of the nervous system

50

The cerebrum Extraordinary complexity

52

The peripheral nervous system A network of sensory and motor nerves

54

The motor functions of the nervous system How the body’s muscles are activated

Neurons The nervous system

Cells that transmit nerve impulses The nervous system is based on neurons. These highly specialized cells are unique in that they can carry electrical signals and transmit them to other cells (nervous, muscular, glandular, etc.). Every motor, sensory, and association neuron is made of a cell body and a number of extensions, including dendrites, which receive electrical impulses, and axons, which transmit these impulses.

Golgi apparatus

axon hillock

Dendrites are extensions of the cell body that receive nerve impulses.

endoplasmic reticulum cell nucleus

mitochondrion The cell body contains the cell nucleus and other organelles.

cell body dendrite axon terminal

Q 0

axon

DIFFERENT TYPES OF NEURONS Neurons are classified into three categories, according to their function. Motor (or efferent) neurons direct nerve impulses toward muscles and glands. Sensory (or afferent) neurons transmit messages from the sensory receptors to the nerve centers. Finally, association neurons (or interneurons) connect two other neurons. About 90% of all neurons in the body are of the last type. Neurons can also be distinguished by their structure. Multipolar neurons Q, the most common, have many dendrites and a long axon. Most are motor neurons and interneurons. Unipolar neurons W, which are always sensory neurons, have a single extension that divides into two branches. Finally, bipolar neurons E have two extensions. 44

dendrite cell body axon terminal W 0

axon dendrite cell body axon terminal E 0

axon

nucleus of Schwann cell

Schwann cell

THE AXON The axon, a structure unique to neurons, is a cellular extension that is attached to the cell body at the axon hillock and is between 1 mm (in the cerebrum) and 1 m (in the leg) long. Most axons are covered with myelin, a white fatty substance. Schwann cells (or oligodendrocytes in the central nervous system) deposit the myelin in layers to form a sheath, which is divided into segments by narrow sections called nodes of Ranvier.

Electrical signals propagate along the axon at a speed of up to 400 km/h.

The nodes of Ranvier, which separate Schwann cells, accelerate propagation of electrical signals.

axonal terminal bouton

Axon terminals have a branching structure.

SYNAPSES The nerve message passes from one neuron to another at a site called the synapse. Usually, two neurons are not in direct contact but are separated by a very thin cleft, so the electrical signal must be converted into a chemical signal in order for transmission to take place.

Some neurons are contacted by up to 30,000 synapses.

When a nerve impulse reaches the terminal bouton, neurotransmitters are released into the synaptic cleft from the vesicles that contain them. When these molecules come into contact with the receptors of the postsynaptic neuron, they generate an electrical signal.

In a chemical synapse, a synaptic cleft separates the two membranes.

axonal terminal bouton synaptic vesicle neurotransmitter neurotransmitter receptor postsynaptic neuron

The nervous system

The myelin sheath improves the electrical insulation of neurons.

The central nervous system The nervous system

The control center for the nerve network The nervous system is the main network for communication in and control of the human body. It is responsible for the actions of organs and muscles, processes sensory messages from the entire body, and provides psychic and intellectual activity. These many functions are made possible by coordination between the peripheral nervous system, which involves all the nerves in the body, and the central nervous system.

With a weight of between 1.3 and 1.4 kg, the cerebrum is the most highly developed part of the central nervous system.

THE CENTRAL NERVOUS SYSTEM The command, control, and processing center for nerve information, the central nervous system is formed of more than 100 billion neurons. It is composed of the brain (which encompasses the cerebrum, cerebellum, and brain stem) and the spinal cord.

The cerebellum is involved mainly in motor coordination, maintenance of balance, muscle tone, and posture.

The main task of the brain stem is to transmit messages between the spinal cord, the cerebrum, and the cerebellum. Housed in the bony canal formed by the spine, the spinal cord extends from the brain stem to the second lumbar vertebra. Its diameter averages 2 cm but is not uniform; there are two swellings, one cervical, the other lumbar.

second lumbar vertebra Below the second lumbar vertebra, the spinal cord is extended by the filum terminale, a long filament of connective tissue.

Each spinal nerve is attached to the spinal cord by two roots, one sensory (in the back), the other motor (in the front).

On each side of the spinal cord, a chain of sympathetic ganglions controls the contraction of visceral muscles.

GRAY MATTER AND WHITE MATTER The spinal cord is composed of two types of substances. Gray matter, which forms an H shape in the center of the cord, is formed of neuron cell bodies. The dorsal horns contain the sensory neurons of the spinal nerves, while the ventral horns are made up of motor neurons. The gray matter is surrounded by white matter, composed of bundles of ascending and descending nerve fibers (extensions of neurons). The ascending bundles bring sensory information received by dorsal horns to the brain, while the descending bundles transmit motor impulses to the ventral horns.

46

THE SPINAL CORD

The pia mater, which covers the white matter of the spinal cord, is a highly vascularized membrane. The epidural cavity, filled with blood vessels and fatty tissues, separates the dura mater from the vertebra and plays a protective role.

Cerebrospinal fluid, similar in composition to blood plasma, circulates slowly throughout the central nervous system.

The outermost meninge, the dura mater, is fused with the tissue that covers the spinal nerves.

The nervous system

The spinal cord provides a link between the brain and the 30 pairs of spinal nerves, which are attached to it by their sensory and motor roots. It is made of a soft, fragile tissue and is protected by various membranes and liquids within the spinal canal formed by the vertebrae.

spinal cord arachnoid sensory root motor root

spinal nerve vertebral body

motor root

dorsal horn

sensory root white matter

gray matter

The spinal ganglion contains the cell bodies of primary sensory neurons.

ventral horn pia mater arachnoid dura mater The meninges are membranes that protect the spinal cord. From the inside to the outside, they are the pia mater, the arachnoid, and the dura mater. 47

The brain The nervous system

The core of the nervous system The brain is the central component of the nervous system. It is housed in the skull and communicates with the rest of the body via cranial nerves and the spinal cord. It is formed of the brain stem, the cerebellum, and the cerebrum, which constitutes almost 90% of its volume.

longitudinal fissure

left hemisphere

right hemisphere

THE SHAPE OF THE BRAIN The cerebrum is a soft mass measuring about 1,400 cm3, divided into two hemispheres by a deep groove, the longitudinal fissure. Other fissures define particular zones, the lobes, while shallower grooves separate convolutions whose patterns vary from individual to individual. The cerebellum, located under the cerebrum and behind the brain stem, is also divided into two hemispheres.

The frontal lobe is responsible for thought, language, emotions, and voluntary movements. The parietal lobe is responsible for perception and interpretation of the sense of touch.

convolution

Visual images are processed in the occipital lobe.

fissure

cerebellum

The neurons of the temporal lobe recognize and interpret sounds, and help form new memories.

brain stem

The superior and inferior colliculi intervene in visual and auditory sensations.

The midbrain is composed of the four colliculi and two cerebral peduncles.

THE BRAIN STEM cerebral Located deep within the heart of the cerebrum, peduncle the brain stem is the extension of the spinal cord and has the same histological structure (white matter covering a core of gray matter). The nerve bundles of the Its three main parts, the medulla oblongata, pons join the cerebrum to the pons, and the midbrain, contain the cerebellum and to the ascending and descending nerve bundles that spinal cord. link the cerebrum and cerebellum to the rest of the body. The brain stem also plays an The medulla oblongata (or spinal essential role in the innervation of the head, bulb) controls some vital functions, since 10 of the 12 pairs of cranial nerves are including respiration, circulation, heart directly attached to it. rhythm, coughing, and swallowing. spinal cord 48

The three meninges of the spinal cord (the dura mater, the arachnoid, and the pia mater) also cover and protect the brain. These membranes are themselves covered by several successive protective envelopes: the bones of the skull, the cranial aponeurosis (a layer of tendons), and the skin. In addition, the cerebral material sits in cerebrospinal fluid, which offers both mechanical and chemical protection. This liquid is formed inside cavities called ventricles (the lateral ventricles, the third ventricle, and the fourth ventricle), then circulates throughout the central nervous system, including the subarachnoid space.

third ventricle fourth ventricle

skin

In the fissures, the two fibrous layers of the dura mater separate to form venous sinuses. Arachnoidal villi allow for exchanges between the cerebrospinal fluid and the blood.

aponeurosis cranial bone dura mater

blood vessel

arachnoid

The subarachnoid space, formed by bays in the arachnoid membrane, contains blood vessels and cerebrospinal fluid.

pia mater cortex fissure

THE CEREBELLUM Located in the back of the brain, the cerebellum is separated from the occipital lobes by a fold in the meninges, the tentorium cerebelli. The hemispheres of the cerebellum, connected by a central projection, the vermis, present a folded surface very different from that of the cerebrum. The role of the cerebellum is very specific: it regulates and coordinates movements. To do this, it continually analyzes the messages sent by the sensory receptors and adjusts tension in the muscles by inhibiting commands issued by the motor area of the cerebrum. Because the cerebellum is linked to the organs of balance, it also regulates the posture of the body by commanding the involved muscles. fourth ventricle

cerebellar hemispheres

occipital lobe tentorium cerebelli

occipital bone

vermis

The white matter is organized in a branching structure. cerebellar cortex

The nervous system

lateral ventricles

PROTECTION OF THE BRAIN

The cerebrum The nervous system

Extraordinary complexity The human cerebrum bears traces of the different stages of animal evolution. Thus, most of the vital early functions are provided by components very deep within it, such as the hypothalamus. Covering this “reptilian” cerebrum is the limbic system, which controls more highly evolved functions: memory, emotions, learning. The cerebral cortex, the most recently developed zone, is responsible for thought, language, voluntary movements, and the conscious representation of sensations. INSIDE THE CEREBRUM Like the spinal cord, the cerebrum is formed of two types of substances. Gray matter, composed of neuron cell bodies, is found in the cerebral cortex and in certain central bodies such as the thalamus. White matter, composed of nerve fibers, provides communication between the different parts of the central nervous system. The cerebrum is covered by a layer of gray matter, the cerebral cortex, between 2 and 5 mm thick. This zone plays a fundamental role in interpreting sensory messages, commanding movements, and intellectual functions.

The gray central cores are involved with motor functions.

white matter

The two cerebral hemispheres are linked by a group of commissures formed of white matter, the larger of which is the corpus callosum.

Buried deep within the cerebrum, the thalamus is composed of two masses located on either side of the third ventricle. It forms a relay between the sense organs and the sensory areas of the cortex. The hypothalamus is formed of a number of small masses that control the body’s vital functions: bodyheat regulation, appetite, sexual activity, and so on. The many neurons of the reticular formation are interwoven with the brain stem, providing a relay between the sensory nerve bundles and the cerebrum. They stimulate the activity of the cortex and maintain it in an alert state.

BRAIN WAVES Electrodes attached to the scalp can measure the electrical activity of the cerebrum, which is then transcribed onto an electroencephalogram. The frequency and intensity of brain waves vary according to the state of consciousness. During deep sleep, the waves have high amplitude and low frequency; their frequency rises when the subject is awake but relaxed. In a state of activity or during dreams, the brain waves have a higher frequency but a low amplitude. 50

cerebellum

THE LIMBIC SYSTEM

The mamillary bodies, attached to the hypothalamus, relay olfactory sensations.

The nervous system

The limbic system, formed of certain parts of the central gray core, including the hypothalamus, parts of the thalamus, and interconnecting bundles of white matter, is superimposed on the primitive structures of the cerebrum. It controls our instinctive and emotional reactions (fear, anger, pleasure) and associates them with the more evolved zones of the cerebral cortex, thus helping to produce complex behaviors. It is also within the limbic system that memories are formed, through mechanisms that are not yet fully understood. The presence of olfactory bulbs in this region of the cerebrum also explains our often emotional reaction to smells.

front nucleus of the thalamus

The callosal convolution, which covers the corpus callosum, is the main cortical zone in the limbic system.

The fornix is a large bundle of white matter that allows communication between the different parts of the limbic system.

The septal nuclei may be linked to the sensation of pleasure.

olfactory bulbs The hippocampus is involved in memory and learning.

The amygdala is interconnected with the cerebral cortex and the hypothalamus and plays a major role in the regulation of emotional reactions. The gyrus hippocampi is involved in emotional reactions such as fear and anger.

GROWTH OF THE CEREBRUM In the embryo’s first weeks of life, it develops a primitive central nervous system. At 7 weeks Q, three zones can already be identified: the forebrain, with eye buds, the midbrain, and the hindbrain, where the cranial nerves begin to grow. At 11 weeks W, the hindbrain is divided into two distinct parts (the cerebellum and the spinal bulb), while the forebrain has grown considerably. At birth E, the cerebrum is the largest part of the brain. Convolutions have formed on its surface.

forebrain

hindbrain

cerebrum

midbrain midbrain

forebrain

cerebellum

eye bud

spinal bulb

cranial nerves

Q 0

spinal cord

W 0

brain stem

E 0

cerebellum 51

The nervous system

The peripheral nervous system A network of sensory and motor nerves The central nervous system communicates with the rest of the body via 43 pairs of nerves: 12 pairs of cranial nerves directly connected to the cerebrum, and 31 pairs of spinal nerves linked to the spinal cord. This network, which constitutes the peripheral nervous system (PNS), branches out to every part of the body. There are two orders of nerve impulses: sensory and motor. In the former case, nerve terminals send messages to the central nervous system (CNS). In the latter case, the CNS commands a muscle to contract. Some nerves perform both types of tasks: these are mixed nerves. CRANIAL NERVES Twelve pairs of nerves (numbered I to XII) are directly linked to the cerebrum. These cranial nerves innervate mainly the head and neck. Some, such as the optic nerve, the auditory nerve, and the olfactory nerve, have solely sensory functions, while others perform motor or mixed tasks.

Olfactory sensations are transmitted by the olfactory nerve (I).

The optic nerve (II) transmits sensory impulses from the eye.

II I

III IV

V

Eye movements are commanded VI by the oculomotor nerve (III), the trochlear nerve (IV), and the abducens nerve (VI).

The trigeminal nerve (V) is a mixed nerve whose motor functions concern mastication.

VIII VII The facial nerve (VII) commands the facial muscles and gland secretions. It is also involved in taste.

The acoustic nerve (VIII) transmits nervous impulses related to hearing and balance. XI X

IX

XII The accessory nerve (XI), an exclusively motor nerve, commands the movements of the neck. The vagus nerve (X) is linked to the thoracic and abdominal organs and plays a very important role in the autonomic nervous system. 52

The glossopharyngeal nerve (IX) and the hypoglossal nerve (XII) innervate the tongue, the salivary glands, and the pharynx and play a role in the sense of taste.

THE ANATOMY OF A NERVE In the peripheral nervous system, the axons of neurons, generally covered with myelin, are grouped in bundles. Several bundles are, in turn, held together by an envelope of connective tissue, the epineurium, to form a nerve.

myelinated axon

epineurium blood vessel

THE SPINAL NERVES

A nerve fiber bundle may contain both sensory and motor neurons.

The nervous system

A sheath of connective tissue called the perineurium covers each bundle.

The brachial plexus branches into three main nerves (radial, median, and ulnar) that innervate most of the arm.

The 62 spinal nerves, linked to the spinal cord by a sensory root and a motor root, are all mixed nerves. They leave the spinal canal by narrow passages between the vertebrae, the vertebral foramens, divide into a number of branches (ventral rami, dorsal rami, rami communicantes), and then join together to form local networks, the plexuses. The 8 pairs of cervical nerves innervate the head, neck, shoulders, and upper limbs. The ventral rami of the 12 pairs of thoracic nerves do not form plexuses but are aligned between the ribs: they are called intercostal nerves. radial nerve median nerve The 5 pairs of lumbar nerves serve mainly the abdomen and the front of the lower limbs. ulnar nerve The genital organs, buttocks, and most of the back of the lower limbs are innervated by the 5 pairs of sacral nerves.

The two coccygeal nerves are relatively undeveloped. The main branch of the sacral plexus is the sciatic nerve, the largest nerve in the body. It has a number of branches (tibial nerve, peroneal nerve, plantar nerves) that innervate the back part of the lower limbs. The front of the thigh is innervated by the femoral nerve. peroneal nerve tibial nerve The internal and external plantar nerves innervate the bottom of the foot.

53

The nervous system

The motor functions of the nervous system How the body’s muscles are activated The human body’s skeletal muscles allow it to perform a wide variety of very specific movements. The motor cortex, an area of the cerebrum located behind the frontal lobes, is responsible for these voluntary motor functions. The smooth muscles that contract and relax the internal organs, on the other hand, are commanded by the autonomic nervous system, controlled mainly by the hypothalamus. Finally, certain muscular actions are not commanded by the cerebrum but result from reflexive stimulation of the motor neurons in the spinal cord. THE AUTONOMIC NERVOUS SYSTEM From contractions of the heart to the secretion of saliva, the actions of the visceral organs and the body’s glands are commanded not consciously but through the autonomic nervous system. This system functions along two distinct paths: the sympathetic system, which goes through the spinal cord and a chain of ganglions, and the parasympathetic system, which mainly uses the nerve bundles of the vagus nerve (cranial nerve X). SYMPATHETIC SYSTEM

PARASYMPATHETIC SYSTEM nasal cavities

tear glands eye brain stem

cervical spinal segments

salivary glands

vagus nerve

lungs

thoracic spinal segments

heart liver lumbar spinal segments

stomach kidneys

sacral spinal segments

spinal cord sympathetic chain

intestines rectum bladder

genital organs 54

The skeletal muscles in the human body can be contracted consciously, through a nerve message from the motor cortex Q. The message reaches the brain stem W, then descends the spinal cord E. It then travels along a spinal nerve that stimulates the targeted muscle R. The sensory receptors of the muscle emit a message to control the movement. This signal returns to the cerebellum T, which compares the movement made with movements learned and remembered since childhood. The cerebellum sends an inhibiting message Y to the muscle to control its activity. In parallel, it acts on the motor cortex through the thalamus U, to adjust the command.

Q 0

U 0

The nervous system

VOLUNTARY MOVEMENTS

motor cortex

thalamus W 0

Y 0 T 0

cerebellum Most of the nerve bundles cross in the brain stem, so that one cerebral hemisphere controls the movements on the opposite side of the body.

spinal cord

E 0

sensory neuron motor neurons

R 0

PAIN: REFLEX AND REACTION When a hand picks up a very hot object Q, receptors in the skin (nociceptors) send a message to the spinal cord W. In a few hundredths of a second, the spinal cord commands a muscular movement E to release the object. This is called a reflex. At the same time, other sensory nerves send a message to the area of sensory processing in the cerebrum R to signal the sensation of touch. One or two seconds later, the nociceptor impulses arrive in the cortex, which causes the sensation of pain T. Because the limbic system is also activated, emotions are felt and the sensation is memorized. The cerebrum may then decide to order a conscious reaction Y, such as blowing on the injury to inhibit the receptors and lessen the pain.

The area of sensory processing is located in the center of the two parietal lobes.

Pain reaches the cerebrum after the sensation of touching. R 0

T 0

spinal cord W 0 E 0

Y 0 Q 0

55

Touch, sight, hearing, taste, and feel. We depend on five complementary perception systems to learn about the world around us: these are the five senses. Detection of physical stimuli is provided by

incredibly

sensitive specialized organs. Transformed into nerve impulses, this information is directed toward the central nervous system, where it is processed to give

of our environment.

us a conscious representation

The five senses 58

Touch How the skin communicates with the cerebrum

60

The eye A tool for capturing light

62

Sight Our most highly developed sense

64

The organ of hearing Inside the ear

66

Perception of sound The path of vibration through the ear

67

Balance A sixth sense?

68

Taste A limited sense

70

Taste receptors A chemical process

72

Smell A little-known sense

Touch The five senses

How the skin communicates with the cerebrum Even though the sensation of pain is not pleasant, its role is vital: it draws the attention of the central nervous system to injuries, burns, and punctures, as well as to all other mechanical, thermal, or chemical assaults on the organism. Without this alarm system, we would be at risk of not noticing that our bodies have been attacked. When the skin’s specialized receptors detect a tactile sensation, they convert the information into nerve impulses that transmit the sensation to the cerebrum via different nerve bundles. It is up to the central nervous system to process the message and order the actions needed (defense, manipulation, change of posture, etc.). TOUCH RECEPTORS The skin is the site of different types of sensations. Tactile sensations (light touch, vibration, pressure) tell us about the weight, size, and consistency of an object; thermal sensations tell us about an object’s temperature; and painful sensations are produced whenever the skin is injured. These stimuli are perceived by receptors located in the dermis and epidermis, most of which specialize in one or a few types of sensations.

epidermis

Located in the lower layer of the epidermis, Merkel cells are capable of detecting very light contact. They therefore specialize in specific kinds of touch and sharp pain. dermis

nerve fiber Krause end bulbs respond to pressure, vibration, and extreme cold. Ruffini’s corpuscles are abundant in the deep dermis of hairy parts of the body. They are sensitive to strong, continuous pressure and to heat. 58

THE NERVE PATHS OF TOUCH

On the other hand, pain signals (free nerve endings) and diffuse touch (Pacinian corpuscles) are routed via the spinothalamic tracts: they are analyzed in the gray matter of the spinal cord, which modulates and integrates them before sending them to the cerebrum. The transmission time is therefore longer: one second lapses between occurrence of the stimulus and its reception by the cortex. The somatosensory cortex is the region of the parietal lobe where tactile sensations become conscious. There, a mental representation of the region touched and the type of contact is formed. This mental image is then compared with previously memorized sensations and integrated with other types of sensations (visual, auditory).

The five senses

Depending on their nature, sensory nerve impulses take one of two different paths to the cerebrum. Specific touch signals (Meissner’s corpuscles) travel to the brain stem directly and thus reach the somatosensory cortex very rapidly (within a few hundredths of a second).

The nerve bundles converge in the thalamus, which leads to the somatosensory cortex.

Free nerve endings, abundant at the surface of the dermis, react to pain: these are nociceptors.

brain stem

Meissner’s corpuscles, which are sensitive to precise touch, are located in the upper part of the dermis of the hands, feet, lips, and genital organs.

spinothalamic tract

Pacinian corpuscles are located in the deep dermis and react to vibrations and strong, continuous pressure.

nerve fiber

spinal cord 59

The eye The five senses

A tool for capturing light Although it weighs just 7 grams and has an average diameter of only 24 millimeters, the human eye is a biological camera whose complexity and capacities surpass those of the most advanced optical apparatuses. This highly evolved optical system includes two lenses and a pupil that are responsible for deflecting a precise quantity of light rays toward the retina, where more than 130 million photoreceptors convert the light into neural signals that can be interpreted by the brain. INSIDE THE EYEBALL The eye, recessed within a bony socket, is a hollow body filled with a gelatinous substance called the vitreous body. It is covered with several layers of tunics that form the coat of the eyeball: the retina, the choroid, and the sclera. At the front of the eye, the sclera becomes perfectly transparent to form the cornea. Light enters the eye through the cornea Q, which is the principal ocular lens. It then travels through the opening in the pupil W. Behind the pupil is the crystalline lens E, which converges light rays toward the retina R.

retina

The choroid is a vascular layer located between the sclera and the retina. It supplies the retina with nutritive substances and oxygen. The sclera, whitish in color, is the thickest layer of the coat of the eyeball. It is covered with a mucous layer, the conjunctiva, and protects the fragile internal structures of the eye. vitreous body Via suspensory ligaments called zonules, the muscles of the ciliary body pull or release the lens to change its curvature. Q 0

The curved shape of the cornea enables it to deflect light at a sharp angle toward the interior of the eye.

W 0

E 0

The opening of the pupil changes size to adapt to the quantity of light rays that reach it. The crystalline lens has two convex curvatures. The iris is a muscle that dilates or contracts to determine the size of the opening of the pupil. Its color varies from individual to individual. The eyeball has six extraocular muscles that move it in different directions. 60

THE ROLE OF THE RETINA

The retina contains two types of photoreceptor cells: cones and rods. There are many more rods (125 million) than cones; although they do not perceive colors, rods are very sensitive to contrasts in light. On the other hand, cones (6 million) perceive colors perfectly.

The disks on the outer segment of the cell contain the photosensitive pigments.

Synaptic terminal branches are in contact with the intermediary neurons. cell nucleus

The five senses

Light rays T that reach the retina pass through several layers of cells before reaching the photoreceptor cells Y, the only cells that have pigments capable of transforming light into electrical impulses. These impulses are transmitted by intermediary neurons U to the optic nerve I, which carries the information to the brain.

rod choroid

cone retina

trajectory of light

T 0 Y 0

U 0

R 0

The fovea, composed mainly of cones, is the part of the retina where visual acuity is strongest. I 0

trajectory of neural signal The photoreceptors are contained in the pigmented epithelium, a cellular layer that absorbs all light crossing the retina.

One million axons (extensions of neurons) originating in the retina converge in the optic nerve. The part of the retina where the nerve fibers converge to form the optic nerve has no photosensitive cells; this zone is the blind spot.

THE EYE’S DEFENSES Tears are constantly being secreted by the lacrimal glands, located above each eye. Every time the eyelid blinks, it causes this lacrimal liquid to flow over the surface. This keeps the eye moist and free of dust and microbes. The eyelids and eyelashes also play a protective role.

upper eyelid The eyelashes trap outside elements: dust, sweat, and direct rays from the Sun.

Tears are evacuated by the lacrimal duct, which leads into the nose.

lacrimal gland

lower eyelid

61

Sight The five senses

Our most highly developed sense Human beings have remarkable visual ability; in fact our sense of sight is vastly superior to our other senses. The perception of shapes, distances, colors, and movements in our environment is a complex process that uses a chain of optical and nervous components, from the cornea to the cortex. HOW THE EYE FOCUSES Light rays emanating from an object that we look at are first deflected by the cornea to the crystalline lens. Unlike the curve of the cornea, the curve of the crystalline lens is variable, so it can cause the images of objects at different distances to converge on the retina. However, the precision of this optical system makes it particularly fragile: the slightest imperfection in the shape of the eyeball or the curve of the cornea leads to an imbalance for which the crystalline lens cannot always compensate. In these cases, because the image is not focused on the retina but in front of or behind it, vision is blurry.

crystalline lens The image of the object is formed on the retina.

light rays object

cornea

Myopia is a defect in which the image of distant objects is formed in front of the retina. This situation is corrected with a concave lens, which pushes the point of convergence of the light rays farther back in the eye. myopic eye

concave lens

In hypermetropia, by contrast, the image is formed behind the retina. To correct this problem, a convex lens is used to bring the point of convergence forward in the eye. hypermetropic eye convex lens

Astigmatism is a defect in the curve of the cornea or the crystalline lens preventing homogeneous convergence of light rays. An asymmetrical lens can correct this problem. astigmatic eye

cornea 62

asymmetrical lens

When an object Q enters our field of vision, each eye perceives it from a slightly different angle, which enables us to evaluate its distance and see its shape in three dimensions. Light rays are deflected as they pass through the cornea and the crystalline lens W so that the object is inverted as it reaches the retina E. The optical image is then converted by photoreceptor cells into electrical impulses that reach the optic nerve R. The two optic nerves meet in the optic chiasm T, which leads to the lateral geniculate bodies Y, outgrowths of the thalamus. The information is then transmitted by optic radiation to the visual cortex U, where the image is reconstructed right side up I.

iris

real object

pupil

The five senses

SIGHT, FROM THE CORNEA TO THE CORTEX

Q 0

The pupil, which is the opening at the center of the iris, can dilate or contract to adjust to the quantity of light reaching it.

crystalline lens

retina W 0

E 0

optic nerve

R 0

In the optic chiasm, some of the nerve fibers from each eye cross over to the opposite cerebral hemisphere. Each hemisphere thus receives information from both eyes.

T 0

optic tract

The lateral geniculate body of the thalamus is a cellular relay.

optic radiation

Y 0

U 0

In the visual cortex, located in the occipital lobes, the image of the real object is reconstructed through a series of complex mechanisms. I 0

reconstructed image

63

The organ of hearing The five senses

Inside the ear From the delicate tinkling of a needle bouncing on a glass table to the deafening roar of a plane taking off, our ears enable us to distinguish almost 400,000 sounds. The organ responsible for hearing is not the visible external ear, but a group of small, fragile internal structures housed in a bony cavity inside the head.

helix

THE THREE PARTS OF THE EAR Our auditory system has three parts. The outer ear is essentially composed of the auricle, which captures sound vibration and directs it to the auditory canal. The middle ear, bounded by a fine membrane (the tympanum), contains a group of three tiny bones only a few millimeters long: the hammer, the anvil, and the stirrup. This chamber is connected to the nose and throat by a narrow passage, the eustachian tube. Finally, the inner ear contains the cochlea, a liquid-filled spiral, and the cochlear nerve.

The auricle contains many cartilaginous and cutaneous folds designed to capture sounds. The external auditory canal is lined with hairs and covered with cerumen, a waxy substance that traps dust.

The hairs of the auditory canal play a protective role.

The earlobe, a fleshy extension of the auricle, is not involved in hearing.

outer ear

64

middle ear

inner ear

THE AUDITORY CORTEX

Wernicke’s area primary auditory cortex

The five senses

Auditory messages, relayed by the auditory nerve, end up in a zone of the cerebral cortex, the auditory cortex, which has two areas. Specific sounds are identified in the primary auditory cortex, while the secondary auditory cortex, which surrounds it, provides a more diffuse representation of sounds perceived. These areas are beside Wernicke’s area, which is involved in language comprehension.

secondary auditory cortex

tympanum The three semicircular canals are responsible for balance. vestibule The vestibular nerve transmits messages related to balance.

The cochlear nerve and the vestibular nerve join in the inner auditory canal to form cranial nerve VIII. The cochlear nerve carries the nerve signals of hearing.

round window

temporal bone

The eustachian tube enables the pressure on either side of the tympanum to be equalized.

hammer anvil

The cochlea, filled with liquid, is housed in a cavity of the temporal bone. A system of membranous and bony partitions defines three canals that spiral around a central axis. One of these canals contains the organ of Corti, which is the true hearing organ and is linked to the cochlear nerve.

stirrup The ossicles of the middle ear (hammer, anvil, stirrup) are the smallest bones in the human body. The stirrup is 4 mm long.

65

Perception of sound The five senses

The path of vibration through the ear Our auditory system functions like a complex trap that routes sound vibrations through several successive elements: air in the outer ear, a solid in the middle ear, and liquid in the inner ear. Only at the end of this series of transmissions does the real receptor, the organ of Corti, detect the frequency and intensity of sounds.

FROM THE TYMPANUM TO THE COCHLEA

helicotrema cochlear canal

Sound, directed from the auricle through the external auditory canal, makes the tympanum vibrate Q. The ossicles W located tympanic behind this membrane amplify the vibration and transmit it to the canal entrance to the inner ear, the oval window E. The sound vibration then travels through the vestibular canal of the cochlea R and stimulates the organ of Corti. High-frequency sounds are felt at the base of the spiral and low-frequency sounds at the apex. When the vibrations arrive at the helicotrema T, they travel up the tympanic canal and leave the cochlea via the round window Y. oval window ossicles W 0 tympanum

Q 0

T 0

E 0 R 0

round window

Y 0

vestibular canal

eustachian tube vestibular canal

INSIDE THE COCHLEA cochlear nerve

tympanic ramp

cochlear canal organ of Corti

The cochlea is composed of three parallel spiral-shaped canals filled with liquid. The cochlear canal is bounded by membranes that separate it completely from the vestibular and tympanic canals. These canals are connected by a passage called the helicotrema, located at the top of the cochlea. Sound waves travel through the vestibular canal and cause the basilar membrane, against which sits the organ of Corti, to vibrate. The ciliated cells in the organ of Corti transform the vibrations into nerve impulses, which are transmitted to the cerebrum by the cochlear nerve. The sound waves leave the cochlea via the tympanic canal. tympanic canal The stiffness of the basilar membrane varies from the base of the cochlea to the apex.

vestibular canal tectorial membrane vestibular membrane 66

cochlear canal

Located between the basilar membrane and the tectorial membrane, the hair cells of the organ of Corti react to the slightest change in displacement by generating a nerve impulse.

Balance Our five senses inform us about our environment, but they do not tell us everything about the position of our body in relation to the space around us. This information, however, is essential in order for us to keep our balance and to move effectively. The organ responsible for this “sixth sense” is located in the inner ear, where it sits beside the auditory organ.

The five senses

A sixth sense?

DYNAMIC BALANCE Three semicircular canals, corresponding to the three dimensions of space, evaluate the position of the head when it is in angular movement. Each canal, filled with endolymph, ends in an ampulla. This swelling contains hair cells whose cilia are enveloped in a cone-shaped gelatinous mass, the cupula. endolymph When the head is still, the cupula does not move. cupula

superior semicircular canal

posterior semicircular canal ampulla cupula

hair cell When the head moves, the cupula moves and stimulates the hair cells, which send a nerve message via the vestibular nerve. horizontal semicircular canal If the movement stops suddenly, the endolymph continues to move for a few moments, causing an imbalance or dizziness.

utricle

vestibular nerve saccule

STATIC BALANCE Static balance, which evaluates the position of the head in relation to the ground, is obtained through hair cells in the utricle and the saccule, two membranous pockets in the inner ear. The cilia of the hair cells Q are immersed in a gelatinous mass that contains small particles, otoliths. When the head is tilted, these particles are subjected to gravity and move the gelatinous mass W. As they tilt, the cilia modify the nerve impulses generated by the cells. This mechanism enables the body to detect a variation of 0.5° in the tilt of the head.

gelatinous mass

gelatinous mass

W 0 Q 0

otolith hair cell

cilia nerve fiber 67

Taste The five senses

A limited sense Food lovers may find it hard to believe, but the extent of our ability to taste is limited to four basic flavors (sweet, salty, sour, and bitter), and its acuity is very low. A chemical substance must be 25,000 times more concentrated to be perceived by taste receptors than by smell receptors. What we call the “flavor” of a food is often a combination of smell and taste, perceived by the olfactory receptors in the nasal cavity and the gustatory receptors in the tongue, palate, and oropharynx. To this combination of sensations are added the tactile (consistency) and thermal (temperature) sensations that inform us about the nature of what we put in our mouths. The palatine tonsils, located on either side of the posterior base of the tongue, contribute to immune-system defense by imprisoning bacteria that penetrate the organism through air or food passages. The palatoglossal arch is a muscular fold that links the tongue to the palate.

WHAT DOES SALIVA DO? Sapid substances (substances that produce a taste) must be in liquid form for the taste buds to react to them. Saliva dissolves substances and allows taste to be perceived. This liquid is produced by three pairs of major salivary glands (the left and right parotids, sublinguals, and submandibulars) and by numerous minor salivary glands located in the mucosae of the oral cavity. These glands are controlled by sympathetic and parasympathetic nerves and function reflexivly when stimulated by various receptors. Many visual, psychological, tactile, olfactory, and gustatory stimuli may cause a reflex flow of saliva.

nasal cavity The palate is the partition between the mouth and the nasal cavities. It is composed of a bony part in the front (the vault, or hard palate) and a musculo-membranous part in the back (the velum, or soft palate). tongue parotid gland sublingual gland submandibular gland

The lingual tonsils, which sit behind the tongue, contribute to immune defense.

The palatopharyngeal arch joins the palate to the epiglottis.

The five senses

The epiglottis is a cartilaginous appendix that closes the entrance to the larynx during swallowing.

The glossopharyngeal nerve is a mixed nerve (cranial nerve IX), whose sensory fibers innervate the mucosa of the pharynx and the back of the lingual mucosa.

terminal sulcus

The back of the tongue has a clearly visible line of 10 circumvallate papillae that form a V-shaped row.

The corpus linguae (body of the tongue) is composed mainly of muscles covered with a mucosa. Thousands of filiform papillae on its surface give it a velvety texture. fungiform papilla

THE TONGUE The tongue is the main taste organ. The papillae located on its surface allow us to perceive four basic taste modalities (sweet, salty, sour, and bitter), which when combined form many nuances of flavor. Contrary to popular belief, these basic taste modalities are not perceived differently on different parts of the tongue. All areas of the tongue, as long as they have taste buds, can perceive all taste modalities. However, the ability to perceive tastes is very uneven: some sweet substances may have to be 10,000 times greater in concentration than a bitter substance to be sensed with the same intensity. 69

Taste receptors The five senses

A chemical process The sense of taste uses a very large number of receptors, housed in the folds of the taste papillae. Each individual has between 200,000 and 500,000 tastereceptor cells spread over the top of the tongue, in the throat, on the insides of the cheeks, on the back part of the palate, and on the epiglottis. These cells are constantly being replaced, since they decline over about 10 days. With age, the taste-receptor cells regenerate more slowly, which causes a diminution in the sense of taste. The filiform papillae, which are distributed over almost the entire dorsal surface of the tongue, have a mainly tactile function. fungiform papilla

foliate papilla

taste bud

nerve fiber

THE LINGUAL PAPILLAE The top of the tongue has a bumpy surface due to protuberances called lingual papillae. These irregular projections, some of which are also found on the palate and in the throat, have a number of different shapes, though they are difficult to differentiate with the naked eye. The largest papillae, the circumvallate papillae, form a wide V at the back of the tongue, with a groove on either side. Smaller but more numerous are the fungiform papillae, which look like small red balls scattered on the top of the tongue. The filiform papillae, which are conical in shape with a ridge at the top, are spread over the entire top of the tongue. Finally, the foliate papillae are found on either side of the top of the tongue, where they form series of parallel frond-like grooves. The taste buds are most numerous in the folds of the circumvallate, much less so among the fungiform papillae.

70

The epithelium (the top cellular layer), with its circumvallate and fungiform papillae, contains many gustatory cells. Grouped into small buds with a maximum diameter of 0.05 mm, these cells have cilia, or microvilli, at their tips, which protrude from the epithelium and are immersed in saliva. When the terminal ends of the microvilli come into contact with molecules corresponding to one of the four basic taste modalities, a cascade of biochemical reactions takes place. The gustatory cells then generate a nerve message that is transmitted to the cerebral cortex where it is interpreted as taste. A taste bud contains 50 to 100 gustatory cells.

gustatory cell

gustatory pore

The five senses

THE TASTE BUDS

The gustatory cells terminate in tiny cilia, microvilli.

circumvallate papilla

epithelium

connective tissue Nerve fibers transmit the taste message to the cerebrum.

The gustatory area of the cortex is located in the insula.

brain stem

thalamus U 0

Y 0

hypothalamus

T 0 R 0

salivary gland

The papillae are bordered by a groove filled with saliva.

FROM THE TONGUE TO THE CEREBRUM Three cranial nerves share transportation of taste sensations: the lingual nerve, a branch of the facial nerve (VII) Q, the glossopharyngeal nerve (IX) W, and the vagus nerve (X) E. These three nerves converge in the brain stem R. After a preliminary analysis, the nerve impulses are shared between the hypothalamus T, which regulates appetite, and the thalamus Y, where a second analysis is conducted. The signals ultimately reach the cerebral cortex U, where the conscious perception of tastes and flavors takes place.

Q 0

W 0

E 0

71

Smell The five senses

A little-known sense Smell is perhaps the most mysterious sense. Its mechanisms are not yet completely understood, and its organs, hidden within the nose, are usually invisible. The olfactory epithelium, the cellular layer responsible for detection of odors, covers an area of 5 cm2 to 10 cm2 of our nasal cavities and contains from 10 million to 100 million receptors. Although the human sense of smell is not as highly developed as that of other animals, an adult is able to distinguish more than 10,000 odors. This sensitivity, which helps us defend ourselves against dangers (such as fire and natural gas), also enables us to better appreciate the flavors of the foods we eat.

gland of Bowman Mucus produced by the glands of Bowman humidifies the tiny cilia at the tips of the olfactory cells and dissolves odoriferous molecules to facilitate chemical reactions.

THE NASAL CAVITIES The nasal cavities, which communicate with the outside environment via the two nostrils, constitute the main entrance for the respiratory system. During respiration, odoriferous molecules contained in the inhaled air activate the olfactory receptors of the two nasal cavities, which are separated from the mouth by the palate but connected to it via the nasopharynx. Odors from foods reach the olfactory epithelium by this route.

olfactory bulb

The cells responsible for detection of odors are found in a mucosa, the olfactory epithelium, which covers the upper part of the nasal cavities. nasal cavity The protruding part of the nose is structured around bony and cartilaginous elements. nostril palate nasopharynx

72

Supporting cells, which form the bulk of the olfactory epithelium, do not have a sensory function.

THE NERVE PATHS FOR SMELL

R 0

E 0 Q 0

W 0

The five senses

From the olfactory bulbs Q, nerve impulses travel to the limbic system of the cerebrum, where they come into contact with the zones assigned to emotions and memory, such as the mamillary bodies W. This explains why a simple odor can instantaneously trigger very strong emotional reactions, provoking a memory or even influencing sexual behavior. Another part of the olfactory nerve travels through the thalamus E to the orbitofrontal cortex R, where a conscious representation of the perceived odor is created.

olfactory bulb Mitral cells relay the nerve impulse to the cerebrum. ethmoid bone connective tissue The axons of the olfactory cells are grouped in bundles to cross the ethmoid bone.

Basal cells constantly produce new olfactory cells.

olfactory epithelium olfactory cell

The mechanism that converts a chemical stimulus into a nerve impulse is located on the surface of the olfactory cilia. mucous layer

odoriferous molecule

THE SMELL RECEPTORS The olfactory cells are neurons whose axons cross the ethmoid bone and enter the olfactory bulb, where they form a synaptic connection with interneurons called mitral cells. At their other end is a dendrite with a dozen sensory cilia. The olfactory cells are unique in that they are regenerated by the organism; this occurs for no other neurons. Their life span is about two months.

73

Blood, propelled by

regular contractions of the cardiac muscle, plays a

number of very important roles in the organism. As it flows through

the vast network of veins,

arteries, and capillaries, it carries oxygen and nutritive elements that are indispensable to cells, and it drains some waste matter, such as carbon dioxide. It also carries hormones and white blood cells to most parts of the body.

Blood circulation 76

Blood A means of transport and defense

78

The cardiovascular system Two blood circuits

80

Arteries and veins A closed circuit for irrigation

82

The heart A tireless pump

84

The cardiac cycle A remarkably regular rhythm

86

The lymphatic system Drainage and cleansing of the body’s fluids

88

Immunity How the body defends itself against infection

90

The endocrine system Hormones: the body’s chemical messengers

92

The hypothalamus and the pituitary gland The control centers of the endocrine system

94

The urinary system How the kidneys filter the blood

Blood Blood circulation

A means of transport and defense Blood, which comprises 8% of our body weight, moves through a vast closed network of arteries and veins. It infuses all the tissues of the body, provides them with oxygen and nutritive substances, and removes their waste. Blood also carries white blood cells and hormones. THE COMPOSITION OF BLOOD

plasma (54%)

Blood is composed of cells and cell fragments floating in a watery liquid called plasma. There are two types of blood cells: red blood cells (erythrocytes) and white blood cells (leucocytes). There are relatively few white blood cells, and they take various forms: neutrophils, lymphocytes, monocytes, eosinophils, and basophils. Finally, platelets are not true cells but fragments of giant cells.

white blood cells and platelets (1%)

red blood cells (45%)

blood vessel

Plasma is a yellowish liquid that is 90% water. It also contains proteins, vitamins, and other solutes.

Monocytes are the largest white blood cells. The blood carries them to the tissues, to which they adhere. Blood platelets (or thrombocytes) are fragments of megacaryocytes, giant blood cells in the bone marrow. They have a very short life span (five to 10 days) and are involved in blood coagulation and promoting the formation of scars. fibrin

COAGULATION When a blood vessel is damaged, several mechanisms combine to stop the hemorrhage. First, the platelets stick to each other to plug small holes. Then the plasma produces a filamentous protein, fibrin, which forms a net capable of holding red blood cells together to make a scab.

76

red blood cell

Red blood cells, platelets, and white blood cells such as neutrophils all come from the same type of cell, hemocytoblasts, produced by the red bone marrow. Lymphocytes and monocytes, which also come from hemocytoblasts, complete their differentiation in the lymphoid tissues. The red bone marrow is located in flat bones (skull, sternum) and the epiphyses of long bones.

platelet

Blood circulation

THE FORMATION OF BLOOD CELLS

red blood cell Stem cells of the bone marrow, hemocytoblasts, can be transformed into several types of blood cells. neutrophil

red blood cell

The lymphocytes play several roles in the immune system. There are only a small number of them in the blood.

The neutrophils are white blood cells that contribute to immune defense by ingesting bacteria.

A molecule of oxygen can unite RED BLOOD CELLS with an iron ion in a heme. Our bodies contain an average of 25 000 billion red blood cells globin (or erythrocytes), cells with no nucleus that are capable of stretching and deforming to pass through the narrowest blood vessels. Each red blood cell contains about 250 million molecules heme of hemoglobin, a substance formed of a protein (globin) and four pigments (the hemes). Hemoglobin plays an essential role in gas exchanges by transporting oxygen and carbon dioxide through hemoglobin the blood. Each heme has an iron ion that oxidizes to give the molecule oxygenated blood its red color.

BLOOD GROUPS

The plasma contains antibodies, which react to antigens that are normally absent from our blood. In the event of a blood transfusion, it is therefore essential that the blood types of the donor and the receiver be compatible in order to prevent rejection.

A donor

Red blood cells carry antigens, substances that can be attacked by antibodies, on their surface. Among the 100 known antigens, two are used to determine different blood types. Types A and B designate the carriers of the antigens A and B, respectively, while type AB designates carriers of both antigens. Finally, type O refers to those that carry neither.

COMPATIBILITY OF BLOOD GROUPS

B AB O A

B

AB

O

receiver

77

Blood circulation

The cardiovascular system Two blood circuits The blood, continually propelled by the heart, flows through all blood vessels in the body in one minute, via two distinct circuits: the pulmonary and systemic bloodstreams. All of the blood vessels, the heart, and the blood constitute the circulatory, or cardiovascular, system. A HUGE CLOSED-CIRCUIT NETWORK The blood vessels in the human body form a vast network with a total length of about 150,000 km. Blood, pumped by the heart, constantly circulates through the arteries (vessels leading from the heart) and veins (vessels leading to the heart). The arteries and veins branch off into smaller vessels (arterioles and venules), which, in turn, branch further into tiny channels, the capillaries.

The carotid artery perfuses the head and neck. axillary artery aortic arch pulmonary artery brachial artery internal thoracic artery aorta

The internal jugular vein gathers blood from the face, neck, and sinuses of the brain. subclavian vein superior vena cava heart cephalic vein The basilic vein and the cephalic vein are the main superficial veins in the arm.

renal artery radial artery The aorta divides into two iliac arteries at hip level.

inferior vena cava renal vein iliac vein

femoral vein In the thigh, the iliac artery is called the femoral artery.

The popliteal artery is located behind the knee joint.

The long saphenous vein is the longest vein in the body. popliteal vein

tibial vein tibial artery Arteries are generally represented in red because oxygen makes the blood red by linking with the ferrous pigment of hemoglobin. 78

In the veins, on the other hand, the blood is oxygen-poor. It thus has a darker color, which is depicted as the color blue in diagrams.

THE TWO CARDIOVASCULAR CIRCUITS

The systemic bloodstream is composed of all the other blood vessels in the body, including the aorta and the vena cava. The blood is expelled from the left ventricle and circulates through all body tissues except for the lungs.

superior vena cava The pulmonary arteries are the only arteries that carry oxygen-poor blood.

Blood circulation

The cardiovascular system is composed of two distinct circuits. The pulmonary bloodstream includes the pulmonary arteries, veins, and capillaries. The right ventricle of the heart pumps blood to the lungs, where the blood is oxygenated and the carbon dioxide it contains is removed.

The aorta is the main stem of all the body’s arteries.

pulmonary capillaries

In the lungs, the blood is oxygenated and its carbon dioxide is removed.

The pulmonary veins return the blood oxygenated by the lungs to the heart.

right ventricle

left ventricle

Through the systemic capillaries, the blood supplies oxygen to the tissues and takes away carbon dioxide.

Oxygen-poor blood flows from the lower body up to the heart via the inferior vena cava.

When the body is at rest, the systemic veins and venules contain more than 60% of all the body’s blood.

140

Q

carotid pulse

120 100

brachial pulse

80

E 60

radial pulse

40 20

W

R

0

femoral pulse popliteal pulse

BLOOD PRESSURE Blood pressure (or tension) is the pressure that the blood exerts on the walls of the blood vessels. It is measured in millimeters of mercury. Blood pressure is irregular in the heart Q, is very high in the arteries W, diminishes considerably when the blood reaches the capillaries E, and is even lower when it enters the venous system R.

tibial pulse Each time the blood is expelled from the heart, it creates a wave, the pulse, perceptible in certain superficial arteries. The pulse rate varies according to the amount of physical exertion. 79

Arteries and veins Blood circulation

A closed circuit for irrigation

The blood circulates throughout the human body except for certain very localized areas, such as the enamel of the teeth and the cornea of the eye. It moves through two types of blood vessels, arteries and veins, which are distinguished both by their anatomy and by their respective roles in the cardiovascular system. THE ANATOMY OF BLOOD VESSELS The walls of blood vessels, which must resist variations in blood pressure, are composed of three concentric layers of tissue called tunicas. The tunica intima (inside layer), composed of the endothelium and a basement membrane, defines the lumen, the channel within which blood circulates. It is covered by a layer of smooth muscle and elastic fibers, which forms the tunica media (middle layer), and the tunica adventita (outside layer), made mainly of collagen fibers. basement membrane valve

endothelium

adventice The tunica media contains many elastic fibers.

tunica intima

The thickness of the smooth muscle of arteries enables them to contract to maintain arterial tension and facilitate circulation of blood from the heart.

The wide lumen in veins enables them to carry more blood. The veins have a thinner wall and a wider lumen than do arteries. In the lower limbs, some veins have valves that keep the blood from backflowing due to gravity when the person is upright.

THE CAPILLARIES Capillaries, which are formed of a thin layer of endothelial cells covered by a basement membrane, are very small blood vessels: they measure only 0.3 to 1 mm in length and have a maximum diameter of 0.01 mm. The extreme thinness of their walls encourages exchanges between the blood and the space around them. Through the capillaries, oxygen and nutritive elements are distributed to the tissues and carbon dioxide, the product of cellular metabolic activity, is carried away. basement membrane endothelium

The metarteriole runs straight across the capillary network to intersect with the venule.

venule arteriole

A precapillary sphincter, made of muscle fibers, controls the blood flow at the entrance to a capillary. 80

The 30 billion capillaries contained in the human body form vast networks.

CAPILLARY CIRCULATION

When the muscle is at rest Q, a series of sphincters contract, cutting blood flow in the capillaries.

muscle at rest

active muscle

Q 0

contracted sphincter

W 0

Blood circulation

Blood flow in the capillary networks is based on the tissues’ need for oxygen. A muscle at rest requires less blood than an active muscle. The precapillary sphincters control blood flow in the capillaries through a process of contracting or relaxing.

arteriole When the precapillary sphincters are relaxed, blood is free to irrigate the capillary networks of an active muscle W. capillary network relaxed sphincter

BLOOD CIRCULATION IN THE VEINS open valve

vein

Blood pools in veins in the lower limbs because the force of gravity creates pressure that distends the elastic walls of the veins. Skeletal muscles that lie near the veins facilitate blood circulation from the lower extremities by contracting, compressing the venous walls and forcing the valves located above them to open to let blood flow toward the heart. The valves located below the muscles keep the blood from descending, since they can open only in one direction. This mechanism is called the venous pump or the muscular pump.

relaxed muscle

contracted muscle

closed valve

THE SPEED OF BLOOD FLOW The blood flows more slowly in capillaries than in larger vessels. This slowing makes it possible for exchanges to occur between the blood and the tissues. arteriole artery aorta

capillary

venule vein vena cava

81

The heart Blood circulation

A tireless pump In spite of its small size, the heart is the most active organ in the body. Its muscle fibers contract constantly to propel blood through the body, at an average rate of 70 contractions per minute, for an entire lifetime. With its complex system of chambers and valves, the heart is a formidable dynamo that pumps 2.5 million liters of blood each year.

EXTERNAL SURFACE OF THE HEART The heart is a small organ (10 to 12 cm in diameter and weighing an average of 300 g) located in the ribcage, between the lungs. Its surface is divided by clefts along which run the coronary arteries and veins that are responsible for blood perfusion of the cardiac muscle. These clefts mark the boundaries between the atria and the ventricles.

superior vena cava right pulmonary artery

right atrium

left atrium right pulmonary veins anterior interventricular artery right atrium left ventricle anterior interventricular vein right ventricle right coronary artery

tricuspid valve

The myocardium contracts to expel blood. It is thicker around the ventricles than around the atria.

A relatively inelastic fibrous covering, the pericardium covers the heart and keeps it in position. Pericardial fluid is a lubricant that reduces the friction caused by cardiac pulses.

THE CARDIAC MUSCLE The heart is composed essentially of the myocardium (or cardiac muscle), which forms a thick wall of striated muscle fibers. The endocardium, the interior surface of the myocardium, is lined with a thin layer of cells which are similar to those covering all blood vessels. The cardiac muscle is covered with the epicardium, a thin membrane that is the inside layer of the pericardium. 82

endocardium epicardium

inferior vena cava

The aorta is the largest blood vessel in the human body. Its diameter is between 2.5 and 3 cm.

The heart has two parts, separated by the septum, that do not communicate directly. Each part has two chambers: an atrium and a ventricle. The atrium is the chamber that receives blood from the veins (venae cavae in the right atrium, pulmonary veins in the left atrium), while the larger ventricle expels blood into the arteries (pulmonary trunk from the right ventricle, aorta from the left ventricle). All four chambers have valves designed to impede blood backflow when the heart contracts. The atrioventricular valves (tricuspid and mitral) are located between the atria and the ventricles, and the semilunar valves (pulmonary and aortic) are located at the exits from the ventricles.

Blood circulation

FOUR CHAMBERS, FOUR VALVES

pulmonary trunk left pulmonary artery

left pulmonary veins

left atrium pulmonary valve When the left ventricle contracts, the mitral valve is closed by blood pressure. The aortic valve closes after blood is expelled into the aorta. chordae tendineae left ventricle

Due to the chordae tendineae, the papillary muscles keep the tricuspid and mitral valves from being pushed into the atria when the ventricles contract.

The intraventricular septum separates the two ventricles.

thoracic aorta

right ventricle 83

The cardiac cycle Blood circulation

A remarkably regular rhythm The contractions of the myocardium follow a regular cycle with three distinct phases. Each cycle is triggered by particular cells in the cardiac muscle that are called autorythmic because they are capable of spontaneously generating and propagating electrical impulses. These cardiac stimulators are essential, since proper functioning of the cardiovascular system depends on the regularity and coordination of the heart’s movements. THE CARDIAC CYCLE It takes about 0.8 seconds for a stream of 70 ml of blood to enter the heart, pass through it, and be expelled into the arteries. This cycle includes a rest phase (diastole) and two contraction phases (systoles). THE DIASTOLE right atrium

left atrium

A phase of muscular relaxation, the diastole is marked by generalized dilation. The blood from the veins enters the atria, then, when the atrioventricular valves open, it flows directly into the ventricles, which fill to 70% of their capacity.

When the heart is at rest, the atrioventricular valves are open. The semilunar valves are closed during diastole and atrial systole.

ATRIAL SYSTOLE When the atria contract, they expel the blood that they contain, which fills the ventricles. This first muscular contraction is called the atrial systole.

left ventricle

right ventricle

VENTRICULAR SYSTOLE pulmonary trunk aorta

Ventricular systole is the contraction of the ventricles. The atrioventricular valves close to keep the blood from flowing back to the atria, while the semilunar valves open to let the blood flow into the pulmonary trunk and aorta.

The contraction of the ventricles closes the atrioventricular valves. Blood pressure forces the semilunar valves to open. 84

Although nervous or hormonal messages can change the cardiac rhythm, this rhythm is dictated essentially by certain cells in the myocardium that have the capacity to depolarize spontaneously and to emit electrical impulses 70 to 80 times per minute. This stimulation propagates throughout the entire myocardium and triggers, in succession, contraction of the atria and the ventricles. The sinoatrial node Q, located in the wall of the right atrium, is where cardiac excitation begins. When its cells depolarize (on average every 0.8 seconds), they create an electrical action potential. By propagating rapidly from one cell to the next via the internodal tracts W, this impulse provokes contraction of the atria. When it reaches the atrioventricular node E, the impulse passes through the bundle of His R (or atrioventricular bundle), which is the only electrical conduit between the atria and the ventricles. The impulse descends along the interventricular septum, reaches the apex of the heart, and then propagates rapidly in the muscle mass of the ventricles via the Purkinje network T. The ventricles contract about 0.16 seconds after the atria contract.

Blood circulation

CARDIAC CONDUCTION

left atrium right atrium The sinoatrial node is also known as the pacemaker.

Q 0

The internodal tracts propagate the electrical impulses of the sinoatrial node in the two atria.

W 0

E 0

atrioventricular node

R 0

The bundle of His, divided into two branches, directs the impulse along the septum to the apex. T 0

Large muscle fibers in the outer walls of the myocardium, the Purkinje network, propagate the impulse throughout the ventricular wall.

ventricles interventricular septum

apex

THE ELECTROCARDIOGRAM The electrocardiograph is an apparatus that uses sensors placed on the skin to measure the intensity of electrical currents resulting from depolarization of the heart’s muscular fibers. The graph of the results, an electrocardiogram, shows the deflections (deflections P, Q, R, S, and T) that correspond to the different phases of the cardiac cycle. 1 mV

The P deflection indicates atrial depolarization, which leads to contraction of the atria. It is followed by the QRS sequence, corresponding to depolarization of the ventricles. The T deflection represents ventricular repolarization, which occurs immediately after contraction of the ventricles.

R deflection

0.5

P deflection

T deflection

0

Q deflection

S deflection

-0.5 0

0.4

0.8 s

85

The lymphatic system Blood circulation

Drainage and cleansing of the body’s fluids The lymphatic system is closely connected to the cardiovascular system. Plasma constantly leaks out of the blood capillaries and accumulates in the tissues, where it forms interstitial liquid. Through its network of vessels, the lymphatic system drains this liquid (called lymph at this stage), thus keeping the tissues from swelling. Infectious agents are removed in the lymph nodes, and then the lymph is reintroduced into the cardiovascular system. Other organs, such as the spleen, thymus, and tonsils, play a role similar to that of the lymph nodes, although they do not process lymph directly. DRAINAGE OF LYMPH The lymphatic system consists of a one-way network that collects about three liters of lymph per day from the body’s various tissues. The lymph is evacuated by the lymph capillaries, passes through the nodes to be filtered, then flows into two main canals: the right lymphatic duct, which drains the right upper quarter of the body, and the thoracic duct, which receives lymph from the rest of the organism. These two vessels join, then open into the subclavian vein, through which the lymph is sent into the cardiovascular system.

tissue cells blood capillaries

A system of valves prevents backflow of the lymph.

lymph capillaries

LYMPHATIC VESSELS

The endothelial cells of the lymph capillaries are very thin and permeable to interstitial liquid.

86

The lymphatic vessels run alongside the blood vessels throughout the body, except along the central nervous system and in the top layer of the skin. Lymph capillaries are formed of an extremely thin, permeable membrane that enables interstitial liquid to penetrate by simple pressure. The bacteria that it contains are evacuated and then destroyed by white blood cells.

The tonsils, located on the palate, the pharynx, and the back of the tongue, protect against bacterial infections of the throat.

right lymphatic duct The thymus, composed of lymphoid tissue, is where certain lymphocytes are differentiated. axillary nodes

Blood circulation

cervical nodes

thoracic duct

THE SPLEEN: A FILTER The spleen, located behind the stomach, has two types of tissues: red pulp, which has abundant red blood cells, and white pulp, which forms small masses of lymphocytes along the arteries. Aside from its role in immune defense, the spleen filters the blood by destroying old red blood cells. It also constitutes a large blood reservoir. red pulp white pulp intestinal nodes splenic artery

inguinal nodes

splenic vein

THE LYMPH NODES After being drained by the lymphatic vessels, lymph flows through the lymph nodes, specialized organs containing a great quantity of white blood cells (lymphocytes and macrophages), which filter and clean it. A large number of these small (1 to 25 mm diameter) organs are arranged in bunches along the vessels, mainly in the underarms (axillary nodes), in the neck (cervical nodes), in the groin (inguinal nodes), and in the intestines (intestinal nodes). capsule The germinal centers contain B lymphocytes.

efferent lymphatic vessel

afferent lymphatic vessel

The capsule extends into the node in fibrous trabeculae. 87

Immunity Blood circulation

How the body defends itself against infection To protect itself against foreign bodies, the body has a number of complementary defense modes. The epidermis, which functions as a physical barrier, is seconded by tears, sebum, saliva, and gastric juices, which contain chemical defenses (acids, enzymes, etc.). If a pathogen manages to break through this first line of defense, the body responds to the assault with an inflammatory reaction or a specific immune response. In both cases, white blood cells play a major role, reaching the infected region of the body through the blood and lymphatic vessels and destroying the foreign bodies and affected cells.

THE INFLAMMATORY REACTION histamine Q 0

attacked cell pathogen

blood capillary W 0

E 0

R 0

neutrophil

phagocytosis

macrophage dead neutrophil

When pathogens (bacteria, viruses, parasites, etc.) are introduced into the body, the injured region reacts with a group of nonspecific mechanisms that are called the inflammatory reaction. When a cell is attacked Q, a chain of chemical reactions take place that result in a release of substances, such as histamine, that increase the diameter and permeability of nearby blood vessels, causing the redness, heat, and swelling characteristic of an inflammation. These substances also attract white blood cells to the infection site by a mechanism called chemotaxis. Neutrophils W are the first to appear: in less than one hour, they cross through the walls of the blood capillaries and begin to destroy pathogens by phagocytosis E. They are joined by monocytes, which are transformed into macrophages R. These large cells continue with the destruction of the intruders, and they also destroy infected cells and dead neutrophils. When there is chronic inflammation, the dead white blood cells and debris of microbes form a yellowish liquid, pus, which accumulates in the wound. If the pus is not eliminated quickly, it can form an abscess, which makes its dispersion more difficult.

PHAGOCYTOSIS Neutrophils, eosinophils, and monocytes are phagocytic cells – white blood cells capable of engulfing and digesting other cells. Phagocytosis takes place in several steps. The phagocytic cell contacts a pathogen with its pseudopods Q. The foreign body is pulled toward the cell membrane of the phagocyte, which surrounds and engulfs it W. Lysosomes adhere to the vesicle in which the prey is enclosed E, which allows enzymes to destroy it R. Residues may be used by the phagocytic cell or ejected to the outside. phagocytic cell

lysosome pseudopod

vesicle

pathogen Q 88

W

E

R

SPECIFIC IMMUNE RESPONSES

THE CELLULAR IMMUNE RESPONSE Pathogens Q that enter the body are attacked by macrophages W. Unlike neutrophils, macrophages do not completely digest the cells that they phagocytose, but decompose them into fragments of proteins that they incorporate into their membrane. All T lymphocytes with a receptor specific to this antigen react by becoming active and multiplying. Auxiliary T lymphocytes E secrete cytokines, substances that stimulate the immune response. Cytotoxic T lymphocytes R move to the site of the infection, where they attack the cells infected by the pathogen T. auxiliary T lymphocyte After a pathogen is phagocytosed, its antigen adheres to the membrane of the macrophage.

Pathogens are composed of antigens, proteins foreign to the body.

T lymphocytes that recognize the antigen multiply.

E 0

W 0

Q 0

infected cell T 0

Cytotoxic T lymphocytes destroy infected cells by piercing their membranes.

R 0

The cytokines activate cytotoxic T lymphocytes.

THE HUMORAL IMMUNE RESPONSE In the presence of an antigen, B lymphocytes also multiply, and they differentiate into plasmocytes Q, cells capable of secreting antibodies. The antibodies W act in different ways against pathogens. Some cause microbes to clump and be destroyed by phagocytic cells E. Others attach to the antigen and attract the complement R, a group of proteins. The complement proteins pierce the cell membrane of the pathogen and make it explode T. During the immune reaction, some T and B lymphocytes differentiate into memory cells, long-lived cells that retain a memory of the antigen that activated them. Their presence in the body greatly accelerates the immune response if there is a new infection by the same pathogen. complement

Each plasmocyte secretes 2,000 antibodies per second.

antibody

A phagocytic cell can digest many pathogens before dying.

Q 0

R 0

W 0

pathogen E 0

T 0

Blood circulation

The inflammatory reaction is not adapted to a particular type of assault. It is therefore sometimes insufficient and must be complemented by specific immune responses: the cellular immune response and the humoral immune response.

The endocrine system Blood circulation

Hormones: the body’s chemical messengers The human body secretes and circulates some 50 different hormones. A wide variety of these chemical substances are produced by endocrine cells, most of which are in glands. The hormones then enter the blood system to circulate throughout the body and activate target cells. The endocrine system, tightly linked to the nervous system, controls a large number of the body’s functions: metabolism, homeostasis, growth, sexual activity, and contraction of the smooth and cardiac muscles.

THE ENDOCRINE GLANDS The endocrine system is composed of nine specialized glands (the pituitary, the thyroid, the four parathyroids, the two adrenals, and the thymus) and a number of organs capable of producing hormones (including the pancreas, heart, kidneys, ovaries, testicles, and intestines). The hypothalamus, which is not a gland but a nerve center, also plays a major role in the synthesis and release of hormonal factors. Unlike substances produced by the exocrine glands, which flow through ducts, the hormones are released directly into the space surrounding the secreting cells. The very high vascularization of endocrine glands enables hormones to spread throughout the blood system via the capillaries. Some of them circulate freely in the blood, while others must attach to carrier proteins to reach the target cells.

Generally considered the master endocrine gland, the pituitary secretes 10 different hormones. Some of these substances then act on the other endocrine glands.

The adrenal glands, located above each kidney, are composed of two distinct parts. The adrenal cortex secretes cortical hormones such as aldosterone and cortisol, as well as the male and female sex hormones (androgens and estrogens). The adrenal medulla produces mainly adrenaline and noradrenaline, the hormones that are involved in the body’s response to fearful stimuli.

thyroid gland

adrenal glands

Above each kidney is an adrenal gland.

adrenal medulla

adrenal cortex

pancreas kidney 90

hypothalamus

thymus

The thyroid gland, consisting of two lobes, one on either side of the larynx, is activated by thyroid stimulating hormone (TSH) secreted by the pituitary gland. The thyroid hormones, commonly called T3 and T4, are made from iodide of blood. Their main task is to regulate growth and metabolism.

Thyroid hormones are stored in tiny sacs, the thyroid follicles. trachea

Blood circulation

THE THYROID GLAND

Located behind the thyroid gland, the parathyroid glands produce parathormone, which controls the calcium level in the body.

thyroid gland

THE PANCREAS The pancreas, which plays an important role in digestion by producing enzymes, is also part of the endocrine system. Groups of cells called islets of Langerhans secrete four different hormones, the most important of which are glucagon and insulin, which regulate the glycemic level in the body.

The islets of Langerhans are the seat of endocrine activity in the pancreas. The acini are groups of cells responsible for exocrine production of pancreatic enzymes.

pancreas

HOW HORMONES WORK When a hormone diffuses outside of a capillary, it can act on a target cell – a cell with receptors that correspond to it. There are two types of hormonal activity. A steroid hormone Q is capable of crossing through the cell membrane of the target cell. It unites with a receptor protein located inside the nucleus, which stimulates or blocks the cell’s genetic activity. A proteinic hormone W, on the other hand, cannot penetrate the target cell. It attaches to the cell’s membrane and activates a receptor that releases, in turn, a messenger within the cell. Each target cell has between 5,000 and 100,000 hormone receptors on its surface. Their number may be reduced or increased to adapt to the quantity of hormones in the blood. Hormones belong to different classes of chemical products: steroids (testosterone), proteins (insulin), polypeptides (parathormone), derivatives of amino acids (adrenaline), and eicosanoids (prostaglandin).

target cell

target cell nucleus

proteinic hormone W 0

capillary steroid hormone

Q 0

91

Blood circulation

The hypothalamus and the pituitary gland The control centers of the endocrine system Because it controls the activity of a number of other glands, the pituitary gland is often considered the main gland of the endocrine system. However, it is controlled by the hypothalamus, a nerve center involved in the regulation of many vital functions. Between them, the hypothalamus and the pituitary gland produce one third of all the hormones in the body and influence actions ranging from lactation and urine retention to skin pigmentation and bone growth. THE HYPOTHALAMUS Located under the thalamus, the hypothalamus is composed of several nuclei that control the autonomic nervous system and regulate hunger, thirst, body temperature, and sleep. The hypothalamus also influences sexual behavior and controls the emotions of anger and fear. Closely linked to the pituitary gland, it acts as a coordinator between the nervous and endocrine systems.

THE PITUITARY GLAND A small mass, about 1.3 cm in diameter, the pituitary gland is located in a cavity in the sphenoid bone, the sella turcica. It is composed of two very different structures: the neurohypophysis, which contains the axonal extensions of the secreting neurons of the hypothalamus, and the adenohypophysis, which is composed only of endocrine cells.

sphenoid bone The hypothalamus contains a dozen nervous nuclei. pituitary gland neurohypophysis

adenohypophysis

ACTIVITY OF THE THYROID GLAND: AN EXAMPLE OF HORMONAL FEEDBACK CONTROL The production of thyroid hormones by the thyroid gland is regulated by a chain of hormonal stimulation. First, the hypothalamus Q secretes thyrotropin-releasing hormone (TRH), which travels through the capillary network to stimulate the adenohypophysis W. This body reacts by releasing thyrotropin (or TSH), which, in turn, activates the thyroid gland E, thus provoking production of thyroid hormones. This mechanism is controlled by a feedback system. If the nerve receptors detect signs of too high a concentration of thyroid hormone in the body, production of TRH by the hypothalamus is inhibited. Receiving less stimulation, the pituitary gland reduces its secretion of thyrotropin, which affects the activity of the thyroid gland. This is called negative feedback. In contrast, if there is not enough of a hormone in the body, feedback to the hypothalamus stops, and it then releases TRH.

hypothalamus adenohypophysis

0 Q W 0

feedback

thyroid gland 92

E 0

nervous nucleus The axons of the secreting neurons of the hypothalamus route hormones (vasopressin and ocytocin) to the neurohypophysis.

Controlled by the hypothalamus via a capillary network, the adenohypophysis secretes six different hormones: melanocyte-stimulating hormone, thyrotropin, prolactin, corticotropin, growth hormone, and gonadotropin.

Blood circulation

ADENOHYPOPHYSIC HORMONES

Melanocyte-stimulating hormone governs the synthesis of melanin, the pigment that colors the skin.

NEUROHYPOPHYSIC HORMONES The secreting cells of the hypothalamus synthesize and secrete two hormones: vasopressin and oxytocin, which are released into the blood system by the neurohypophysis.

Thyrotropin governs the secretion of hormones by the thyroid gland.

Prolactin triggers and controls the synthesis of milk by the mammary glands.

neurohypophysis Vasopressin orders the kidneys to reduce the quantity of urine excreted, provokes constriction of arterioles, and reduces perspiration.

adenohypophysis The activity of the adrenal cortexes (especially production of cortisol, which regulates the storage of glucose) is stimulated by corticotropin. Growth hormone is the main pituitary hormone. This protein stimulates general body growth and affects the metabolism.

Oxytocin provokes uterine contractions during childbirth and triggers the release of milk by the breast in reaction to a stimulus: sucking on the nipples.

Follicle-stimulating hormone and luteinizing hormone are gonadotropins. They act on the ovaries and testicles, in particular triggering production of ova and spermatozoa and the secretion of estrogen and testosterone.

93

The urinary system Blood circulation

How the kidneys filter the blood Water, which forms 60% of the weight of the human body, circulates mainly via the blood, carrying nutritive elements and waste. The urinary system allows the body’s volume of water to be controlled and certain substances to be eliminated through the urine. The kidneys function as filters by extracting waste from the blood without depriving it of nutritive elements. The urine produced is stored in the bladder, then evacuated via the urethra. To compensate for this loss of liquid, an adult must ingest two liters of water per day. THE ORGANS OF THE URINARY SYSTEM

aorta renal artery kidney

Located on either side of the aorta and the inferior vena cava, the kidneys are supplied by the renal arteries. They filter the blood and produce urine, which is transported to the bladder by two ureters. The urethra, which carries urine out of the bladder, is longer in men than in women.

vena cava ureter

bladder bladder

The woman’s urethra opens to the exterior above the vaginal opening.

In men, the urethra passes through the penis.

THE BLADDER ureter openings of ureters

detrusor urinae

Before being eliminated, urine is temporarily stored in the bladder. This sac, made of muscle tissue, is spherical in shape when it is full and flat when it is empty. The bladder can hold up to an average of 500 ml, but the micturition (urine evacuation) reflex appears when the bladder contains 200 to 400 ml of urine. The detrusor urinae muscle contracts, while the internal sphincter relaxes, which leads to evacuation of urine via the urethra. The external urethral sphincter, voluntarily controlled, allows micturition to be blocked.

mucosa

urethra empty bladder 94

full bladder

cortex

medulla

THE KIDNEYS

renal artery renal vein

Small bean-shaped organs with an average length of 11 cm, the kidneys are enclosed in a fibrous capsule and surrounded by adipose tissue. They are composed of an outside layer, the cortex, and an internal area, the medulla, in which there are conical structures called pyramids. The pyramids are formed of many renal tubules that converge to form collecting ducts that empty into the small and large calyces. The calyces receive the urine produced by the nephrons (functional units located in both the cortex and the medulla) and drain into the renal pelvis, a cavity that leads to the ureter.

ureter

Blood circulation

renal capsule

pyramid small calyx large calyx

The glomerulus is formed of a mass of capillaries folded inward in Bowman’s capsule.

afferent arteriole Q 0

Bowman’s capsule

W 0

filtrate

renal pelvis

R 0

The renal tubule descends into the medulla, where it forms the loop of Henle.

efferent arteriole

E 0 T 0

Y 0

peritubular capillaries

renal tubule Ducts of Bellini collect the urine made in many renal tubules.

U 0

Urine is 90% water, but it also contains urea, creatinine, uric acid, and ions.

NEPHRONS: FROM BLOOD TO URINE Each kidney contains about 1 million nephrons, units that filter the blood and produce urine. Blood enters the nephrons via an afferent arteriole Q, which subdivides into numerous capillaries to form a glomerulus W, a small sphere enveloped in a Bowman’s capsule. Some of the constituent elements of blood (water, mineral salts, glucose) pass through the walls of the capillaries and form a liquid called filtrate E. The capillaries come together again in an efferent arteriole R, which leaves the glomerulus. The filtrate enters a renal tubule T, which winds through the cortex and the medulla, exchanging substances with peritubular capillaries Y. These exchanges enable the blood to re-absorb some useful products. It is estimated that out of 180 liters of filtrate produced every day, about 179 liters are re-absorbed. What remains of the filtrate becomes urine U, which is drained toward the calyces via the ducts of Bellini. 95

Like all living organisms, the human body needs certain

products to survive and develop.

Two major systems supply it with the elements needed by its metabolism:

the respiratory system

and the digestive system. Respiration puts the oxygen in the air in contact with the blood, while digestion is a process for assimilation of nutritive substances.

Respiration and nutrition 98

The respiratory system Oxygenating the body

100

Respiration Exchanges between air and blood

102

Speech Vibration, resonance, and articulation

104

The digestive system How foods are transformed and absorbed

106

The teeth The first step in digestion

108

The stomach A pouch with an acid environment

109

The intestines A succession of tubes

110

The liver, pancreas, and gallbladder Biochemical laboratories

The respiratory system Respiration and nutrition

Oxygenating the body Because the cells in the human body cannot be deprived of oxygen, the organism must constantly be oxygenated through respiration. This generally involuntary function, governed by specialized neurons of the brain stem, consists of bringing air from outside the body to the depths of the lungs via the branching network of the lower airway system. These innumerable ramifications constitute most of the mass of the lungs, which are the main organs of respiration.

THE ORGANS OF RESPIRATION The respiratory system is composed of a series of passages designed to transport air from outside the body to the alveoli of the lungs, where gas exchanges occur. The upper airway is composed of the nasal cavities and the pharynx. The lower airway is composed of the larynx, trachea, bronchi, and lungs. middle nasal concha

superior nasal concha

orbit

The nasal cavities, separated by the nasal septum, meet in the pharynx, at the back of the nose. The conchae, bony folds covered with mucus, guide the air along canals (or meatuses) where dust is trapped. maxillary sinus inferior nasal concha

nasal septum

hard palate

The pharynx (or throat) connects the nasal cavities, the mouth, and the larynx. larynx The downward extension of the larynx, the trachea consists of a tube about 12 cm long and 1.5 cm wide that divides to form the two bronchia. A series of 15 to 20 horseshoeshaped pieces of cartilage protect the front of the trachea, while a muscle separates the back of the trachea from the esophagus. The right lung has three lobes, while the left lung, on the same side of the chest as the heart, has only two lobes. left lung heart

The diaphragm is a musculotendinous partition that separates the thorax from the abdomen. 98

esophagus

THE LUNGS

The inside of the trachea, like the rest of the bronchial tree, is covered with a ciliated mucosa that directs impurities to the outside. At the carina, the trachea divides into the left and right main bronchi. cilia

Respiration and nutrition

The trachea divides into two main bronchi that lead to the two lungs. These channels, in their turn, subdivide into secondary bronchi that lead to the lobes, then into tertiary bronchi, which ramify into narrower, even more numerous bronchioles. This arborescent structure is the bronchial tree.

left main bronchus upper lobe carina

right main bronchus

secondary bronchus

tertiary bronchus

Each lung has about 250,000 bronchioles.

lower lobe

The lung is enveloped in a double membrane, the pleura. The space between the two layers is filled with a lubricating liquid, the pleural fluid. 99

Respiration Respiration and nutrition

Exchanges between air and blood The diaphragm and the intercostal muscles work together to cause inspiration, which brings air deep into the lungs. No muscle work is needed for expiration, which expels the carbon dioxide produced by the cells. At the ends of the bronchial tree are tiny cavities, the pulmonary alveoli, which are in close contact with the blood capillaries. There are so many alveoli that their total area is more than 100 m2. It is along this surface that gas exchanges between air and blood take place. INSPIRATION AND EXPIRATION The coordinated activity of the diaphragm and the intercostal muscles inflates the lungs. During the inspiration phase, the diaphragm Q and the intercostal muscles W contract. Their contraction enlarges the ribcage and increases the volume of the lungs E. The difference in pressure draws air in through the trachea R. Expiration, on the other hand, is an essentially passive phenomenon, due to the elasticity of the ribcage as the diaphragm and intercostal muscles relax.

trachea

R 0 W 0

lung

intercostal muscles

E 0

Q 0

diaphragm EXPIRATION

INSPIRATION

The sinuses are facial bony cavities that heat the inhaled air and contribute to vocal resonance. nasal cavity nostril

THE ROLE OF THE NOSE IN RESPIRATION Inhaled air enters the body through the nostrils and crosses the nasal cavities to the pharynx. As it passes, the air is filtered by the hairs in the nose, which hold back the largest dust particles. The mucus that lines the nasal cavities also traps undesirable particles and helps to humidify the air. Finally, tiny blood vessels heat cold air before it reaches the lungs.

inhaled air exhaled air

Respiration and nutrition

oxygenated blood

oxygen-poor blood

Q 0

The respiratory bronchioles are the terminal channels of the bronchial tree.

W 0

Individuals have an estimated 600 million alveoli, which give the lungs their spongy consistency. alveolar atrium

oxygen molecule E 0

The blood capillaries coil around the alveolar masses.

carbon dioxide molecule

GAS EXCHANGES OF RESPIRATION At the end of the respiratory bronchioles Q, the inhaled air reaches the alveoli W, small cavities grouped in bunches around an alveolar atrium. The alveoli are wrapped in a dense network of blood capillaries E. The respiratory membrane R that separates an alveolus from the capillaries that surround it is extremely thin and permeable, allowing for gas exchanges between the blood and the air. During inspiration, oxygen molecules pass from the air to the blood, while carbon dioxide molecules transported by the red blood cells T pass through the membranes in the opposite direction to be evacuated during expiration.

throat

R 0

T 0

respiratory membrane capillary red blood cell

COUGHING AND SNEEZING

diaphragm

COUGH

alveolus

nasal cavity

When particles obstruct the airways, special respiratory actions are spontaneously triggered to expel them. Coughing frees the bronchia, trachea, and throat, while sneezing produces a powerful current of air in the nasal cavity. It is estimated that air is expelled at a speed of 150 km/h!

SNEEZE 101

Speech Respiration and nutrition

Vibration, resonance, and articulation To express themselves, humans are capable of producing a great many different speech sounds (the phonetic elements of a language), which are formed into words. This ability is the result of a complex interaction between many parts of the body, including the brain, the lungs, the larynx, the pharynx, and a collection of mobile articulators: the tongue, lips, lower jaw, and soft palate (or velum). The lungs and larynx provide the sound source, which is shaped by the upper airway known as the vocal tract. THE PROCESS OF SPEAKING soft palate

If the vocal folds are close together, pressure supplied by the exhaled air sets them into vibration to produce a tone. This is called phonation. The size and shape of the cavities of the vocal tract (pharynx, nose, and mouth), determined by the positions of the mobile articulators, amplify certain frequencies of the tone. This resonance produces a complex sound that is unique for each phonetic element of the language.

nasal cavities

oral cavity

Attached to the mandible by muscles and tendons, the hyoid bone supports the larynx. lips pharyngeal cavity vocal folds

larynx

The epiglottis is a cartilaginous flap that covers the larynx during swallowing to keep food from entering the lungs.

teeth tongue thyrohyoid membrane

THE LARYNX The larynx, situated on top of the trachea, opens into the pharynx and is considered part of the upper airway. It is composed of cartilages linked by ligaments and muscles and completely covered by mucous membrane. The largest of these cartilages, the thyroid, forms a visible bump in the neck in men, the Adam’s apple. Within the thyroid cartilage are the vocal folds.

thyroid cartilage

cricoid cartilage trachea

102

epiglottis

The vocal folds are long, smooth, rounded bands of muscle tissue that can be lengthened or shortened, tensed or relaxed, and separated or approximated. They are attached to the thyroid cartilage in the front and to the arytenoid cartilages in the back. Activation of the various intrinsic muscles, also attached to the vocal folds arytenoid cartilages, causes the vocal folds to open wide during respiration or to close, tense, and stretch during phonation. For sound to be produced when exhaled air passes through the vocal folds, their edges must be more or less closed: the amount of closing affects voice quality. thyroid cartilage In general, men’s vocal folds are longer and have greater mass than women’s, which is arytenoid cartilage why men have a lower voice. The posterior cricoarytenoid muscle causes the vocal folds to separate. The lateral cricoarytenoid muscle causes the vocal folds to meet in the midline. The cricothyroid muscle stretches the vocal folds, resulting in a rise in the pitch of the voice.

Respiration and nutrition

THE VOCAL FOLDS

The glottis is the space between the vocal cords.

epiglottis

Phonation Q requires the vocal folds to be closely approximated. The tension of the intrinsic muscles, along with the pressure applied from the lungs, determines the quality of the sound that is produced. On the other hand, no sound is produced when the glottis is wide open W and the larynx is used solely for breathing.

Q 0

W 0

ARTICULATION OF CONSONANTS AND VOWELS A large number of muscles act to position the tongue, lips, jaw, and soft palate in various combinations to articulate different consonant or vowel sounds. Many consonants result from the presence of obstructions to the air flow by the tongue and lips with teeth and hard palate. Occlusive consonants (p, t, k) are produced by the complete obstruction and then sudden release of the air flow, while fricative consonants (f, th, s, sh) are produced with an incomplete obstruction, resulting in noise-like sounds. For both of these categories, sounds are also produced while the vocal folds are vibrating, resulting in voiced consonants (b, d, g, v, z, j).

p

t

nasal resonator

oral resonator labial resonator

f

s

g

Articulation of vowels involves no major obstacles to the passage of sounds from the larynx to the mouth opening. Therefore, resonance is what differentiates these sounds. The size and shape of the vocal tract, the degree of lip rounding, and the degree of muscular tension are the most important factors affecting vowel articulation. Changes in oral, labial, and nasal cavities also contribute to vowel articulation. In some languages, such as French, the nasal resonator is involved in articulation of nasal vowels, when the velum of the soft palate moves to let some air pass through, adding a nasal quality to the sound. 103

The digestive system Respiration and nutrition

How foods are transformed and absorbed The energy that the human body needs to function is supplied by food. Working together, the 10 organs that form the digestive system decompose food, absorb its nutrients, and eliminate the waste. The series of conduits and pouches through which food travels before being evacuated in the form of fecal matter is called the digestive tract. This nine-meter-long tract starts at the mouth and continues, in order, through the pharynx, esophagus, stomach, small intestine, large intestine, and anus. Some related organs contribute to digestion although they are not part of the digestive tract. The teeth and tongue help to transform food into alimentary boluses. The salivary glands, liver, pancreas, and gallbladder produce or store digestive substances (including enzymes) and release them into the digestive tract. THE PATH THAT FOOD TAKES The food that we eat begins to be transformed in the mouth, where it is ground up by the teeth, compacted by the tongue, and moistened by the saliva. Amylase, a digestive enzyme contained in the saliva, begins to transform sugars. In less than one minute, the mouthful has become an alimentary bolus Q – a soft, moist ball. Swallowing requires perfect coordination of the muscles of the mouth and pharynx. The alimentary bolus is pushed by the tongue to the back of the mouth, where it enters the pharynx. The tongue is raised toward the velum of the palate, which obstructs the nasal cavity and keeps the bolus from entering it. As it slides into the pharynx W, the alimentary bolus pushes the epiglottis down, closing the entrance to the trachea. The pharynx and tongue combine to propel the alimentary bolus down the esophagus. gallbladder alimentary bolus

velum of the palate nasal cavity

Q 0

tongue W 0

pharynx epiglottis

trachea 104

esophagus

tongue teeth Three pairs of salivary glands produce saliva.

E 0

Most of the digestion and absorption occurs in the small intestine T, where the chyme remains for one to four hours. Bile and pancreatic juices completely decompose the food, and its nutrients are absorbed by the intestinal mucosa. In the large intestine Y, where some of the water and ions are absorbed, the waste is transformed into fecal matter, then stored for at least 10 hours before being evacuated via the anus U.

Respiration and nutrition

Once swallowed, the alimentary bolus descends the esophagus E in a few seconds. It enters the stomach R, where it is mixed with gastric juices containing enzymes that begin to decompose the sugars and proteins. This step, which lasts two to four hours, transforms the alimentary bolus into chyme.

The esophagus, a tube about 25 cm long, propels the alimentary bolus to the stomach with a series of involuntary muscular contractions called peristalsis.

The liver is the largest organ in the human body except for the skin. It contributes to digestion by producing a number of substances. R 0

The stomach can contain up to four liters of food.

The pancreas controls the sugar level in the body and releases digestive substances.

T 0

The small intestine looks like a folded inner tube. It is between 4 and 7 meters long.

Y 0

The chyme is transformed into fecal matter in the large intestine.

rectum U 0

The sphincters that surround the anus relax to permit defecation.

105

The teeth Respiration and nutrition

The first step in digestion Before being decomposed by gastric and intestinal juices, food undergoes a physical transformation in the mouth. The teeth, numbering 20 in children and 32 in adults, play a crucial role, as they fragment food and transform it into a lubricated alimentary bolus for swallowing. Mastication (or chewing) is thus the first step in preparing food for digestion. TYPES OF TEETH Thirty-two teeth in total make up the adult human dentition, 16 in the upper jaw and 16 in the lower jaw. There are four tooth types (incisors, canines, premolars, and molars), which all have differently shaped crowns and roots. Each crown shape plays a different role in mastication. upper jaw

crown root

All four canines have a single, long root. Their pointy crowns are used to grip and rip food.

The eight incisors have sharp edges that enable them to cut foods. They are either central or lateral in position.

occlusal surface

The eight premolars replace the deciduous molars. They have an occlusal surface that can grind foods.

Q

lower jaw

Twelve molars erupt into the bony arches of the growing jaws. They have two or three roots and a broad occlusal surface.

DEVELOPMENT OF THE DENTITION permanent molar temporary incisor

W

wisdom tooth

The formation of teeth, which begins when the fetus is only a few weeks old, continues until adulthood. At birth Q, the teeth are not visible, but the jawbones contain tooth buds that develop, grow, and finally pierce through the gums at six months of age. By five years of age W, the child has 20 temporary teeth (or milk teeth): eight incisors, four canines, and eight molars. The permanent teeth are already developing in the jaws, pushing toward the oral cavity and absorbing the roots of deciduous teeth. Replacement of the milk teeth by permanent teeth takes place over several years, generally between the ages of six and 12. An adult’s dentition E has 32 permanent teeth. The four last molars (wisdom teeth) do not emerge before 17 years of age, and they often remain in the bone if the jaw does not grow enough.

E

106

THE HARDNESS OF TEETH

The tooth crown is covered with a protective layer of enamel, the hardest substance in the human body.

The crowns of molars and premolars have points called cusps.

Pulp, highly innervated and vascularized, is a gelatinous organ that occupies the center of the crown and the root canal.

The crown is the visible part of the tooth.

Respiration and nutrition

The permanent teeth, which appear during childhood, must be able to chew food for decades. They are hard and strong because of the nature of their tissues: enamel, composed mainly of calcium phosphate and calcium carbonate, contains less than 1% organic materials.

gum The narrow part of the tooth root, just below where the enamel margin ends, is called the neck.

The teeth are formed mainly of calcified connective tissue, dentine. root canal

The root of the tooth extends below the gum.

jawbone The roots of teeth are covered with a layer of cement, which resembles bony tissue.

A layer of fibrous connective tissue, the periodontal ligament, holds the tooth root firmly in the jawbone.

The apical foramen is a narrow opening through which nerves and blood vessels enter the tooth.

TREATMENT OF A CARIES CAVITY When bacteria attack the enamel of the tooth, they create a hole called a cavity Q, which increases as the caries (decay) progress through the enamel into the dentine W. After drilling the tooth to remove all traces of infection, the dentist fills the hole with a sealing compound E. If left untreated, the caries continue to propagate, infecting the living tissues in the pulp R, and may even form an abscess T. In this case, a root canal treatment must be performed Y to stop the infection. This consists of completely removing the pulp tissue, then sealing the root canals permanently with an inert substance. This operation deprives the tooth of its innervation and blood vessels, but the periodontal ligament, root, and crown remain. enamel cavity

Q 0

sealing material

dentine

W 0

E 0

root canal

pulp

R 0

T 0

Y 0

abscess 107

The stomach Respiration and nutrition

A pouch with an acid environment The alimentary bolus moves from the esophagus into the stomach, an elastic pouch about 25 cm long that secretes extremely acid juices. Mixed together through constant movement of the stomach’s muscle layers, foods are slowly transformed into a mush called chyme, which is expelled into the duodenum in small quantities. THE MUCOSA OF THE STOMACH The interior mucosa of the stomach consists of an epithelium that is invaginated to form many folds. The gastric glands that are located in the stomach release different substances (hydrochloric acid, enzymes, mucus, hormones) that combine to form gastric juices. The mucosa sits on a vascularized submucosa, which covers three muscle layers. The fibers in each layer are oriented in a different direction, which ensures that foods are well mixed.

esophagus

The pylorus has a sphincter, a small ringshaped muscle, to control the exit of chyme from the stomach. duodenum The mucosa of the stomach includes many cavities, called crypts, at the bottom of which are the gastric glands.

muscle layers

The stomach is covered by the peritoneum, a transparent membrane that surrounds all viscera.

The gastric glands produce a number of different substances, including hydrochloric acid, which sterilize and break up the alimentary bolus.

Separated from the mucosa by a thin layer of muscle, the submucosa of the stomach contains many blood and lymphatic vessels.

THE GASTRIC CYCLE When it reaches the stomach, the alimentary bolus is kneaded, mixed with the gastric juices, and transformed into a whitish mush: chyme Q . Regular contractions of the stomach push the chyme toward the closed pylorus W. The sphincter opens repeatedly to release small quantities of chyme into the duodenum E.

chyme

Q 108

pylorus

W

duodenum

E

The intestines After being kneaded in the stomach, the chyme enters the intestines, a long series of tubes where most of the digestion process occurs. The small intestine absorbs nutrients, and the large intestine transforms the chyme into fecal matter. Muscular contractions of the intestines evacuate the waste via the anus.

THE SMALL INTESTINE The small intestine, formed of the duodenum, the jejunum, and the ileum, is a very long, folded tube, where intestinal juices secreted by its mucosa, pancreatic enzymes, and bile perform most of the digestive process. Absorption also takes place in the small intestine through epithelial cells. The many villi on the internal lining considerably increase the absorptive surface.

The bile duct transports bile from the liver and gallbladder to the duodenum.

Respiration and nutrition

A succession of tubes

gallbladder stomach

villi The duodenum receives the chyme released by the stomach.

pancreas

The small intestine is lined with rounded folds covered with villi.

absorbent cell

transverse colon

jejunum right colon ileum

capillary network

descending colon

cecum

sigmoid colon Sometimes, the appendix, rich in lymphatic tissue, suffers an acute inflammation, appendicitis. The liquid part of the chyme, the chyle, enters the lymphatic network via the chyliferous vessels of the villi.

THE LARGE INTESTINE

The rectum is a canal 12 to 16 cm in length.

The chyme passes from the ileum to the cecum, the first part of the large intestine, then to the colon, where bacteria complete its degradation. As water is absorbed by the mucosa of the colon, the chyme solidifies and is transformed into fecal matter. The colon pushes the fecal matter to the rectum, which triggers the reflexive opening of the internal anal sphincters. The external sphincters, contraction of which is voluntary, enable defecation to be controlled.

anal canal external sphincter

The opening of the anus is triggered by the internal and external sphincters.

internal sphincter

109

Respiration and nutrition

T h e l i v e r, p a n c r e a s , and gallbladder Biochemical laboratories The digestive tract could not perform all of its functions without the assistance of certain organs related to the digestive system. The liver, pancreas, and gallbladder manufacture many digestive substances, store them, then release them into the duodenum. THE LIVER The liver, which weighs almost 1.5 kg, is the largest gland in the human body. Located on the right side of the abdomen, it is composed of two asymmetrical lobes separated by the falciform ligament. It is an effective biochemical laboratory, involved in more than 500 different chemical reactions due to the large quantity of blood that it receives from the hepatic artery, which comes from the heart, and the hepatic portal vein, which rises from the small intestine (1.5 liters of blood per minute). Among other things, the liver makes bile, cholesterol, and proteins, stores glucose, iron, and vitamins, and degrades certain toxic products contained in the blood, such as alcohol. falciform ligament

left lobe

hepatic vein right lobe The liver has an amazing capacity to regenerate itself if part of it is amputated. common hepatic duct cystic duct

The muscles of the gallbladder contract to eject the bile that it holds.

hepatic artery hepatic portal vein

Formed by the junction of the cystic duct and the hepatic duct, the common bile duct transports bile to the duodenum.

THE GALLBLADDER The liver synthesizes almost one liter of bile per day. This yellowish-green liquid is temporarily stored in the gallbladder, an organ 7 to 10 cm long, which concentrates the bile and then releases it into the duodenum at mealtime. The bile salts contained in bile emulsify fats (fragment them into tiny droplets), making them easier to digest. duodenum 110

pancreatic duct

LIVER LOBULES

Respiration and nutrition

The liver looks like a grouping of hexagonal units, each measuring about 1 mm in diameter: the liver lobules. These lobules, irrigated by branches of the hepatic portal vein and the hepatic artery, are made of specialized cells, hepatocytes, radiating out from the central vein of the lobule. hepatocytes The sinusoids, spaces between the hepatocytes, act as capillaries by linking veins and arteries. liver lobule central vein of the lobule

Kupffer cells destroy dead cells and bacteria. central vein of the lobule hepatocyte bile duct

canaliculus

The branches of the hepatic artery bring oxygenated blood to the liver.

E 0

R 0

Blood loaded with nutrients from the small intestine circulates in the branches of the hepatic portal vein.

W 0

T 0

sinusoid

Q 0

bile duct

HOW BILE IS MADE branch of the hepatic portal vein

pancreas

The cells that produce pancreatic juices are grouped in masses called acini.

islet of Langerhans

Flowing in blood vessels Q that surround the lobule, blood is carried toward the central vein via the sinusoids W. The hepatocytes E near the sinusoids extract the nutrients contained in the blood and make bile, which is ejected into the canaliculi R, then into the bile ducts T. These ducts join to form a branching network that leaves the liver via the hepatic canals. The blood flows into the central vein of the lobule, then into the inferior portal vein.

THE PANCREAS The pancreas, located behind the stomach, is an elongated gland that secretes two types of substances. The acinar cells produce pancreatic juices, rich in enzymes (amylase, lipase). These juices are transported by the pancreatic duct to the duodenum, where they help with digestion. The islets of Langerhans, much less numerous, make hormones (insulin, glucagon) and are part of the endocrine system. 111

What are the

anatomical and physiological differences between men and

women? How is the ovum fertilized by a spermatozoid? What are the steps in development of the fetus? How does childbirth take place? Because they affect

the origin and transmission

of life, the questions around sexuality and reproduction are particularly interesting.

Reproduction 114

The male genital organs Making and transporting spermatozoa

116

The female genital organs Organs mainly hidden inside the body

118

Fertilization The fusion of sexual cells

120

The life of the embryo The first weeks

122

Maternity Gestation, childbirth, and nursing

Reproduction

The male genital organs Making and transporting spermatozoa Like other sexual animals, human beings reproduce by mating. The man’s reproductive apparatus includes two testicles, supported outside of the abdomen by the scrotum, a group of additional ducts and glands, and the penis. The testicles are essentially inactive during childhood and begin to mature at puberty, which occurs generally between the ages of 12 and 15. Up to the end of a man’s life, the testicles produce male sexual cells called spermatozoa. They also play an endocrine role by secreting the main male sexual hormone, testosterone. The genital organs grow during puberty. Stimulated by testosterone, facial hair and a deep voice are secondary sexual characteristics among men. When it is not sexually stimulated, the penis is flaccid.

ADOLESCENT

CHILD

ADULT

The scrotum is a sac of skin that holds and supports the two testicles. It has several muscular layers capable of letting the testicles drop from or bringing them closer to the body, thus regulating their temperature. The ideal temperature for development of spermatozoa is 34°C.

R 0

seminal vesicle

T 0

prostate

THE PATH THAT SPERMATOZOA TAKE

Y 0

Cowper’s gland

Spermatozoa, produced constantly by the testicles Q, are stored in epididymides W, where they mature. Sexual excitement causes them to flow up the vas deferens E. They are combined with secretions from the seminal vesicles R, the prostate T, and the Cowper’s glands Y to form a whitish liquid, semen. If stimulation intensifies, the sperm is ejected from the urethra U through rhythmic contractions of muscles at the root of the penis; this is ejaculation.

E 0 U 0

114

The tip of the penis, the glans, is partially covered by a fold of skin, the foreskin.

vas deferens urethra

W 0

epididymis

Q 0

testicle

vas deferens

THE PENIS

bladder

The cylindrical bodies (two lateral corpora cavernosa and one central corpus spongiosum) that form the penis become engorged with blood during sexual excitement. The penis undergoes a major transformation called an erection: it hardens, becomes thicker and longer, and stands up. The urethra, located in the center of the corpus spongiosum, routes the semen to the end of the penis, where it is ejected via the meatus of the urethra. corpus spongiosum corpus cavernosum urethra The glans is part of the corpus spongiosum.

Cowper’s gland prostate

The urethra opens through a narrow channel, the meatus of the urethra. foreskin efferent ductule epididymis lobule THE TESTICLES

testicle seminal vesicle

scrotum

The testicles, contained in the layers of muscle and skin forming the scrotum, are oval masses 3 to 5 cm long, divided into about 250 lobules. Each lobule contains small ducts called testicular tubules, within which the male sexual cells, spermatozoa, develop. rete testis The tubules converge in the back of the testicle to form the rete testis. When they leave the testicle via the efferent ductules, the spermatozoa reach the epididymis.

testicular tubules lumen

membrane of the testicular tubule flagellum

SPERMATOGENESIS The immature cells that line the membrane of the tubules, the spermatogonia Q, multiply by mitosis. Some stay close to the membrane, while others detach and differentiate into primary spermatocytes W. These grow and divide by meiosis, recombining their genetic material. The cells that result, secondary spermatocytes E, are haploid – they contain not 46 but 23 chromosomes. They divide once more to become spermatids R, then spermatozoa T, which are drawn into the lumen of the tubule. This process, known as spermatogenesis, takes about 74 days.

A spermatozoid, about 0.06 mm long, is formed of three parts: the head, which contains the nucleus; the middle piece, where the mitochondria are concentrated; and the flagellum, which supplies propulsive force.

Q 0 W 0 E 0 R 0 T 0

head

middle piece

115

Reproduction

The female genital organs Organs mainly hidden inside the body Like men, women have a pair of specialized sexual glands. These glands, the ovaries, are responsible for the production of oocytes (sexual cells) and steroid hormones (estrogen and progesterone). They are located deep within the pelvis, but they are connected to the exterior via a system of ducts and cavities that includes the fallopian tubes, the uterus, and the vagina. Women’s external genital organs, commonly referred to as the vulva, include the labia majora, labia minora, and clitoris. Although the breasts are not directly involved in reproduction, they are also considered organs of the reproductive system.

Aside from their role in nursing, the breasts are an erogenous zone. The woman’s wider hips facilitate childbirth.

In women, distribution of hair is concentrated mainly at the pubis.

CHILD

ADOLESCENT

ADULT

The labia majora are two rounded folds of fat that meet in the midline.

labia minora

THE WOMAN’S REPRODUCTIVE APPARATUS The ovaries are the female sexual glands: they are responsible for the production of ova and the main sexual hormones. Two ducts, the fallopian tubes, link the ovaries to the uterus, a muscular organ in which the embryo develops. The wall of the uterus is formed of a thick layer of muscle, the myometrium. The cavity of the uterus is lined with a mucosa called the endometrium. The uterus is connected to the vagina, a fibromuscular tube about 7 to 10 cm long at the level of the cervix. The vagina has very elastic walls and can dilate to receive the penis during sexual relations and to allow the baby to pass through during childbirth.

fallopian tube uterus

myometrium

ovary

endometrium cervix

vagina labia minora

The hymen, a thin membrane that partially blocks the entrance to the vagina, is usually broken during the first time a woman has sexual relations. 116

labia majora

endometrium The muscle layer of the uterus, the myometrium, contracts strongly during childbirth to expel the baby.

bladder pubic bone Adipose tissues that cover the pubis form a protective cushion called the mons veneris. The clitoris, made of erectile tissues similar to those in the penis, plays an important role in sexual excitement in women. The urethra opens between the clitoris and the vagina.

labia majora labia minora

vagina

OOGENESIS At birth, a girl has 1 to 2 million oocytes (immature sexual cells) in her ovaries. These cells are contained in tiny sacs, the primordial follicles Q. Each month, starting in puberty, the sexual hormones cause 20 to 25 follicles to ripen and be transformed into primary follicles W. Most of them degenerate; only one continues to mature and becomes a secondary follicle E. This follicle grows rapidly: in a few days, its wall thickens and liquid accumulates around the oocyte that it contains. It is then known as a graafian follicle R. When the wall of the follicle ruptures, the oocyte is expelled from the ovary and captured by the fimbriae of the fallopian tube: this is ovulation T. At this point, the oocyte is called an ovum.

The mucosa of the fallopian tube is covered with cilia whose movements draw in the expelled ovum.

If the ovum is not fertilized by a spermatozoid, it degenerates after a few days.

As it develops, the graafian follicle forms a bump on the surface of the ovary. The oocyte inside the secondary follicle divides by meiosis; it has only 23 chromosomes. T 0

primary follicle

R 0 E 0

primordial follicles

corpus albicans

W 0 Q 0

ovary

nucleus

blood plug After ovulation, the follicle is transformed into a corpus luteum, which secretes progesterone and estrogen before being reabsorbed and degenerating into a corpus albicans. 117

Fertilization Reproduction

The fusion of sexual cells For fertilization to occur – for a spermatozoid to unite with an ovum – the man must ejaculate into the woman’s vagina. This expulsion occurs during sexual relations, when a man achieves an intense moment of pleasure called orgasm. However, an ejaculation does not necessarily lead to fertilization, since the period of fertility lasts only a few days in the ovarian cycle. If the ovum is not fertilized during this short period, it degenerates and is eliminated with the menstrual flow. SEXUAL RELATIONS Many different types of sensory and psychic stimulation may cause sexual excitement. In men, this stimulation causes an erection of the penis, while in women the vagina secretes lubricating mucus, and the clitoris, labia majora, and nipples also become erect. Sexual relations (or coitus) begin when the man’s penis enters the woman’s vagina. Both partners then have heightened pleasurable sensations. When the man’s pleasure reaches a climax, muscular spasms expel the sperm contained in his urethra: this is ejaculation. The woman may also feel an orgasm, but it is not accompanied by ejaculation. However, the contraction of the muscle walls of her vagina may provoke her partner’s orgasm. During ejaculation, 300 to 500 million spermatozoa are deposited deep within the vagina. Propelled by undulations of their flagella, the spermatozoa migrate into the uterus and up the fallopian tubes, where one of them may fertilize an egg.

prostate

Only several thousand spermatozoa reach the fallopian tubes. clitoris uterus

testicle urethra The in-and-out movements of the erect penis in the vagina create pleasant sensations for the man and the woman. 118

The lubricants secreted by the vagina facilitate the penetration and movement of the penis.

Between puberty and menopause, a woman ovulates between 400 and 500 times, in a cycle lasting an average of 28 days. In the pre-ovulatory phase, a follicle develops in one of the ovaries and releases estrogen that encourages the endometrium, the internal lining of the uterus, to thicken. The rise in estrogen level also causes a surge in the release of luteinizing hormone by the pituitary gland, which provokes ovulation. Once the ovum is expelled into the fallopian tube, the follicle that produced it is transformed into a corpus luteum. It then secretes large quantities of progesterone and estrogen, which increases vascularization of the endometrium and prepares it for a possible pregnancy. If the ovum is not fertilized, the corpus luteum degenerates after about eight days. The resulting drop in hormone levels causes the blood vessels in the endometrium to constrict, and its top layer begins to detach 14 days after ovulation. A small amount of blood, mucus, and tissues, the menstrual flow, flows out of the vagina for three to seven days. Then the cycle starts again. Due to the effect of the estrogen and progesterone the endometrium thickens by several millimeters.

The degradation of the endometrium triggers the beginning of the menstrual flow.

1

2

3

4

menstrual phase

5

6

7

8

Reproduction

THE MENSTRUAL CYCLE

9

10

11

12

pre-ovulatory phase

13

14

15

16

17

ovulation

18

19

20

21

22

23

24

25

26

27

28

post-ovulatory phase ovum

estrogen level progesterone level spermatozoid Q 0

Certain follicle cells form a sort of protective cap over the ovum, the corona radiata.

The ovum is surrounded by a gelatinous layer made of protein, the zona pellucida.

FERTILIZATION

cytoplasm The spermatozoa and ovum generally meet in the upper part of a fallopian tube. When a spermatozoid Q comes into contact with the corona radiata W, it releases enzymes that enable it to penetrate. It crosses through flagellum the zona pellucida E and reaches the cell membrane of the ovum. When the head of the spermatozoid enters the cytoplasm R, the ovum secretes enzymes that make head of the it impenetrable to the other spermatozoa. The flagellum spermatozoid of the spermatozoid detaches itself and remains outside the ovum, while the head, which contains its nucleus, nucleus of the unites with the nucleus of the ovum T. ovum

W 0

E 0

R 0

T 0

119

The life of the embryo Reproduction

The first weeks Only 12 weeks elapse between fertilization of the ovum by a spermatozoid and the appearance of the future baby’s fingernails. During these first three months, the fertilized egg develops considerably and is gradually transformed into a fetus – a being that looks human. FROM FERTILIZATION TO IMPLANTATION Expelled by the ovary Q, the ovum is released into the fallopian tube W, where it encounters spermatozoa. When fertilization E takes place, the nuclei of the ovum and the spermatozoid merge to form a single nucleus with 46 chromosomes. This fertilized egg, called a zygote R, divides immediately after fertilization and begins to descend the fallopian tube. The cellular divisions continue at a quickening pace, and after four days the zygote forms a solid ball of 64 cells: the morula T. The next day, the morula enters the uterus and becomes a blastocyst Y. Seven days after fertilization, the blastocyst attaches to the endometrium and implantation U begins. Several days later, the blastocyst is completely buried in the endometrium, which supplies it with the nutrients it needs. The zona pellucida gradually degenerates.

zygote

R 0

morula

E 0

T 0

blastocyst

fertilization W 0

fallopian tube Y 0

Q 0

uterus ovary

If the ovum is fertilized, hormones prepare the endometrium to receive the egg.

The embryo develops from an embryonic disk, a mass of cells nested inside the blastocyst.

The trophoblast, the cellular covering of the blastocyst, will become the placenta, umbilical cord, and amniotic sac. 120

U 0

GROWTH OF THE EMBRYO

Reproduction

Two weeks after fertilization, the blastocyst is deeply anchored in the endometrium and the embryonic disk begins to develop; it is now called an embryo. The systems of the body (nervous system, cardiovascular system, etc.) are created after the first weeks, while the limbs are slower to develop.

Even though it is only about 5 mm long, the four-week-old embryo already has a model of the spine and nervous system. Its heart begins to beat and the limbs begin to form.

At the end of the sixth week, the embryo is about 1.3 cm long. Its head, as big as the rest of the body, contains models for the eyes, ears, and mouth.

model of the eye

The arms develop and rudimentary hands appear.

umbilical cord The six-week-old embryo still has an obvious tail.

THE FETUS After eight weeks, the embryo is called a fetus. It is beginning to look more like a human baby, even though it is still only 3 cm long and weighs only a few grams. During the rest of the pregnancy, the different organs of the fetus finish developing and its body grows considerably: its weight is multiplied almost 1,000 times between the eighth week and birth. The nine-week-old fetus has well-formed limbs. Its head is still large compared to the rest of its body, but it already has eyes, covered by fused eyelids. The ossification of cartilage has begun.

At nine weeks, the fingers have separated. At 12 weeks, the fetus is 8 cm long. The features of the face become better defined, the eyelids will soon be able to open, and the external ears are clearly visible. Oxygen, nutrients, and antibodies pass into the fetus’s body via the umbilical cord, composed of two arteries and one large vein.

The fingernails begin to form in the 12th week.

umbilical cord 121

Maternity Reproduction

Gestation, childbirth, and nursing During the nine months of gestation, the future baby develops inside the mother’s body and is totally dependent on her. The baby becomes physically separate from the mother at childbirth but maintains a special bond with her, mainly through nursing. NINE MONTHS OF GESTATION In general, 40 weeks (about 9 months) pass between fertilization of the ovum and childbirth. This period is called gestation. During the first trimester, the pregnancy is not yet visible but the woman experiences nausea and her breasts begin to swell. In the second trimester, growth of the fetus causes the abdomen to swell. This growth continues in the third trimester, and the resulting compression of organs may cause minor problems, such as incontinence or heartburn. The pregnant woman’s heart rhythm and blood volume increase as the fetus develops, and so do her pulmonary volume and her appetite.

FIRST TRIMESTER

SECOND TRIMESTER

THIRD TRIMESTER

breast

bladder

uterus

The placenta is a highly vascularized organ that forms against the wall of the uterus and provides nutrition to the fetus.

The fetus is surrounded by a liquid-filled pouch, the amniotic sac.

THE BREASTS

pectoral muscle

The breasts, which develop at puberty, are glands that cover the pectoral muscles and are surrounded by adipose tissu. Each mammary gland is formed of 20 lobes arranged in bunches. The breasts grow larger during pregnancy and produce milk after childbirth when stimulated by a hormone, prolactin. The mother’s milk is routed by the mammary ducts to reservoirs, the lactiferous sinuses, where it is stored until it is ejected from the nipples through tiny orifices. Each lobe is connected to the nipple by a mammary duct. mammary duct The areola, which forms a circle around the nipple, contains sebaceous glands. nipple lactiferous sinus 122

adipose tissue

CHILDBIRTH

muscles of the uterus DILATATION Childbirth begins when the combined action of a number of hormones provokes rhythmic and painful contractions of the uterus. These uterine contractions, which propagate from top to bottom, gradually dilate the cervix and cause the amniotic sac to rupture.

Reproduction

In the weeks preceding childbirth, the fetus, which usually presents head first, gradually descends between the bones of the pelvis and rests on the cervix.

vagina cervix EXPULSION Several hours may pass before the cervix and vagina are sufficiently dilated to allow the baby to pass through. When the opening is about 10 cm, the baby’s head enters the vagina. With strong contractions of the mother’s abdominal muscles, the child is expelled in less than an hour.

amniotic sac DELIVERY OF PLACENTA After childbirth, the uterine muscles continue to contract in order to expel the placenta. These contractions also prevent hemorrhage by compressing the damaged blood vessels. Complete retraction of the uterus and vagina may take several weeks.

placenta

umbilical cord

NURSING After childbirth, the mother can nurse her baby – nourish it with milk produced by her breasts. Mother’s milk is easily digestible, contains nutritive substances, and boosts the newborn’s immune defenses. Stimulation of the nipples also provokes uterine contractions, helping the uterus to return to normal size.

The baby’s sucking action is sensed by the receptors in the nipple. The nervous information is transmitted to the pituitary gland, which secretes prolactin, a hormone that stimulates production of milk by the mammary glands, and oxytocin, a hormone that causes the milk to be ejected from the mammary glands. 123

Glossary abdomen Region of the body between the thorax and the pelvis. adipose tissue Connective tissue formed mainly of adipose cells, or fat cells. afferent Describing a path (nerve, blood vessel, canal) that leads to an organ. amino acid Organic acid that is the basic structural unit of proteins. amniotic sac Sac filled with amniotic fluid within which the fetus is immersed. anatomy Science that studies the shape and structure of organs and organisms. antibody Soluble protein capable of attaching itself to a specific foreign substance and helping to destroy it. antigen Foreign substance that causes an antibody to react when it is introduced into the organism. apex Tip of an organ. aponeurosis A sheet of dense connective tissue, resembling a tendon, that links a muscle to another muscle or to a bone. arrector muscle of hair Smooth muscle attached to a hair; its contraction causes the hair to rise to a vertical position. bacterium Single-celled micro-organism. callus Mass of soft bone tissue that forms in a fracture and is gradually replaced by mature bone tissue.

124

cartilage Strong semi-opaque connective tissue composed of chondrocytes covered with a dense network of collagen and elastic fibers. chemotaxis The effect of attraction or repulsion exerted by certain chemical substances on a cell that is capable of moving. collagen Fibrous protein that is an essential component of connective tissue. commissure Band of tissue joining two parts of the body, especially in the central nervous system. concave Curved inward. convex Curved outward. cortex Outside layer of an organ or structure, especially in the cerebrum, cerebellum, kidneys, and adrenal glands. distal Designating the end of an organ or structure that is the farthest from the center of the body. efferent Describing a path (nerve, blood vessel, canal) that leads from an organ. electroencephalogram Graph made by an apparatus that records the electrical activity of neurons in the cerebral cortex. endolymph Potassium-rich liquid that fills the cavities of the inner ear and surrounds the organs of hearing and balance.

enzyme Protein that acts as a catalyst for a chemical reaction. erogenous zone Part of the body susceptible to sexual excitement. fiber Substance formed of a large number of filaments; the main component of certain tissues. follicle Small pocket or gland. genetic Having to do with genes and heredity. glycemia Level of sugar in the blood. haploid cell Cell that has undergone meiosis and, in the human species, has only 23 chromosomes instead of 46. Only sexual cells are haploid. hemorrhage Blood leakage outside the blood vessels. homeostasis Maintenance of the internal normal state of an organism. hyaline Resembling glass. intrinsic muscle Muscle contained entirely within an organ or a part of the body. juices Organic liquids containing enzymes. limb One of the four parts of the body detached from the trunk (upper and lower limbs). lipid Organic water-insoluble substance that makes up a fatty body.

Glossary matrix Homogeneous intercellular substance in all tissues.

nociceptor Nerve ending sensitive to pain stimuli.

refraction Deflection of light when it changes milieus.

meatus Opening from a canal to the outside of the body.

orbit Pyramid-shaped bony cavity that holds the eyeball and its associated organs.

sebum Substance secreted by the sebaceous glands, intended to lubricate the skin and hairs.

organ Part of the body composed of a number of different tissues, with a definite shape and performing a particular function.

sinus Cavity inside a bone.

meiosis Type of cellular division producing exclusively sexual cells. It involves a phase of random distribution of genetic material and a phase of division that leads to the reduction by half of the number of chromosomes (from 46 to 23 in the human species). membrane Thin layer of tissue. metabolism The group of biochemical reactions that enable the exchange of materials and energy within the body. It includes synthesis reactions (anabolic) and organic degradation reactions (catabolic). microvillus Microscopic extension of the cellular membrane of certain epithelial cells, notably on the intestinal mucosa. molecule Particle formed of two or more atoms. mucosa Mucus-secreting membrane lining the cavities and canals of the body. neurotransmitter Molecule serving as a chemical messenger between two neurons. Synthesized in an axonal ending, the neurotransmitter is released into the synaptic cleft in response to a nerve impulse. nitrogenous base Nitrogen-bearing organic molecule that is involved in composition of nucleotides.

photoreceptor Cell in the retina capable of converting light into nerve impulses. physiology Science that studies the functioning of organs or organisms. pigment Substance responsible for the coloration of a tissue. placenta Spongy, highly vascularized organ that forms in the uterus during pregnancy and is connected to the fetus via the umbilical cord. pore Small orifice on the surface of the skin, of a membrane, or of a mucosa. protein Organic substance made of long chains of amino acids, found in abundance in living matter. proximal Describing the end of an organ or structure that is the closest to the center of the body. puberty Period of life, generally between 11 and 16 years of age, during which physiological processes transform the body so that it is able to reproduce.

solute Substance dissolved in a solvent. stem cell Immature cell capable of multiplying indefinitely and differentiating into all cell types in the human body. steroid Type of hormone secreted mainly by cortico-adrenal glands and sexual glands. Steroids belong to the group of sterols, which also include substances such as cholesterol and vitamin D. stimulus Environmental element capable of stimulating a sensory receptor. trabecula Fine cord of connective tissue extending within an organ and supporting it. Bony trabeculae are interwoven to form spongy bone tissue. vertebral body Main part of a vertebra. villus Small protuberance at the surface of a mucosa or organ. virus Very small micro-organism composed of a chain of nucleic acid; it cannot live except as a parasite on another living being, from which it draws enzymes and amino acids.

125

Index A Achilles’ tendon 35 actin 37 adenine 11 adenohypophysis 93 adrenal gland 90 agonist muscle 40 alimentary bolus 104 amino acid [G] 13 amniotic sac [G] 123 ampulla 67 amygdala 51 ankle 31 antagonist muscle 40 antibody [G] 89 antigen [G] 89 anus 105, 109 aorta 79, 83 apophysis 28 ARTERY 78, 80 articulation 102 astigmatism 62 astrocyte 15 atlas 28 atrium 83, 84 auricle 64 autonomic nervous system 54 axon 45

B BALANCE 67 basement membrane 14 biceps 35 bile 111 bile duct 109, 110 bladder 94 blastocyst 120 BLOOD 76, 78, 80, 82, 84, 94 blood groups 77 blood platelet 76 blood pressure 79 BLOOD VESSELS 78, 80 BONE GROWTH 22 BONE STRUCTURE 20 BONE TYPES 26 BONES 20, 22, 24, 26, 28, 30, 32 bony callus [G] 23 BRAIN 46, 48 brain stem 46, 48 breast 122 bronchial tree 99 bronchiole 99, 101 bronchus 99 buccinator 39

C calcaneus 31 callosal convolution 51 canine 106

capillary 80 cardiac conduction 85 CARDIAC CYCLE 84 cardiac ventricle 83, 84 CARDIOVASCULAR SYSTEM 78 caries 107 carpal bones 30 cartilage [G] 22 CELL 8, 10, 12, 14 cell cycle 12 cell division 12 cell membrane 8 cement 107 CENTRAL NERVOUS SYSTEM 46 centromere 10 cerebellum 46, 49 cerebral cortex 50, 59, 63 cerebral hemisphere 48 cerebral lobe 48 cerebral ventricle 49 cerebrospinal fluid 47, 49 CEREBRUM 46, 50 childbirth 123 chromatid 10 chromatin 9, 11 CHROMOSOME 10, 12 chyme 108 clavicle 26 clitoris 117 coagulation 76 coccyx 28 cochlea 65, 66 cochlear nerve 65 codon 13 coitus 118 collagen [G] 14 colon 109 compact bone tissue 21 complement 89 connective tissue 14 contraction of a muscle 37, 40 convolution 48 cornea 60 corona radiata 119 corpus luteum 117 coughing 101 cranial nerves 52 crown 107 crystalline lens 60 cupula 67 cytokines 89 cytoplasm 8 cytosine 11

D dendrite 44 dentine 107 deoxyribonucleic acid 10 dermis 18 diaphragm 98, 100 diastole 84

DIGESTIVE SYSTEM 104, 106, 108, 110 digestive tract 104 DNA 10, 12 duodenum 109

E EAR 64 ejaculation 114, 118 elbow 32 electrocardiogram 85 electroencephalogram [G] 50 EMBRYO 120 enamel 107 endocrine gland 90 ENDOCRINE SYSTEM 90, 92 endometrium 116, 119, 120 endoplasmic reticulum 9 epicranial aponeurosis [G] 39 epidermis 18 epididymis 115 epidural cavity 47 epiglottis 100, 102 epiphysis 20 epithelium 14 erection 115 esophagus 105 estrogen 119 ethmoid bone 27 eustachian tube 65 expiration 100 external oblique muscle 34 EYE 40, 60 eye muscles 40 eyelash 61

F face 27 fallopian tube 116, 118, 120 FEMALE GENITAL ORGANS 116 FERTILIZATION 118 fetus 121, 122 fiber [G] 14 filaments 37 filum terminale 46 finger 30, 41 flat bone 26 follicle 117 fontanel 27 FOOT 30 foramen magnum 27 forearm 41 foreskin 114 fornix 51 fovea 61 fracture 23 free nerve endings 59 frontal bone 27 frontal muscle 38

Terms in CAPITAL LETTERS and page numbers in boldface type refer to a main entry. The symbol [G] indicates a Glossary listing.

126

Index G

L

N

GALLBLADDER 110 gastric juices [G] 108 genes 13 genetic heritage 11 GENITAL ORGANS 114, 116 gestation 122 glans 114 glial cell 15 glomerule 95 glottis 103 Golgi apparatus 9 graafian follicle 117 gray matter 46, 50 greatest gluteal 35 growth plate 23 guanine 11 gum 107

labia majora 116 labia minora 116 lacrimal gland 61 large intestine 105, 109 larynx 102 lateral geniculate body 63 ligament 32 limb [G] 25 limbic system 51 lingual papillae 70 LIVER 105, 110 liver lobule 111 long bone 20, 26 lumen 80 lung 99, 100 lymph 86 lymph node 87 lymph vessels 86 LYMPHATIC SYSTEM 86 lymphocytes 89

nail 31 nasal cavity 72, 98 nephron 95 nerve impulse 44 nerve tissue 15 NERVES 52 nervous system 44, 46, 48, 50, 52, 54 neurohypophysis 93 NEURONS 15, 44 neurotransmitter [G] 45 neutrophil 77, 88 nipple 122 nitrogenous base [G] 11 nose 72, 100 nostrils 72 nuclear membrane 9, 10 nucleolus 9 nucleotide 11 nucleus 9, 10 nursing 123

H hair 19 HAND 22, 30, 41 HEAD 27, 38 HEARING 64, 66 HEART 82, 84 heel 31 hematoma 23 hemoglobin 77 hepatic portal vein 110 heredity 11 hippocampus 51 hormonal feedback control 92 hormones 90, 92 humerus 26 hymen 116 hypermetropia 62 HYPOTHALAMUS 50, 90, 92

IJ iliac bone 24, 26 IMMUNITY 88 incisor 106 inflammatory reaction 88 inspiration 100 intercostal muscles 100 intervertebral disk 29 INTESTINES 109 intraventricular septum 83 iris 60 irregular bone 26 islets of Langerhans 91 jaw 27 JOINTS 32

K keratin 18, 31 kidney 95 knee 32 kneecap 26, 32

M

O

macrophage 14, 88 MALE GENITAL ORGANS 114 malleolus 31 mamillary bodies 51 mammary gland 122 marrow 21 masseter 39 MATERNITY 122 medulla oblongata 48 medullary canal 21, 23 melanin 19 meninges 47, 49 menstrual cycle 119 metacarpal bones 30 metaphysis 20 metatarsus 31 microgliocyte 15 microvilli [G] 14 mitochondria 8 mitosis 12 molar 106 monocyte 76 morula 120 MOTOR FUNCTIONS OF THE NERVOUS SYSTEM 54 mouth 105 MOVEMENTS 40, 41, 55 mucus 72 muscle fiber 36 MUSCLES 15, 34, 36, 38, 40 MUSCLE TISSUE 15, 36 myelin 45 myocard 82 myofibril 36 myopia 62 myosin 37

occipital bone 27 olfactory bulb 73 olfactory cell 73 olfactory cilia 73 olfactory epithelium 72 oligodendrocyte 15 optic chiasm 63 optic nerve 61, 63 orbicular of the eye 38 orbicular of the mouth 38 organ of Corti 66 organelles 9 orgasm 118 ossicles 65, 66 ossification 22 osteocyte 21 osteon 21 ovary 117 ovulation 117, 119 ovum 117, 119

P pacemaker 85 pain 55 palate 68 palm 30 palmar aponeurosis [G] 41 PANCREAS 91, 105, 110 pancreatic juices [G] 111 parasympathetic system 54 parathyroid gland 91 parietal bone 26 pathogen 88 pectoralis major 34 pelvis 24 penis 94, 115, 118

Terms in CAPITAL LETTERS and page numbers in boldface type refer to a main entry. The symbol [G] indicates a Glossary listing.

127

Index

quadriceps 35 rectum 109 red blood cell 77 red bone marrow 20, 77 reflex 55 reproduction 114, 116, 118, 120, 122 RESPIRATORY SYSTEM 98, 100 retina 61, 62 rib 29 ribcage 29 ribonucleic acid 13 ribosome 9, 13 risorius 39 RNA 13 root of the tooth 107

sclera 60 scrotum 114 sebaceous gland 19 sebum [G] 18 semen 114 semicircular canals 67 senses 58, 60, 62, 64, 66, 68, 70, 72 sexual relations 118 shaft 20 short bone 26 shoulder 32 shoulder blade 26 SIGHT 60, 62 sinus [G] 100 SKELETAL MUSCLES 34, 36, 40 SKELETON 24 SKIN 18, 58 skull 27 small intestine 105, 109 SMELL 72 sneezing 101 somatosensory cortex 59 SPEECH 102 spermatozoid 115, 119 sphenoid bone 27 sphincter 39 spinal bulb 48 spinal cord 46 spinal ganglion 47 spinal nerves 46, 53 SPINE 28 spinothalamic tract 59 spleen 87 spongy bone tissue 20 stem cell [G] 77 sternocleidomastoid muscle 38 sternum 29 STOMACH 105, 108 swallowing 104 sweat gland 18 sympathetic ganglion 46, 54 sympathetic system 54 synapse 45 synaptic cleft 45 synovial fluid 32 systemic bloodstream 79 systole 84

S

T

saccule 67 sacrum 28 saliva 68, 105 salivary gland 68 sarcomere 36 sartorius 34 scar 19 Schwann cell 45 sciatic nerve 53

tactile receptor 18, 55 talus 26, 31 target cell 91 tarsus 31 TASTE 68, 70 taste bud 71 tear 61 TEETH 106 temporal bone 27

perichondrium 22 periosteum 20, 22 PERIPHERAL NERVOUS SYSTEM 52 peritoneum 108 phagocytic cell 88 phagocytosis 88 phalanges 30 pharynx 98 phonation 102 photoreceptor [G] 60 PITUITARY GLAND 90, 92 placenta [G] 122 plasma 76 plasmocyte 89 pleura 99 precapillary sphincter 80 premolar 106 progesterone 119 prostate 114 protein synthesis 9, 12 pubis 116 pulmonary alveoli 101 pulmonary artery 79 pulmonary bloodstream 79 pulmonary vein 79 pulp 107 pulse 79 pupil 60, 63 pus 88 pylorus 108

QR

temporal muscle 39 tendon 35, 36, 41 testicle 115 thalamus 50 thumb 30, 41 thymine 11 thyroid gland 91, 92 TISSUES 14 toe 31 tongue 69, 70 tonsils 68, 87 TOUCH 58 trachea 98 trapezius 35 triceps 35 trophoblast 120 tympanum 65, 66

U umbilical cord 121, 123 uracil 13 ureter 94 urethra 94 URINARY SYSTEM 94 urine 95 uterus 116, 120 utricle 67

V vagina 116, 118 valve 80, 83 VEINS 78, 80 vena cava 78 vertebra 26, 28 vertebral foramen 29 vestibular nerve 65 visual cortex 63 vitreous body 60 vocal folds 103 vulva 116

W white blood cell 76, 88 white matter 46, 50 wrist 30, 32

YZ yellow bone marrow 21 zona pellucida 119 zygomatic muscle 38 zygote 120

Terms in CAPITAL LETTERS and page numbers in boldface type refer to a main entry. The symbol [G] indicates a Glossary listing.

128