191

Claude Bernard, a French physiol-

ogist, realized in 1859 that while

animals, such as humans, live in

an external environment, the cells

of the body are surrounded by an

internal environment composed

of tissue fluid, which is serviced

by blood. Later, Walter Cannon,

an American physiologist, intro-

duced the term homeostasis to

emphasize the dynamic equilib-

rium that keeps the composition

of tissue fluid, and various vital

signs like blood pressure and

body temperature, within normal

limits. The systems of the body

discussed in this part either add

substances to and/or remove sub-

stances from the blood. In this way,

they contribute to homeostasis.

Maintenance of the Human Body

11 Human Organization 193Humans are organized. A limited numberof tissues make up organs that formsystems. Organ systems work together tomaintain homeostasis.

13 Cardiovascular System239

One of the cardiovascular system’s mainfunctions is to transport nutrients to andcarry wastes away from the body’s cells.

15 Respiratory System 283The respiratory system exchanges gasesbetween the external environment andblood. Ridding the body of carbondioxide helps maintain blood pH.

12 Digestive System andNutrition 213

The digestive system breaks down foodand absorbs nutrients into thebloodstream.

14 Lymphatic System andImmunity 263

The lymphatic system absorbs fat, drainstissue fluid, and helps provide immunityagainst foreign invaders.

16 Urinary System andExcretion 303

The urinary system maintains the volumeand chemical composition of blood withinnormal limits.

Pharmacist filling prescriptions.

Physicians assistant preparing a cast.

Registered nurse taking blood pressure.

Careers in Human Anatomy & Physiology

192

Pharmacists measure, count, mix, and dispense drugs and medi-cines prescribed by physicians, physician assistants, and dentists,among others. Pharmacists must understand the use, composition,and effects of drugs, and how they are tested for purity andstrength.

Respiratory therapists evaluate, treat, and care for patients withbreathing disorders. They treat many types of patients from pre-mature infants whose lungs are not fully developed to elderly peo-ple whose lungs are diseased. Therapists run ventilators and cancheck on them for mechanical problems.

Home health aides help elderly, disabled, or ill persons to live intheir own homes instead of a health facility. They assist thesepatients with their daily routines, check their vital signs, overseetheir exercise needs, and assist with medication routines.

Physician assistants (P.A.s) are formally trained to perform manyof the routine but time-consuming tasks physicians usually do.They take medical histories, perform physical examinations, orderlaboratory tests and X rays, make preliminary diagnoses, and giveinoculations. They also treat minor injuries by suturing, splinting,and casting.

Dental hygienists provide preventive dental care and teachpatients how to practice good oral hygiene. They remove calculus,stain, and plaque from above and below the gumline; apply caries-preventive agents such as fluorides and pit and fissure sealants;and expose and develop dental X rays.

Registered nurses (R.N.s) care for the sick and injured and helppeople stay well. They observe, assess, and record symptoms, reac-tions, and progress; assist physicians during treatments and exam-inations; and administer medications. Licensed practical nurses(L.P.N.s) provide basic bedside care.

Dietitians and nutritionists plan nutrition programs and super-vise the preparation and serving of meals in institutions such ashospitals and schools. Working in such places as public health clin-ics, home health agencies, and health maintenance organizations,dietitians and nutritionists determine individual needs, establishnutritional care plans, instructing both individuals and families.

193

HumanOrganization

Chapter Concepts

11.1 Types of Tissues• Animal tissues can be categorized into four major

types: epithelial, connective, muscular, andnervous tissues. 194

• Epithelial tissues line body cavities and coversurfaces. 194

• Connective tissues protect, support, and bindother tissues. 196

• Muscular tissues make body parts move. 199• Nervous tissues coordinate the activities of the

other tissues and body parts. 200

11.2 Body Cavities and Body Membranes• The internal organs occur within cavities lined by

membranes that also cover the organs themselves.201

11.3 Organ Systems• Organs are grouped into organ systems, each of

which has specialized functions. 202

11.4 Skin as an Organ System• The skin contains various tissues and has

accessory organs. It is sometimes called theintegumentary system. 204

11.5 Homeostasis• Humans have a marked ability to maintain a

relatively stable internal environment. All organsystems contribute to homeostasis. 208

The cilia of epithelial cells lining the trachea sweep particlestoward the throat. This action helps prevent impurities from reach-ing the lower air passages and helps prevent respiratory infections.

11.1 Types of TissuesA tissue is composed of similarly specialized cells that per-form a common function in the body. The tissues of thehuman body can be categorized into four major types:epithelial tissue, which covers body surfaces and lines bodycavities; connective tissue, which binds and supports bodyparts; muscular tissue, which moves body parts; and nervoustissue, which receives stimuli and conducts impulses fromone body part to another.

Cancers are classified according to the type of tissuefrom which they arise. Carcinomas, the most common type,are cancers of epithelial tissues; sarcomas are cancers arisingin muscle or connective tissue (especially bone or cartilage);leukemias are cancers of the blood; and lymphomas are can-cers of lymphoid tissue. The chance of a cancer developingin a particular tissue shows a positive correlation to the rateof cell division; new blood cells arise at a rate of 2,500,000 cells per second, and epithelial cells also repro-duce at a high rate.

Epithelial TissueEpithelial tissue, also called epithelium, consists of tightlypacked cells that form a continuous layer or sheet lining theentire body surface and most of the body’s inner cavities. Onthe external surface, it protects the body from injury, dryingout, and possible pathogen (virus and bacterium) invasion.On internal surfaces, epithelial tissue may be specialized for

other functions in addition to protection. For example,epithelial tissue secretes mucus along the digestive tract andsweeps up impurities from the lungs by means of cilia (sing.,cilium). It efficiently absorbs molecules from kidney tubulesand from the intestine because of minute cellular extensionscalled microvilli.

There are three types of epithelial tissue (Fig. 11.1).Squamous epithelium is composed of flattened cells and isfound lining the lungs and blood vessels. Cuboidal epithe-lium contains cube-shaped cells and is found lining the kid-ney tubules. Columnar epithelium has cells resembling rec-tangular pillars or columns, and nuclei are usually locatednear the bottom of each cell. This epithelium is found liningthe digestive tract. Ciliated columnar epithelium is foundlining the oviducts, where it propels the egg toward theuterus or womb.

An epithelium can be simple or stratified. Simple meansthe tissue has a single layer of cells, and stratified means thatthe tissue has layers of cells piled one on top of the other. Thewalls of the smallest blood vessels, called capillaries, are com-posed of a single layer of epithelial cells. The nose, mouth,esophagus, anal canal, and vagina are all lined by stratifiedsquamous epithelium. As we shall see, the outer layer of skinis also stratified squamous epithelium, but the cells havebeen reinforced by keratin, a protein that provides strength.

Pseudostratified epithelium appears to be layered; how-ever, true layers do not exist because each cell touches thebase line. The lining of the windpipe, or trachea, is calledpseudostratified ciliated columnar epithelium. A secreted cover-ing of mucus traps foreign particles, and the upward motionof the cilia carries the mucus to the back of the throat, whereit may either be swallowed or expectorated. Smoking cancause a change in mucous secretion and inhibit ciliaryaction, and the result is a chronic inflammatory conditioncalled bronchitis.

A so-called basement membrane often joins an epitheli-um to underlying connective tissue. We now know that thebasement membrane is glycoprotein, reinforced by fibersthat are supplied by connective tissue.

An epithelium sometimes secretes a product, in whichcase it is described as glandular. A gland can be a singleepithelial cell, as in the case of mucus-secreting goblet cellsfound within the columnar epithelium lining the digestivetract, or a gland can contain many cells. Glands that secretetheir product into ducts are called exocrine glands, and thosethat secrete their product directly into the bloodstream arecalled endocrine glands. The pancreas is both an exocrinegland, because it secretes digestive juices into the smallintestine via ducts, and an endocrine gland, because itsecretes insulin into the bloodstream.

Epithelial tissue is named according to the shapeof the cell. These tightly packed protective cellscan occur in more than one layer, and the cellslining a cavity can be ciliated and/or glandular.

194 Part 3 Maintenance of the Human Body 11-02

S ally busily hammers nails into the roof of a new build-ing. And except for the sweat dripping into her eyes,she hardly notices the heat of this midsummer morning

even though it is about 100°F. She had skipped breakfastand then gulped down three jelly doughnuts and a cup ofcoffee the construction foreman had provided for the car-penters. At lunchtime, Sally heads for the local diner, wherethe temperature is a chilly 68°F. Remembering her diet, sheorders a salad without dressing and a diet cola. Then shegoes back to work.

The remarkable thing is that although Sally wasexposed to environmental temperatures of both 68°F and100°F, her body kept a relatively constant temperature ofabout 98.6°F. And even though she loaded her blood withsugar after she came to work and deprived it of sugar atlunch, her body maintained a relatively constant blood sug-ar level of about 100 mg/100 ml blood. And her bloodremained at a pH of about 7.4 even though she consumedcarbonic acid in the diet cola. As we will find out in this chap-ter, tissues and organs work together to maintain relativelyconstant levels of internal temperature and blood chemistry,regardless of whether the external temperature is hot orcold, or meals are high or low in sugar or acids. This chapterdiscusses the basic organization of the body, revealing thetruly amazing way the body maintains normal conditionswithout us even thinking about it.

Chapter 11 Human Organization 195

Figure 11.1 Epithelial tissue.The three types of epithelial tissue—squamous, cuboidal, and columnar—are named for the shape of their cells. They all have a protectivefunction, as well as the other functions noted.

11-03

Simple squamous epithelium• has flattened cells.• occurs in air sacs of lungs,

walls of capillaries, and lining of blood vessels.

• functions in protection, diffusion, filtration.

Cuboidal epithelium• has cube-shaped cells.• occurs in lining of kidney tubules

and on surfaces of ovaries.• functions in protection,

secretion, absorption.

Columnar epithelium• has rectangle-shaped cells.• occurs in lining of intestine and uterus.• functions in protection,

secretion, absorption.

Pseudostratified ciliatedcolumnar epithelium• appears to be layered.• occurs in lining of

respiratory tract.• functions in protection,

secretion, movement of mucus.

goblet cell

goblet cell

microvilli

nucleus

basement membrane

basement membrane

basement membrane

20 µm 20 µm

20 µm 20 µm

Junctions Between CellsThe cells of a tissue can function in a coordinated mannerwhen the plasma membranes of adjoining cells interact. Thejunctions that occur between cells help cells function as a tis-sue (Fig. 11.2). A tight junction forms an impermeable barri-er because adjacent plasma membrane proteins actuallyjoin, producing a zipperlike fastening. In the intestine, thegastric juices stay out of the body, and in the kidneys, theurine stays within kidney tubules because epithelial cells arejoined by tight junctions.

A gap junction forms when two adjacent plasma mem-brane channels join. This lends strength, but it also allowsions, sugars, and small molecules to pass between the twocells. Gap junctions in heart and smooth muscle ensure syn-chronized contraction. In an adhesion junction (desmo-some), the adjacent plasma membranes do not touch but areheld together by intercellular filaments firmly attached tobuttonlike thickenings. In some organs—like the heart,stomach, and bladder, where tissues get stretched—adhesion junctions hold the cells together.

Connective TissueConnective tissue binds organs together, provides supportand protection, fills spaces, produces blood cells, and storesfat. As a rule, connective tissue cells are widely separated bya matrix, consisting of a noncellular material that varies inconsistency from solid to semifluid to fluid. The matrix mayhave fibers of three possible types: Collagen (white) fiberscontain collagen, a protein that gives them flexibility andstrength. Reticular fibers are very thin collagen fibers thatare highly branched and form delicate supporting networks.Elastic (yellow) fibers contain elastin, a protein that is not asstrong as collagen but is more elastic.

Loose Fibrous and Dense Fibrous TissuesBoth loose fibrous and dense fibrous connective tissues havecells called fibroblasts that are located some distance fromone another and are separated by a jellylike matrix contain-ing white collagen fibers and yellow elastic fibers.

Loose fibrous connective tissue supports epitheliumand also many internal organs (Fig. 11.3a). Its presence inlungs, arteries, and the urinary bladder allows theseorgans to expand. It forms a protective covering enclosingmany internal organs, such as muscles, blood vessels, andnerves.

Dense fibrous connective tissue contains many colla-gen fibers that are packed together. This type of tissue hasmore specific functions than does loose connective tissue.For example, dense fibrous connective tissue is found in ten-dons, which connect muscles to bones, and in ligaments,which connect bones to other bones at joints.

Adipose Tissue and Reticular Connective TissueIn adipose tissue (Fig. 11.3b), the fibroblasts enlarge andstore fat. The body uses this stored fat for energy, insulation,and organ protection. Adipose tissue is found beneath theskin, around the kidneys, and on the surface of the heart.Reticular connective tissue, also called lymphoid tissue, ispresent in lymph nodes, the spleen, and the bone marrow.These organs are a part of the immune system because theystore and/or produce white blood cells, particularly lym-phocytes. All types of blood cells are produced in red bonemarrow.

CartilageIn cartilage, the cells lie in small chambers called lacunae(sing., lacuna), separated by a matrix that is solid yet flexi-ble. Unfortunately, because this tissue lacks a direct bloodsupply, it heals very slowly. There are three types of carti-lage, distinguished by the type of fiber in the matrix.

Hyaline cartilage (Fig. 11.3c), the most common type ofcartilage, contains only very fine collagen fibers. The matrixhas a white, translucent appearance. Hyaline cartilage isfound in the nose and at the ends of the long bones and the

196 Part 3 Maintenance of the Human Body 11-04

Figure 11.2 Junctions between epithelial cells.Epithelial tissue cells are held tightly together by (a) tight junctions;(b) gap junctions that allow materials to pass from cell to cell; and(c) adhesion junctions that allow tissues to stretch.

intercellularspace

filaments ofcytoskeleton

cytoplasmicplaque

intercellular filaments

plasma membranes

c. Adhesion junction

plasma membranes

membranechannels

intercellularspace

b. Gap junction

plasmamembranes

intercellular space

a. Tight junction

tight junctionproteins

ribs, and it forms rings in the walls of respiratory passages.The fetal skeleton also is made of this type of cartilage. Later,the cartilaginous fetal skeleton is replaced by bone.

Elastic cartilage has more elastic fibers than hyaline car-tilage. For this reason, it is more flexible and is found, forexample, in the framework of the outer ear.

Fibrocartilage has a matrix containing strong collagenfibers. Fibrocartilage is found in structures that withstandtension and pressure, such as the pads between the verte-brae in the backbone and the wedges found in the knee joint.

BoneBone is the most rigid connective tissue. It consists of anextremely hard matrix of inorganic salts, chiefly calciumsalts, deposited around protein fibers, especially collagenfibers. The inorganic salts give bone rigidity, and the proteinfibers provide elasticity and strength, much as steel rods doin reinforced concrete.

Compact bone makes up the shaft of a long bone (Fig.11.3d). It consists of cylindrical structural units called

osteons (Haversian systems). The central canal of eachosteon is surrounded by rings of hard matrix. Bone cells,called osteocytes, are located in spaces called lacunaebetween the rings of matrix. Blood vessels in the centralcanal carry nutrients that allow bone to renew itself. Thenutrients can reach all of the cells because canaliculi (minutecanals) containing thin processes of the osteocytes connectthe cells with one another and with the central canals.

The ends of a long bone contain spongy bone, which hasan entirely different structure. Spongy bone containsnumerous bony bars and plates, separated by irregularspaces. Although lighter than compact bone, spongy bonestill is designed for strength. Just as braces are used for sup-port in buildings, the solid portions of spongy bone followlines of stress.

Connective tissues, which bind and support bodyparts, differ according to the type of matrix and theabundance of fibers in the matrix.

Chapter 11 Human Organization 19711-05

Figure 11.3 Connective tissue examples.a. In loose connective tissue, cells called fibroblasts are separated by a jellylike matrix, which contains both collagen and elastic fibers.b. Adipose tissue cells have nuclei (arrow) pushed to one side because the cells are filled with fat. c. In hyaline cartilage, the flexible matrix has awhite, translucent appearance. d. In compact bone, the hard matrix contains calcium salts. Concentric rings of osteocytes in lacunae form anelongated cylinder called an osteon (Haversian system). An osteon has a central canal that contains blood vessels and nerve fibers.

central canal50 µm

elasticfiber

collagenfiber

fibroblast

a. Loose fibrous connective tissue• has space between components.• occurs beneath skin and most epithelial layers.• functions in support and binds organs.

b. Adipose tissue• cells are filled with fat.• occurs beneath skin, around organs and heart.• functions in insulation, stores fat.

osteon

canaliculi

osteocytewithin a lacuna

d. Compact bone• has cells in concentric rings.• occurs in bones of skeleton.• functions in support and protection.

matrix

cell withina lacuna

c. Hyaline cartilage• has cells in lacunae.• occurs in nose and walls of respiratory passages; at ends of bones including ribs.• functions in support and protection.

50 µm50 µm

50 µm

BloodThe functions of blood include transporting molecules, reg-ulating the tissues, and protecting the body. Blood trans-ports nutrients and oxygen to cells and removes carbondioxide and other wastes. It helps distribute heat and alsoplays a role in fluid, ion, and pH balance. Various compo-nents of blood, as discussed below, help protect us from dis-ease, and its ability to clot prevents fluid loss.

If blood is transferred from a person’s vein to a test tubeand prevented from clotting, it separates into two layers(Fig. 11.4). The upper liquid layer, called plasma, representsabout 55% of the volume of whole blood and contains a vari-ety of inorganic and organic substances dissolved or sus-pended in water (Table 11.1). The lower layer consists of redblood cells (erythrocytes), white blood cells (leukocytes),and blood platelets (thrombocytes). Collectively, these arecalled the formed elements and represent about 45% of thevolume of whole blood. Formed elements are manufacturedin the red bone marrow of the skull, ribs, vertebrae, andends of long bones.

The red blood cells are small, biconcave, disk-shapedcells without nuclei. The presence of the red pigmenthemoglobin makes the cells red, and in turn, makes theblood red. Hemoglobin is composed of four units; each iscomposed of the protein globin and a complex iron-con-taining structure called heme. The iron forms a loose asso-ciation with oxygen, and in this way red blood cells trans-port oxygen.

White blood cells may be distinguished from redblood cells by the fact that they are usually larger, have anucleus, and without staining would appear to be translu-cent. White blood cells characteristically appear bluishbecause they have been stained that color. White bloodcells, which fight infection, function primarily in two ways.Some white blood cells are phagocytic and engulf infec-tious pathogens, while other white blood cells produceantibodies, molecules that combine with foreign substancesto inactivate them.

Platelets are not complete cells; rather, they are frag-ments of giant cells present only in bone marrow. When ablood vessel is damaged, platelets form a plug that seals thevessel and along with injured tissues release molecules thathelp the clotting process.

Blood is unlike other types of connective tissue in thatthe matrix (i.e., plasma) is not made by the cells. Some peo-ple do not classify blood as connective tissue; instead, theysuggest a separate tissue category for blood called vasculartissue.

Blood is a connective tissue in which the matrix isplasma.

198 Part 3 Maintenance of the Human Body 11-06

Water (92% of Total)

Solutes (8% of Total)

Inorganic ions (salts) Na�, Ca2�, K�, Mg2�; Cl�, HCO3�,PO4

3�, SO42�

Gases O2, CO2

Plasma proteins Albumin, globulins, fibrinogen

Organic nutrients Glucose, fats, phospholipids,amino acids, etc.

Nitrogenous waste Urea, ammonia, uric acidproducts

Regulatory substances Hormones, enzymes

Table 11.1 Blood Plasma

Figure 11.4 Blood, a fluid tissue.a. In a test tube, a blood sample separates into its two components:blood cells and plasma. b. Microscopic examination of a bloodsmear shows that there are red blood cells, white blood cells, andplatelets. Platelets are fragments of a cell. Red blood cells transportoxygen, white blood cells fight infections, and platelets are involvedin initiating blood clotting.

a. Blood sample

b. Blood smear

plasma

blood cells

white bloodcells

platelets

red blood cells

Muscular TissueMuscular (contractile) tissue is composed of cells that arecalled muscle fibers. Muscle fibers contain actin filamentsand myosin filaments, whose interaction accounts for move-ment. There are three types of vertebrate muscles: skeletal,smooth, and cardiac.

Skeletal muscle, also called voluntary muscle (Fig.11.5a), is attached by tendons to the bones of the skeleton,and when it contracts, body parts move. Contraction ofskeletal muscle is under voluntary control and occurs fasterthan in the other muscle types. Skeletal muscle fibers arecylindrical and quite long—sometimes they run the lengthof the muscle. They arise during development when severalcells fuse, resulting in one fiber with multiple nuclei. Thenuclei are located at the periphery of the cell, just inside theplasma membrane. The fibers have alternating light anddark bands that give them a striated appearance. Thesebands are due to the placement of actin filaments andmyosin filaments in the cell.

Smooth (visceral) muscle is so named because the cellslack striations. The spindle-shaped cells form layers inwhich the thick middle portion of one cell is opposite thethin ends of adjacent cells. Consequently, the nuclei form anirregular pattern in the tissue (Fig. 11.5b). Smooth muscle isnot under voluntary control and therefore is said to beinvoluntary. Smooth muscle, found in the walls of viscera(intestine, stomach, and other internal organs) and bloodvessels, contracts more slowly than skeletal muscle but canremain contracted for a longer time. When the smooth mus-cle of the intestine contracts, food moves along its lumen(central cavity). When the smooth muscle of the blood ves-sels contracts, blood vessels constrict, helping to raise bloodpressure.

Cardiac muscle (Fig. 11.5c) is found only in the walls ofthe heart. Its contraction pumps blood and accounts for theheartbeat. Cardiac muscle combines features of both smoothmuscle and skeletal muscle. It has striations like skeletalmuscle, but the contraction of the heart is involuntary for themost part. Cardiac muscle cells also differ from skeletalmuscle cells in that they have a single, centrally placednucleus. The cells are branched and seemingly fused onewith the other, and the heart appears to be composed of onelarge interconnecting mass of muscle cells. Actually, cardiacmuscle cells are separate and individual, but they are boundend to end at intercalated disks, areas where folded plasmamembranes between two cells contain desmosomes and gapjunctions.

All muscular tissue contains actin filaments andmyosin filaments; these form a striated pattern inskeletal and cardiac muscle, but not in smoothmuscle.

Chapter 11 Human Organization 19911-07

Figure 11.5 Muscular tissue.a. Skeletal muscle is voluntary and striated. b. Smooth muscle is invol-untary and nonstriated. c. Cardiac muscle is involuntary and striated.Cardiac muscle cells branch and fit together at intercalated disks.

nucleus

nucleus

nucleus

Cardiac muscle • has branching striated cells, each with a single nucleus.• occurs in the wall of the heart.• functions in the pumping of blood.• involuntary.

a.

striation

Skeletal muscle • has striated cells with multiple nuclei.• usually attached to skeleton.• functions in voluntary movement.

intercalateddisk

smoothmusclecell

b.

c.

Smooth muscle • has spindle-shaped cells, each with a single nucleus.• occurs in walls of hollow internal organs.• functions in movement of substances in lumens of body.• no cross striations, involuntary.

20 µm

20 µm

12 µm

Nervous TissueNervous tissue, which contains nerve cells called neurons,is present in the brain and spinal cord. A neuron is a special-ized cell that has three parts: dendrites, cell body, and anaxon (Fig. 11.6). A dendrite is a process that conducts signalstoward the cell body. The cell body contains the major con-centration of the cytoplasm and the nucleus of the neuron.An axon is a process that typically conducts nerve impulsesaway from the cell body. Axons can be quite long, and out-side the brain and the spinal cord, long fibers, bound by con-nective tissue, form nerves.

The nervous system has just three functions: sensoryinput, integration of data, and motor output. Nerves con-duct impulses from sensory receptors to the spinal cord andthe brain where integration occurs. The phenomenon calledsensation occurs only in the brain, however. Nerves alsoconduct nerve impulses away from the spinal cord andbrain to the muscles and glands, causing them to contractand secrete, respectively. In this way, a coordinated responseto the stimulus is achieved.

In addition to neurons, nervous tissue contains neu-roglial cells.

Neuroglial CellsThere are several different types of neuroglial cells in the brain(Fig. 11.6), and much research is currently being conducted todetermine how much “glial” cells contribute to the function-ing of the brain. Neuroglial cells outnumber neurons nine toone and take up more than half the volume of the brain, butuntil recently, they were thought to merely support and nour-ish neurons. Three types of neuroglial cells are oligodendro-cytes, microglial cells, and astrocytes. Oligodendrocytes formmyelin; and microglial cells, in addition to supporting neurons,phagocytize bacterial and cellular debris. Astrocytes providenutrients to neurons and produce a hormone known as glial-derived growth factor, which someday might be used as a curefor Parkinson’s disease and other diseases caused by neurondegeneration. Neuroglial cells don’t have a long process, buteven so, researchers are now beginning to gather evidence thatthey do communicate among themselves and with neurons!

Nerve cells, called neurons, have fibers (processes)called axons and dendrites. Axons are found innerves. Neuroglial cells support and service neurons.

200 Part 3 Maintenance of the Human Body 11-08

Figure 11.6 Neuron and neuroglial cells.Neurons conduct nerve impulses. Neuroglial cells, which support and service neurons, have various functions: microglial cells are phagocytesthat clean up debris. Astrocytes lie between neurons and a capillary; therefore, substances entering neurons from the blood must first passthrough astrocytes. Oligodendrocytes form the myelin sheaths around fibers in the brain and spinal cord.

footprocesses

axonAstrocyte

Neuron

Oligodendrocyte

nucleus

cell body

axon

dendrite

myelin sheath

Microglial cells

11.2 Body Cavities and BodyMembranesThe internal organs are located within specific body cavities(Fig. 11.7). During human development, there is a largeventral cavity called a coelom, which becomes divided intothe thoracic (chest) and abdominal cavities. Membranesdivide the thoracic cavity into the pleural cavities, contain-ing the right and left lungs, and the pericardial cavity, con-taining the heart. The thoracic cavity is separated from theabdominal cavity by a horizontal muscle called thediaphragm. The stomach, liver, spleen, gallbladder, andmost of the small and large intestines are in the upper por-tion of the abdominal cavity. The lower portion contains therectum, the urinary bladder, the internal reproductiveorgans, and the rest of the large intestine. Males have anexternal extension of the abdominal wall, called the scro-tum, containing the testes.

The dorsal cavity also has two parts: the cranial cavitywithin the skull contains the brain; and the vertebral col-umn, formed by the vertebrae, contains the spinal cord.

Body MembranesIn this context, we are using the term membrane to refer to a thinlining or covering composed of an epithelium overlying a looseconnective tissue layer. Body membranes line cavities andinternal spaces of organs and tubes that open to the outside.

Mucous membranes line the tubes of the digestive, re-spiratory, urinary, and reproductive systems. The epitheliumof this membrane contains goblet cells that secrete mucus.This mucus ordinarily protects the body from invasion bybacteria and viruses; hence, more mucus is secreted andexpelled when a person has a cold and has to blow her/hisnose. In addition, mucus usually protects the walls of thestomach and small intestine from digestive juices, but thisprotection breaks down when a person develops an ulcer.

Serous membranes line the thoracic and abdominalcavities and the organs that they contain. They secrete awatery fluid that keeps the membranes lubricated. Serousmembranes support the internal organs and compartmen-talize the large thoracic and abdominal cavities. This helpsto hinder the spread of any infection.

The pleural membranes are serous membranes that linethe pleural cavity and lungs. Pleurisy is a well-known infec-tion of these membranes. The peritoneum lines the abdomi-nal cavity and its organs. In between the organs, there is adouble layer of peritoneum called mesentery. Peritonitis, alife-threatening infection of the peritoneum, is likely if aninflamed appendix bursts before it is removed.

Synovial membranes line freely movable joint cavities.They secrete synovial fluid into the joint cavity; this fluidlubricates the ends of the bones so that they can move freely.

In rheumatoid arthritis, the synovial membrane becomesinflamed and grows thicker, restricting movement.

The meninges are membranes found within the dorsalcavity. They are composed only of connective tissue andserve as a protective covering for the brain and spinal cord.Meningitis is a life-threatening infection of the meninges.

Chapter 11 Human Organization 20111-09

Figure 11.7 Mammalian body cavities.a. Side view. There is a dorsal cavity, which contains the cranialcavity and the vertebral canal. The brain is in the cranial cavity, andthe spinal cord is in the vertebral canal. There is a well-developedventral cavity, which is divided by the diaphragm into the thoraciccavity and the abdominal cavity. The heart and lungs are in thethoracic cavity, and most other internal organs are in the abdominalcavity. b. Frontal view of the thoracic cavity.

a.

b.

cranialcavity

brain

dorsalcavity

vertebralcanal

diaphragm

spinal cord

vertebra

thoraciccavity

abdominalcavity

ventralcavity

rib

thoraciccavity

pleural cavitycontains a lung

pericardial cavitycontains heart

diaphragm

abdominalcavity

11.3 Organ SystemsThe body contains a number of organ systems (Fig. 11.8). Theskin, which is sometimes called the integumentary system,is discussed in this chapter. The other organ systems con-tribute to either maintenance of the human body, integrationand control of the human body, or continuance of the species.

Maintenance of the BodyThe internal environment of the body consists of the bloodwithin the blood vessels and the tissue fluid that surroundsthe cells. Five systems add substances to and remove sub-stances from the blood: the digestive, cardiovascular, lym-phatic, respiratory, and urinary systems.

The digestive system consists of the mouth, esophagus,stomach, small intestine, and large intestine (colon) alongwith the associated organs: teeth, tongue, salivary glands,liver, gallbladder, and pancreas. This system receives foodand digests it into nutrient molecules, which can enter thecells of the body.

The cardiovascular system consists of the heart andblood vessels that carry blood through the body. Bloodtransports nutrients and oxygen to the cells, and removes

their waste molecules that are to be excreted from the body.Blood also contains cells produced by the lymphatic system.

The lymphatic system consists of lymphatic vessels,lymph, nodes, and other lymphoid organs. This system pro-tects the body from disease by purifying lymph and storinglymphocytes, the white blood cells that produce antibodies.Lymphatic vessels absorb fat from the digestive system andcollect excess tissue fluid, which is returned to the cardio-vascular system.

The respiratory system consists of the lungs and thetubes that take air to and from the lungs. The respiratorysystem brings oxygen into the lungs and takes carbon diox-ide out of the lungs.

The urinary system contains the kidneys and the uri-nary bladder. This system rids the body of nitrogenouswastes and helps regulate the fluid level and chemical con-tent of the blood.

The digestive system, cardiovascular system,lymphatic system, respiratory system, and theurinary system all perform specific processing andtransporting functions to maintain the normalconditions of the body.

202 Part 3 Maintenance of the Human Body 11-10

Figure 11.8 Organ systems of the body.

Digestive system Breakdown and absorption of food materials

Cardiovascular systemTransport of nutrientsto body cells, and transport of wastes from cells

Lymphatic systemImmunity; absorptionof fats; drainage of tissue fluid

Respiratory systemGaseous exchangebetween external environment and blood

Integration and Control of the BodyThe nervous system consists of the brain, spinal cord, andassociated nerves. The nerves conduct nerve impulses fromsensory receptors to the brain and spinal cord. They alsoconduct nerve impulses from the brain and spinal cord tothe muscles and glands, allowing us to respond to bothexternal and internal stimuli.

The musculoskeletal system, consisting of the bonesand muscles of the body, protects other body parts. Forexample, the skull forms a protective encasement for thebrain, as does the rib cage for the heart and lungs. The skele-ton, as a whole, serves as a place of attachment for the skele-tal muscles. Contraction of muscles accounts for movementof the body and also body parts.

The endocrine system consists of the hormonal glandsthat secrete chemicals that serve as messengers betweenbody parts. Homeostasis is a dynamic equilibrium of theinternal environment. Both the nervous and endocrine sys-tems help maintain homeostasis by coordinating and regu-lating the functions of the body’s other systems. The

endocrine system also helps maintain the proper function-ing of male and female reproductive organs.

The nervous and endocrine systems coordinateand regulate the activities of the body’s othersystems, including the musculoskeletal system.

Continuance of the SpeciesThe reproductive system involves different organs in themale and female. The male reproductive system consists ofthe testes, other glands, and various ducts that conductsemen to and through the penis. The female reproductivesystem consists of the ovaries, oviducts, uterus, vagina, andexternal genitals.

The reproductive system in males and in femalescarries out those functions that give humans theability to reproduce.

Chapter 11 Human Organization 20311-11

Figure 11.8 Organ systems of the body.

Excretory systemFiltration of blood;maintenance of volume and chemical composition of the blood

Nervous systemRegulation of all body activities;learning and memory

Musculoskeletal systemInternal support and protection; bodymovement

Endocrine systemRegulation of bodyactivities; maintenance ofreproductive system

11.4 Skin as an Organ SystemThe outer covering of the body, called skin, can be used as anexample of an organ system (it is sometimes called the integu-mentary system) because it contains accessory structures suchas nails, hair, and glands (Fig. 11.9). Skin covers the body, pro-tecting underlying tissues from physical trauma, pathogeninvasion, and water loss. Skin helps to regulate body temper-ature, and because it contains sensory receptors, skin also

204 Part 3 Maintenance of the Human Body 11-12

helps us to be aware of our surroundings and to communicatewith others by touch. The skin even synthesizes certain chem-icals such as vitamin D that affect the rest of the body.

Regions of the SkinThe skin has two regions: the epidermis and the dermis. Asubcutaneous layer is found between the skin and anyunderlying structures, such as muscle or bone.

Figure 11.9 Human skin anatomy.Skin consists of two regions, the epidermis and dermis. A subcutaneous layer lies below the dermis.

dermis

epidermis

subcutaneouslayer

nerve

nerve

artery

vein

adiposetissue

sensory receptor

sensoryreceptor

sensoryreceptor

fat

hairroot

sweatgland

arrectorpili muscle

oilgland

free nerveendings

hair shaft

hair follicle

melanocytes

capillaries

Chapter 11 Human Organization 205

The epidermis of skin is made up of stratified squamousepithelium. New cells derived from basal cells become flat-tened and hardened as they push to the surface. Hardeningoccurs because the cells produce keratin, a waterproof pro-tein. Dandruff occurs when the rate of keratinization is twoor three times the normal rate. A thick layer of dead kera-tinized cells, arranged in spiral and concentric patterns,form fingerprints and footprints. Specialized cells in the epi-dermis called melanocytes produce melanin, the pigmentresponsible for skin color.

The dermis is a region of fibrous connective tissuebeneath the epidermis. The dermis contains collagenous andelastic fibers. The collagenous fibers are flexible but offergreat resistance to overstretching; they prevent the skin frombeing torn. The elastic fibers maintain normal skin tensionbut also stretch to allow movement of underlying musclesand joints. (The number of collagen and elastic fibersdecreases with exposure to the sun, and the skin becomesless supple and is prone to wrinkling.) The dermis also con-tains blood vessels that nourish the skin. When blood rush-es into these vessels, a person blushes, and when blood isminimal in them, a person turns “blue.”

Sensory receptors are specialized nerve endings in thedermis that respond to external stimuli. There are receptorsfor touch, pressure, pain, and temperature. The fingertipscontain the most touch receptors, and these add to our abili-ty to use our fingers for delicate tasks.

The subcutaneous layer, which lies below the dermis, iscomposed of loose connective tissue and adipose tissue,which stores fat. Fat is a stored source of energy for the body.Adipose tissue helps to thermally insulate the body fromeither gaining heat from the outside or losing heat from theinside. A well-developed subcutaneous layer gives the bodya rounded appearance and provides protective paddingagainst external assaults. Excessive development of the sub-cutaneous layer accompanies obesity.

Skin has two regions: the epidermis and the dermis.A subcutaneous layer lies beneath the dermis.

Accessory Structures of the SkinNails, hair, and glands are structures of epidermal origineven though some parts of hair and glands are largely foundin the dermis.

Nails grow from special epithelial cells at the base of thenail in the portion called the nail root. These cells becomekeratinized as they grow out over the nail bed. The visibleportion of the nail is called the nail body. The cuticle is a foldof skin that hides the nail root. The whitish color of the half-moon-shaped base, or lunula, results from the thick layer ofcells in this area (Fig. 11.10).

Hair follicles begin in the dermis and continue throughthe epidermis where the hair shaft extends beyond the skin.Epidermal cells form the root of hair, and their divisioncauses a hair to grow. The cells become keratinized and deadas they are pushed farther from the root. Each hair folliclehas one or more oil (sebaceous) glands, which secretesebum, an oily substance that lubricates the hair within thefollicle and the skin itself. If the sebaceous glands fail to dis-charge, the secretions collect and form “whiteheads” or“blackheads.” The color of blackheads is due to oxidizedsebum. Contraction of the arrector pili muscles attached tohair follicles causes the hairs to “stand on end” and causesgoose bumps to develop.

Sweat (sudoriferous) glands are quite numerous andare present in all regions of skin. A sweat gland begins as acoiled tubule within the dermis, but then it straightens outnear its opening. Some sweat glands open into hair follicles,but most open onto the surface of the skin. Acne is aninflammation of the sebaceous glands that most often occursduring adolescence. Hormonal changes during this timecause the sebaceous glands to become more active.

Regulation of Body TemperatureIf the body temperature starts to rise, the blood vessels dilateso that more blood is brought to the surface of the skin andthe sweat glands become active. Sweat absorbs body heat asit evaporates. If the outer temperature is cool, the blood ves-sels constrict so that less blood is brought to the surface ofthe skin. Whenever the body’s temperature falls below nor-mal, the muscles start to contract, causing shivering, whichproduces heat.

11-13

Figure 11.10 Nail.Cells produced by the nail root become keratinized, forming the nailbody.

nail body

lunula

cuticlenail bed

nail root

The earth’s atmosphere is divided into layers. The troposphereenvelops us as we go about our day-to-day lives. When ozone(O3) is present in the troposphere (called ground-level ozone), itis considered a pollutant because it adversely affects a plant’sability to grow and our ability to breathe oxygen (O2). In thestratosphere, some 50 kilometers above the earth, ozone forms ashield that absorbs much of the ultraviolet (UV) rays of the sunso that fewer rays strike the earth.

UV radiation causes mutations that can lead to skin cancerand can make the lens of the eyes develop cataracts. It also isbelieved to adversely affect the immune system and our abilityto resist infectious diseases. Crop and tree growth is impaired,and UV radiation also kills off small plants (phytoplankton) andtiny shrimplike animals (krill) that sustain oceanic life. Withoutan adequate ozone shield, our health and food sources arethreatened.

Depletion of the ozone layer within the stratosphere in recentyears is, therefore, of serious concern. It became apparent in the1980s that some worldwide depletion of ozone had occurredand that there was a severe depletion of some 40–50% above theAntarctic every spring. A vortex of cold wind (a whirlpool in theatmosphere) circles the pole during the winter months, creatingice crystals where chemical reactions occur that break downozone. Severe depletions of the ozone layer are commonlycalled “ozone holes.” Detection devices now tell us that theozone hole above the Antarctic is about the size of the UnitedStates and growing. Of even greater concern, an ozone hole hasnow appeared above the Arctic as well, and ozone holes couldalso occur within northern and southern latitudes, where manypeople live. Whether or not these holes develop depends on pre-vailing winds, weather conditions, and the type of particles inthe atmosphere. A United Nations Environment Program reportpredicts a 26% rise in cataracts and nonmelanoma skin cancersfor every 10% drop in the ozone level. A 26% increase translatesinto 1.75 million additional cases of cataracts and 300,000 moreskin cancers (see reading next page) every year, worldwide.

The cause of ozone depletion can be traced to the release ofchlorine atoms (Cl) into the stratosphere (Fig. 11A). Chlorineatoms combine with ozone and strip away the oxygen atoms,one by one. One atom of chlorine can destroy up to 100,000 mol-ecules of ozone before settling to the earth’s surface as chlorideyears later. These chlorine atoms come from the breakdown ofchlorofluorocarbons (CFCs), chemicals much in use by humans.The best known CFC is Freon, a heat transfer agent found inrefrigerators and air conditioners. CFCs are also used as clean-ing agents and foaming agents during the production of styro-foam found in coffee cups, egg cartons, insulation, and

Stratospheric Ozone Depletion Threatens the Biosphere

Figure 11A Ozone depletion.CFCs release chlorine atoms that lead to the breakdown of ozone(O3) and the buildup of oxygen (O2) in the stratosphere. Oxygendoes not absorb UV radiation and does not protect the earth.

paddings. Formerly, CFCs were used as propellants in spraycans, but this application is now banned in the United Statesand several European countries.

Most countries of the world have agreed to stop using CFCsby the year 2000. The United States halted production in 1995.Computer projections suggest that an 85% reduction in CFCemissions is needed to stabilize CFC levels in the atmosphere.Otherwise they keep on increasing. Scientists are now searchingfor CFC substitutes that will not release chlorine atoms (norbromine atoms) to harm the ozone shield.

chlorineCl

Same chlorine (Cl)breaks down moreozone (O3) againand again.

troposphere

ozonestratosphere

UV rays

ClO + O2

Cl

O3

O3

O2 + Cl

ClO2+

O2

shades can be seen in the same mole, and they can itch, hurt, orfeel numb. The skin around the mole turns gray, white, or red.Melanoma is most apt to appear in persons who have fair skin,particularly if they have suffered occasional severe sun burns aschildren. The chance of melanoma increases with the number ofmoles a person has. Most moles appear before the age of 14, andtheir appearance is linked to sun exposure. Melanoma rateshave risen since the turn of the century, but the incidence hasdoubled in the last decade. Most often, malignant moles areremoved surgically; if the cancer has spread, chemotherapy andvarious other treatments are also available.

Since the incidence of skin cancer is related to UV exposure,scientists have developed a UV index to determine how power-ful the solar rays are in different U.S. cities. In general, the moresouthern the city, the higher the UV index, and the greater therisk of skin cancer. Regardless of where you live, for every 10%decrease in the ozone layer, the risk of skin cancer rises 13–20%.To prevent the occurrence of skin cancer, observe the following:

• Use a broad-spectrum sunscreen, which protects youfrom both UV-A and UV-B radiation, with an SPF (sunprotection factor) of at least 15. (This means, for example,that if you usually burn after a 20-minute exposure, itwill take 15 times that long before you will burn.)

• Stay out of the sun altogether between the hours of10 A.M. and 3 P.M. This will reduce your annual exposureby as much as 60%. Wear protective clothing. Choosefabrics with a tight weave and wear a wide-brimmed hat.

• Wear sunglasses that have been treated to absorb bothUV-A and UV-B radiation. Otherwise, sunglasses canexpose your eyes to more damage than usual becausepupils dilate in the shade.

• Avoid tanning machines. Although most tanningdevices use high levels of only UV-A, UV-A rays causethe deep layers of the skin to become more vulnerable toUV-B radiation when you are later exposed to the sun.

207

In the nineteenth century and earlier, it was fashionable for Cau-casian women (those who did not labor outdoors) to keep theirskin fair by carrying a parasol when they went out. But early inthis century, some fair-skinned people began to prefer the golden-brown look, and they took up sunbathing as a way to achieve atan. A few hours after exposure to the sun, pain and redness dueto dilation of blood vessels occur. Tanning occurs when melaningranules increase in keratinized cells at the surface of the skin asa way to prevent any further damage by ultraviolet (UV) rays.The sun gives off two types of UV rays: UV-A rays and UV-Brays. UV-A rays penetrate the skin deeply, affect connective tis-sue, and cause the skin to sag and wrinkle. UV-A rays are alsobelieved to increase the effects of the UV-B rays, which are thecancer-causing rays. UV-B rays are more prevalent at midday.

Skin cancer is categorized as either nonmelanoma ormelanoma. Nonmelanoma cancers are of two types. Basal cellcarcinoma, the most common type, begins when UV radiationcauses epidermal basal cells to form a tumor, while at the sametime suppressing the immune system’s ability to detect thetumor. The signs of a tumor are varied. They include an opensore that will not heal, a recurring reddish patch, a smooth, cir-cular growth with a raised edge, a shiny bump, or a pale mark(Fig. 11B). In about 95% of patients the tumor can be excised sur-gically, but recurrence is common.

Squamous cell carcinoma begins in the epidermis proper.Squamous cell carcinoma is five times less common than basalcell carcinoma, but if the tumor is not excised promptly it ismore likely to spread to nearby organs. The death rate fromsquamous cell carcinoma is about 1% of cases. The signs ofsquamous cell carcinoma are the same as for basal cell carcino-ma, except that the former may also show itself as a wart thatbleeds and scabs.

Melanoma that starts in pigmented cells often has the appear-ance of an unusual mole. Unlike a mole that is circular and con-fined, melanoma moles look like spilled ink spots. A variety of

Skin Cancer on the Rise

Figure 11B Skin cancer.a. Basal cell carcinoma occurs when basal cells proliferate abnormally. b. Squamous cell carcinoma arises in epithelial cells derived frombasal cells. c. Malignant melanoma is due to a proliferation of pigmented cells. About one-third develop from pigmented moles.

a. Basal cell carcinoma b. Squamous cell carcinoma c. Melanoma

11.5 HomeostasisHomeostasis means that the internal environment remainswithin normal limits or values, regardless of the conditionsin the external environment. In humans, for example:

1. The blood glucose concentration remains at about 100 mg/100 ml.

2. The pH of blood is always near 7.4.3. Blood pressure in the brachial artery averages near

120/80 mm Hg.4. Body temperature averages around 37°C (98.6°F).

Because body conditions do fluctuate somewhat, homeosta-sis is often called a dynamic equilibrium of normal values.The ability of the body to keep the internal environmentwithin a certain range allows humans to live in a variety ofhabitats, such as the Arctic regions, the deserts, or the tropics.

This internal environment consists of tissue fluid, whichbathes all the cells of the body. Tissue fluid is refreshed whenmolecules such as oxygen and nutrients exit blood andwastes enter blood (Fig. 11.11). Tissue fluid remains constantonly as long as blood composition remains constant.Although we are accustomed to using the word environmentto mean the external environment of the body, it is impor-tant to realize that it is the internal environment of tissuesthat is ultimately responsible for our health and well-being.

The internal environment of the body consists oftissue fluid, which bathes the cells.

Most systems of the body contribute toward maintain-ing a relatively constant internal environment. The cardio-vascular system conducts blood to and away from capillar-ies, the smallest of the blood vessels, whose thin wallspermit exchanges to occur. Blood pressure aids the move-ment of water out of capillaries, and osmotic pressure aidsthe movement of water into capillaries. Blood pressure iscreated by the pumping of the heart, while osmotic pressureis maintained by the protein content of plasma. The formedelements also contribute to homeostasis. Red blood cellstransport oxygen and participate in the transport of carbondioxide. White blood cells fight infection, and platelets par-ticipate in the clotting process. The lymphatic system isaccessory to the circulatory system. Lymphatic capillariescollect excess tissue fluid, and this is returned via lymphaticveins to the circulatory veins.

The digestive system takes in and digests food, provid-ing nutrient molecules that enter blood and replace thenutrients that are constantly being used by the body cells.The respiratory system adds oxygen to and removes carbondioxide from the blood. The chief regulators of blood com-position are the liver and the kidneys. They monitor thechemical composition of plasma (see Table 11.1) and alter itas required. Immediately after glucose enters the blood, it

can be removed by the liver for storage as glycogen. Later,the glycogen can be broken down to replace the glucoseused by the body cells; in this way, the glucose compositionof blood remains constant. The hormone insulin, secreted bythe pancreas, regulates glycogen storage. The liver alsoremoves toxic chemicals, such as ingested alcohol and otherdrugs. The liver makes urea, a nitrogenous end product ofprotein metabolism. Urea and other metabolic waste mole-cules are excreted by the kidneys. Urine formation by thekidneys is extremely critical to the body, not only because itrids the body of unwanted substances, but also because itoffers an opportunity to carefully regulate blood volume,salt balance, and the pH of the blood.

Most systems of the body contribute tohomeostasis, that is, maintaining the dynamicequilibrium of the internal environment.

208 Part 3 Maintenance of the Human Body 11-16

Figure 11.11 Tissue fluid composition.Cells are surrounded by tissue fluid, which is continually refreshedbecause oxygen and nutrient molecules constantly exit, and wastemolecules continually enter the bloodstream as shown.

arteriole

venule

redbloodcell

oxygen andnutrients

wastes

capillary

tissuefluid

tissuecellblood flow

blood flow

Coordination of Organ SystemsThe nervous system and endocrine system are ultimately incontrol of homeostasis. The endocrine system is slower act-ing than the nervous system, which rapidly brings about aparticular response.

Previously, we mentioned that the liver is involved inhomeostasis because it stores glucose as glycogen. But actu-ally there is a hormone produced by an endocrine gland thatregulates storage of glucose by the liver. When the glucosecontent of the blood rises after eating, the pancreas secretesinsulin, a hormone that causes the liver to store glucose asglycogen. Now the glucose level falls, and the pancreas nolonger secretes insulin. This is called control by negativefeedback because the response (low blood glucose) negatesthe original stimulus (high blood glucose). In someinstances, an endocrine gland is sensitive to the blood levelof a hormone whose concentration it regulates. For example,the pituitary gland produces a hormone that stimulates thethyroid gland to secrete its hormone. When the blood levelof this hormone rises to a certain level, the pituitary glandno longer stimulates the thyroid gland.

A negative feedback system can regulate itself becauseit has a sensing device which detects changes in environ-mental conditions. For example, consider the feedbackmechanism that functions to maintain the room tempera-ture of a house. In this feedback system, the thermostat is adevice that is sensitive to room temperature. The furnaceproduces heat, and when the temperature of a room reach-es a certain point, the thermostat signals a switching devicethat turns the furnace off. On the other hand, when the tem-perature falls below that indicated on the thermostat, itsignals the switching device, which turns the furnace onagain.

Figure 11.12a shows that in the body there are sensoryreceptors that fulfill the role of sensing devices. When areceptor is stimulated, it signals a regulatory center that thenturns on an effector. The effector brings about a responsethat negates the original conditions that stimulated thereceptor. In the absence of suitable stimulation, the receptorno longer signals the regulatory center.

Figure 11.12b gives an example involving the nervoussystem. When blood pressure rises, receptors signal a regu-latory center, which then sends out nerve impulses to thearterial walls, causing them to relax, and the blood pressurenow falls. Therefore, the sensory receptors are no longerstimulated, and the system shuts down. Notice that negativefeedback control results in a fluctuation above and below anaverage. Thus, there is a dynamic equilibrium of the internalenvironment.

Positive feedback also occurs on occasion. In theseinstances, certain events increase the likelihood of a particu-

lar response. For example, once the childbirth processbegins, each succeeding event makes it more likely that theprocess will continue until completion.

Homeostasis of internal conditions is a self-regulatory mechanism that usually results in slightfluctuations above and below an average.

Chapter 11 Human Organization 20911-17

Figure 11.12 Negative feedback control.a. A stimulus causes a receptor to signal a regulatory center in thebrain. The regulatory center signals effectors to respond, and theresponse cancels the stimulus. b. For example, when blood pressurerises, special sensory receptors in blood vessels signal a particularcenter in the brain. The brain signals the arteries to relax, and bloodpressure falls.

b.

sensoryreceptors(in aortic andcarotid sinuses)

cancels

arterialwalls relax

regulatorycenter in brain

a.

bloodpressure falls

bloodpressure rises

response

stimulus

sensoryreceptor

cancels

effector

regulatorycenter

210 Part 3 Maintenance of the Human Body 11-18

Summarizing the Concepts

11.1 Types of TissuesHuman tissues are categorized into four groups. Epithelial tissue coversthe body and lines its cavities. The different types of epithelial tissue(squamous, cuboidal, and columnar) can be stratified and have cilia ormicrovilli. Also, columnar cells can be pseudostratified. Epithelial cellssometimes form glands that secrete either into ducts or into blood.

Connective tissues, in which cells are separated by a matrix, oftenbind body parts together. Loose fibrous connective tissue has both col-lagen and elastic fibers. Dense fibrous connective tissue, like that oftendons and ligaments, contains closely packed collagen fibers. In adi-pose tissue, the cells enlarge and store fat. Both cartilage and bone havecells within lacunae, but the matrix for cartilage is more flexible thanthat for bone, which contains calcium salts. In bone, the lacunae lie inconcentric circles within an osteon (or Haversian system) about a cen-tral canal. Blood is a connective tissue in which the matrix is a liquidcalled plasma.

Muscular tissue is of three types. Both skeletal and cardiac muscleare striated; both cardiac and smooth muscle are involuntary. Skeletalmuscle is found in muscles attached to bones, and smooth muscle isfound in internal organs. Cardiac muscle makes up the heart.

Nervous tissue has one main type of conducting cell, the neuron,and several types of neuroglial cells. Each neuron has dendrites, a cellbody, and an axon. The brain and spinal cord contain complete neu-rons, while the nerves contain only neuron fibers. Axons are special-ized to conduct nerve impulses.

11.2 Body Cavities and Body MembranesThe internal organs occur within cavities; the thoracic cavity containsthe heart and lungs; the abdominal cavity contains organs of the diges-

tive, urinary, and reproductive systems, among others. Membranesline body cavities and internal spaces of organs. As an example,mucous membrane lines the tubes of the digestive system; serousmembrane lines the thoracic and abdominal cavities and covers theorgans they contain.

11.3 Organ SystemsThe skin is sometimes called the integumentary system. The digestive,cardiovascular, lymphatic, respiratory, and urinary systems performprocessing and transporting functions that maintain the normal condi-tions of the body. The nervous system receives sensory input from sen-sory receptors and directs the musculoskeletal system and glands torespond to outside stimuli. The musculoskeletal system supports thebody and permits movement. The endocrine system produces hor-mones, some of which influence the functioning of the reproductivesystem, which allows humans to make more of their own kind.

11.4 Skin as an Organ SystemThe skin can be used as an example of an organ system because it con-tains accessory structures such as nails, hair, and glands. Skin protectsunderlying tissues from physical trauma, pathogen invasion, and waterloss. Skin helps regulate body temperature, and because it containssensory receptors, skin also helps us to be aware of our surroundings.

Skin is a two-layered organ that waterproofs and protects thebody. The epidermis contains basal cells that produce new epithelialcells that become keratinized as they move toward the surface. Thedermis, a largely fibrous connective tissue, contains epidermallyderived glands and hair follicles, nerve endings, and blood vessels.Sensory receptors for touch, pressure, temperature, and pain are pres-ent. Sweat glands and blood vessels help control body temperature. Asubcutaneous layer, which is made up of loose connective tissue con-taining adipose cells, lies beneath the skin.

Transplantation of the kidney, heart,liver, pancreas, lung, and other

organs is now possible due to two majorbreakthroughs. First, solutions have beendeveloped that preserve donor organs forseveral hours. This made it possible forone young boy to undergo surgery for 16hours, during which time he received fivedifferent organs. Second, rejection oftransplanted organs is now prevented byimmunosuppressive drugs; therefore,organs can be donated by unrelated indi-viduals, living or dead. . After death, it ispossible to give the “gift of life” to some-one else—over 25 organs and tissues fromone cadaver can be used for transplants.Survival rate after a transplant operationis good. So many heart recipients are nowalive and healthy they have formed bas-ketball and softball teams, demonstrating

the normalcy of their lives after surgery.One problem persists however, and

that is the limited availability of organs fortransplantation. At any one time, at least27,000 Americans are waiting for a donat-ed organ. Keen competition for organs canlead to various bioethical inequities. Whenthe governor of Pennsylvania received aheart and lungs within a relatively shortperiod of time, it appeared that his socialstatus may have played a role. WhenMickey Mantle received a liver transplant,people asked if it was right to give anorgan to an older man who had a diseasedliver due to the consumption of alcohol. Ifa father gives a kidney to a child, he has toundergo a major surgical operation thatleaves him vulnerable to possible seriousconsequences in the future. If organs aretaken from those who have just died, who

guarantees that the individual is indeeddead? And is it right to genetically alteranimals to serve as a source of organs forhumans? Such organs will most likely befor sale, and does this make the wealthymore likely to receive a transplant thanthose who cannot pay?

Questions1. Is it ethical to ask a parent to donate an

organ to his or child? Why or why not?2. Is it ethical to put a famous person at the

top of the list for an organ transplant?Why or why not?

3. Is it ethical to remove organs from anewborn who is brain dead but whoseorgans are still functioning? Why or whynot?

4. When xenotransplants (transplants forhumans from other animals) are available,should they be for sale? Why or why not?

11.5 HomeostasisHomeostasis is the dynamic equilibrium of the internal environment.All organ systems contribute to the constancy of tissue fluid and blood.Special contributions are made by the liver, which keeps blood glucoseconstant, and the kidneys, which regulate the pH. The nervous andhormonal systems regulate the other body systems. Both of these arecontrolled by a negative feedback mechanism, which results in fluctu-ation above and below the desired levels. Body temperature is regulat-ed by a center in the hypothalamus.

Studying the Concepts

1. Name the four major types of tissues. 1942. Name the different kinds of epithelial tissue, and give a loca-

tion and function for each. 1943. What are the functions of connective tissue? Name the differ-

ent kinds, and give a location for each. 196–984. What are the functions of muscular tissue? Name the differ-

ent kinds, and give a location for each. 1995. Nervous tissue contains what type of cell? Which organs in

the body are made up of nervous tissue? 2006. In what cavities are the major organs located? 2017. Distinguish between plasma membrane and body membrane.

2018. Describe the structure of skin, and state at least two functions

of this organ. 204–059. What is homeostasis, and how is it achieved in the human

body? 208–0910. Give an example of a negative feedback system. 209

Testing Yourself

Choose the best answer for each question.1. Which of these is mismatched?

a. epithelial tissue—protection and absorptionb. muscular tissue—contraction and conductionc. connective tissue—binding and supportd. nervous tissue—conduction and message sendinge. nervous tissue—neuroglial cells

2. Which of these is not epithelial tissue?a. simple cuboidal and stratified columnarb. bone and cartilagec. stratified squamous and simple squamousd. pseudostratifiede. All of these are epithelial tissue.

3. Which tissue is more apt to line a lumen?a. epithelial tissueb. connective tissuec. nervous tissued. muscular tissuee. epidermis tissue

4. Tendons and ligaments area. connective tissue.b. associated with the bones.c. found in vertebrates.d. subject to injury.e. All of these are correct.

5. Which tissue has cells in lacunae?a. epithelial tissueb. fibrous connectivec. cartilaged. bonee. Both c and d are correct.

6. Cardiac muscle isa. striated.b. involuntary.c. smooth.d. voluntary.e. Both a and b are correct.

7. Which of these components of blood fights infection?a. red blood cellsb. white blood cellsc. plateletsd. plasminogene. All of these are correct.

8. Which of these body systems contribute to homeostasis?a. digestive and excretory systemsb. respiratory and nervous systemsc. nervous and endocrine systemsd. All of these are correct.e. Body systems are not involved in homeostasis.

9. With negative feedback,a. the output cancels the input.b. there is a fluctuation above and below the average.c. there is self-regulation.d. sensory receptors communicate with a regulatory center.e. All of these are correct.

10. Which of these correctly describes a layer of the skin?a. The epidermis is simple squamous epithelium in which

hair follicles develop and blood vessels expand when weare hot.

b. The subcutaneous layer lies between the epidermis and thedermis. It contains adipose tissue, which keeps us warm.

c. The dermis is a region of connective tissue which containssensory receptors, nerve endings, and blood vessels.

d. The skin has a special layer, still unnamed, in which thereare all the accessory structures like nails, hair, and variousglands.

e. All of these are correct.11. Give the name, the location, and the function for each of these

tissues.a. Type of epithelial tissueb. Type of muscular tissuec. Type of connective tissue

Chapter 11 Human Organization 21111-19

a.

c.

b.

212 Part 3 Maintenance of the Human Body 11-20

Understanding the Terms

adhesion junction 196adipose tissue 196basement membrane 194blood 198bone 197carcinoma 194cardiac muscle 199cardiovascular system 202cartilage 196cilium 194coelom 201collagen fiber 196columnar epithelium 194compact bone 197connective tissue 196cuboidal epithelium 194dense fibrous connective

tissue 196dermis 205digestive system 202elastic cartilage 197elastic fiber 196endocrine system 203epidermis 205epithelial tissue 194fibroblast 196fibrocartilage 197gap junction 196gland 194hair follicle 205homeostasis 203hyaline cartilage 196integumentary system 202intercalated disk 199

lacuna 196ligament 196loose fibrous connective

tissue 196lymphatic system 202matrix 196melanocyte 205meninges 201microvillus 194mucous membrane 201muscular (contractile)

tissue 199musculoskeletal system 203negative feedback 209nerve 200nervous system 203nervous tissue 200neuroglial cell 200neuron 200oil gland 205pathogen 194peritonitis 201plasma 198platelet 198pleural membrane 201pleurisy 201positive feedback 209red blood cell 198reproductive system 203respiratory system 202reticular fiber 196serous membrane 201skeletal muscle 199skin 204

smooth (visceral) muscle 199spongy bone 197squamous epithelium 194striated 199subcutaneous layer 205sweat gland 205

synovial membrane 201tendon 196tight junction 196tissue 194urinary system 202white blood cell 198

Match the terms to these definitions:a. Fibrous connective tissue that joins bone to bone

at a joint.b. Outer region of the skin composed of stratified

squamous epithelium.c. Having bands such as in cardiac and skeletal

muscle.d. Self-regulatory mechanism that is activated by an

imbalance and results in a fluctuation above and below amean.

e. Porous bone found at the ends of long boneswhere blood cells are formed.

Using Technology

Your study of human organization is supported by these availabletechnologies:

Essential Study Partner CD-ROMAnimals ££ Body OrganizationVisit the Mader web site for related ESP activities.

Exploring the InternetThe Mader Home Page provides resources and tools asyou study this chapter.

http://www.mhhe.com/biosci/genbio/mader

Dynamic Human 2.0 CD-ROM Human Body

Thinking Scientifically

1. Considering the differentiation of tissue:a. How is the structure of an epithelial cell suited to its

function?b. Tight junctions between epithelial cells are equivalent to

which feature of endodermal cells in plants (page 157)?How are they equivalent?

c. How is the structure of a skeletal muscle cell suited tomuscle contraction? If, upon contraction, muscular tissuealways shortens from right to left, what would happen toan object attached at the right?

d. How is the structure of a nerve cell suited to its function?

2. Your task is to show that humans have a greater mental ca-pacity than other animals. Mention these features as possiblesupportive data.a. number of neuronsb. size of the overall brainc. organization of the braind. organization of the nervous system