1173

58Maintaining the Internal

Environment

Concept Outline

58.1 The regulatory systems of the body maintainhomeostasis.

The Need to Maintain Homeostasis. Regulatorymechanisms maintain homeostasis through negativefeedback loops.Antagonistic Effectors and Positive Feedback.Antagonistic effectors cause opposite changes, whilepositive feedback pushes changes further in the same way.

58.2 The extracellular fluid concentration is constantin most vertebrates.

Osmolality and Osmotic Balance. Vertebrates have tocope with the osmotic gain or loss of body water.Osmoregulatory Organs. Invertebrates have a variety oforgans to regulate water balance; kidneys are theosmoregulatory organs of most vertebrates.Evolution of the Vertebrate Kidney. Freshwater bonyfish produce a dilute urine and marine bony fish produce anisotonic urine. Only birds and mammals can retain so muchwater that they produce a concentrated urine.

58.3 The functions of the vertebrate kidney areperformed by nephrons.

The Mammalian Kidney. Each kidney containsnephrons that produce a filtrate which is modified byreabsorption and secretion to produce urine.Transport Processes in the Mammalian Nephron.The nephron tubules of birds and mammals have loops ofHenle, which function to draw water out of the tubule andback into the blood.Ammonia, Urea, and Uric Acid. The breakdown ofprotein and nucleic acids yields nitrogen, which is excretedas ammonia in bony fish, as urea in mammals, and as uricacid in reptiles and birds.

58.4 The kidney is regulated by hormones.

Hormones Control Homeostatic Functions.Antidiuretic hormone promotes water retention and theexcretion of a highly concentrated urine. Aldosteronestimulates the retention of salt and water, whereas atrialnatriuretic hormone promotes the excretion of salt and water.

The first vertebrates evolved in seawater, and the physi-ology of all vertebrates reflects this origin. Approxi-

mately two-thirds of every vertebrate’s body is water. If theamount of water in the body of a vertebrate falls muchlower than this, the animal will die. In this chapter, we dis-cuss the various mechanisms by which animals avoid gain-ing or losing too much water. As we shall see, these mecha-nisms are closely tied to the way animals exploit the variedenvironments in which they live and to the regulatory sys-tems of the body (figure 58.1).

FIGURE 58.1Regulating body temperature with water. One of the ways anelephant can regulate its temperature is to spray water on itsbody. Water also cycles through the elephant’s body in enormousquantities each day and helps to regulate its internal environment.

input from a temperature sensor, like a thermometer (asensor) within the wall unit. It compares the actual temper-ature to its set point. When these are different, it sends asignal to an effector. The effector in this case may be an airconditioner, which acts to reverse the deviation from theset point.

In a human, if the body temperature exceeds the setpoint of 37°C, sensors in a part of the brain detect this de-viation. Acting via an integrating center (also in the brain),these sensors stimulate effectors (including sweat glands)that lower the temperature (figure 58.3). One can think ofthe effectors as “defending” the set points of the bodyagainst deviations. Because the activity of the effectors isinfluenced by the effects they produce, and because thisregulation is in a negative, or reverse, direction, this type ofcontrol system is known as a negative feedback loop.

The nature of the negative feedback loop becomes clearwhen we again refer to the analogy of the thermostat andair conditioner. After the air conditioner has been on forsome time, the room temperature may fall significantlybelow the set point of the thermostat. When this occurs,the air conditioner will be turned off. The effector (air con-ditioner) is turned on by a high temperature; and, when ac-tivated, it produces a negative change (lowering of the tem-perature) that ultimately causes the effector to be turnedoff. In this way, constancy is maintained.

1174 Part XIV Regulating the Animal Body

The Need to MaintainHomeostasisAs the animal body has evolved, special-ization has increased. Each cell is a so-phisticated machine, finely tuned tocarry out a precise role within the body.Such specialization of cell function ispossible only when extracellular condi-tions are kept within narrow limits.Temperature, pH, the concentrations ofglucose and oxygen, and many other fac-tors must be held fairly constant for cellsto function efficiently and interact prop-erly with one another.

Homeostasis may be defined as thedynamic constancy of the internal envi-ronment. The term dynamic is used be-cause conditions are never absolutelyconstant, but fluctuate continuouslywithin narrow limits. Homeostasis isessential for life, and most of the regu-latory mechanisms of the vertebratebody that are not devoted to reproduc-tion are concerned with maintaininghomeostasis.

Negative Feedback Loops

To maintain internal constancy, the vertebrate body musthave sensors that are able to measure each condition of theinternal environment (figure 58.2). These constantly moni-tor the extracellular conditions and relay this information(usually via nerve signals) to an integrating center, whichcontains the “set point” (the proper value for that condi-tion). This set point is analogous to the temperature settingon a house thermostat. In a similar manner, there are setpoints for body temperature, blood glucose concentration,the tension on a tendon, and so on. The integrating centeris often a particular region of the brain or spinal cord, butin some cases it can also be cells of endocrine glands. It re-ceives messages from several sensors, weighing the relativestrengths of each sensor input, and then determineswhether the value of the condition is deviating from the setpoint. When a deviation in a condition occurs (the “stimu-lus”), the integrating center sends a message to increase ordecrease the activity of particular effectors. Effectors aregenerally muscles or glands, and can change the value ofthe condition in question back toward the set point value(the “response”).

To return to the idea of a home thermostat, suppose youset the thermostat at a set point of 70°F. If the temperatureof the house rises sufficiently above the set point, the ther-mostat (equivalent to an integrating center) receives this

58.1 The regulatory systems of the body maintain homeostasis.

SensorConstantlymonitors

conditions

Negativefeedback loop

completed

–

ResponseReturn toset point

StimulusDeviation from

set point

Perturbingfactor

EffectorCauses changesto compensate

for deviation

IntegratingcenterComparesconditions to set point

FIGURE 58.2A generalized diagram of a negative feedback loop. Negative feedback loops maintaina state of homeostasis, or dynamic constancy of the internal environment, by correctingdeviations from a set point.

Chapter 58 Maintaining the Internal Environment 1175

Integratingcenter

Sensor

Effector

Blood vesselsdilate

Glands releasesweat

Response

Body temperaturedrops

Response

Body temperaturerises

Effector

Blood vesselsconstrict

Skeletal musclescontract, shiver

Stimulus

Body temperaturedrops

Stimulus

Body temperaturerises

Perturbing factor

Sun

Perturbing factor

Snow and ice

To increasebody temperature

To decreasebody temperature

Negative feedback

—

Negative feedback

—

FIGURE 58.3Negative feedback loops keep the body temperature within a normal range. An increase (top) or decrease (bottom) in bodytemperature is sensed by the brain. The integrating center in the brain then processes the information and activates effectors, such assurface blood vessels, sweat glands, and skeletal muscles. When the body temperature returns to normal, negative feedback preventsfurther stimulation of the effectors by the integrating center.

Regulating Body Temperature

Humans, together with other mammals and with birds, areendothermic; they can maintain relatively constant body tem-peratures independent of the environmental temperature.When the temperature of your blood exceeds 37°C (98.6°F),neurons in a part of the brain called the hypothalamus detectthe temperature change. Acting through the control ofmotor neurons, the hypothalamus responds by promotingthe dissipation of heat through sweating, dilation of bloodvessels in the skin, and other mechanisms. These responsestend to counteract the rise in body temperature. When bodytemperature falls, the hypothalamus coordinates a differentset of responses, such as shivering and the constriction ofblood vessels in the skin, which help to raise body tempera-ture and correct the initial challenge to homeostasis.

Vertebrates other than mammals and birds are ectother-mic; their body temperatures are more or less dependent onthe environmental temperature. However, to the extentthat it is possible, many ectothermic vertebrates attempt tomaintain some degree of temperature homeostasis. Certainlarge fish, including tuna, swordfish, and some sharks, forexample, can maintain parts of their body at a significantlyhigher temperature than that of the water. Reptiles attemptto maintain a constant body temperature through behav-ioral means—by placing themselves in varying locations ofsun and shade (see chapter 29). That’s why you frequentlysee lizards basking in the sun. Sick lizards even give them-selves a “fever” by seeking warmer locations!

Most invertebrates do not employ feedback regulationto physiologically control their body temperature. Instead,they use behavior to adjust their temperature. Many butter-flies, for example, must reach a certain body temperaturebefore they can fly. In the cool of the morning they orientso as to maximize their absorption of sunlight. Moths andmany other insects use a shivering reflex to warm their tho-racic flight muscles (figure 58.4).

Regulating Blood Glucose

When you digest a carbohydrate-containing meal, you ab-sorb glucose into your blood. This causes a temporary risein the blood glucose concentration, which is brought backdown in a few hours. What counteracts the rise in bloodglucose following a meal?

Glucose levels within the blood are constantly moni-tored by a sensor, the islets of Langerhans in the pancreas.When levels increase, the islets secrete the hormone in-sulin, which stimulates the uptake of blood glucose intomuscles, liver, and adipose tissue. The islets are, in thiscase, the sensor and the integrating center. The muscles,liver, and adipose cells are the effectors, taking up glucoseto control the levels. The muscles and liver can convert theglucose into the polysaccharide glycogen; adipose cells canconvert glucose into fat. These actions lower the blood glu-cose (figure 58.5) and help to store energy in forms that thebody can use later.

Negative feedback mechanisms correct deviations froma set point for different internal variables. In this way,body temperature and blood glucose, for example, arekept within normal limits.

1176 Part XIV Regulating the Animal Body

0 1–1 2 3 4

Time (minutes)

Tem

pera

ture

(�C

) of

thor

ax m

uscl

es

25

40

35

30

PreflightNo wing

movement

Warm upShiver-likecontractionof thoraxmuscles

FlightFull rangemovement

of wings

FIGURE 58.4Thermoregulation in insects. Some insects, such as the sphinxmoth, contract their thoracic muscles to warm up for flight.

Eating

Blood glucose

Islets of Langerhans

Stops insulinsecretion

Insulin

Cellular uptake of glucose

Blood glucose

Negativefeedback

loop

–

FIGURE 58.5The negative feedback control of blood glucose. The rise inblood glucose concentration following a meal stimulates the secre-tion of insulin from the islets of Langerhans in the pancreas. In-sulin is a hormone that promotes the entry of glucose in skeletalmuscle and other tissue, thereby lowering the blood glucose andcompensating for the initial rise.

Antagonistic Effectors and PositiveFeedbackThe negative feedback mechanisms that maintain home-ostasis often oppose each other to produce a finer degree ofcontrol. In a few cases positive feedback mechanisms,which push a change further in the same direction, are usedby the body.

Antagonistic Effectors

Most factors in the internal environment are controlled byseveral effectors, which often have antagonistic actions.Control by antagonistic effectors is sometimes described as“push-pull,” in which the increasing activity of one effectoris accompanied by decreasing activity of an antagonistic ef-fector. This affords a finer degree of control than could beachieved by simply switching one effector on and off.

Room temperature can be maintained, for example, bysimply turning an air conditioner on and off, or by just turn-ing a heater on and off. A much more stable temperature,however, can be achieved if the air conditioner and heaterare both controlled by a thermostat (figure 58.6). Then theheater is turned on when the air conditioner shuts off, andvice versa. Antagonistic effectors are similarly involved in thecontrol of body temperature and blood glucose. Whereas in-sulin, for example, lowers blood glucose following a meal,other hormones act to raise the blood glucose concentrationbetween meals, especially when a person is exercising. Theheart rate is similarly controlled by antagonistic effectors.Stimulation of one group of nerve fibers increases the heartrate, while stimulation of another group slows the heart rate.

Positive Feedback Loops

Feedback loops that accentuate a disturbance are calledpositive feedback loops. In a positive feedback loop, pertur-bations cause the effector to drive the value of the con-trolled variable even farther from the set point. Hence, sys-tems in which there is positive feedback are highlyunstable, analogous to a spark that ignites an explosion.They do not help to maintain homeostasis. Nevertheless,such systems are important components of some physiolog-ical mechanisms. For example, positive feedback occurs inblood clotting, where one clotting factor activates anotherin a cascade that leads quickly to the formation of a clot.Positive feedback also plays a role in the contractions of theuterus during childbirth (figure 58.7). In this case, stretch-ing of the uterus by the fetus stimulates contraction, andcontraction causes further stretching; the cycle continuesuntil the fetus is expelled from the uterus. In the body,most positive feedback systems act as part of some largermechanism that maintains homeostasis. In the exampleswe’ve described, formation of a blood clot stops bleedingand hence tends to keep blood volume constant, and expul-sion of the fetus reduces the contractions of the uterus.

Antagonistic effectors that act antagonistically to eachother are more effective than effectors that act alone.Positive feedback mechanisms accentuate changes andhave limited functions in the body.

Chapter 58 Maintaining the Internal Environment 1177

Effectors

AirconditionerFurnace

Thermostat

Sensor

Set pointfor heating

Set pointfor cooling

73 786863 83

FIGURE 58.6Room temperature is maintained by antagonistic effectors. Ifa thermostat senses a low temperature, the heater is turned on andthe air conditioner is turned off. If the temperature is too high,the air conditioner is activated, and the heater is turned off.

Integrating centers in brain Increased neural

and hormonal signalsContinued increased

neural stimulation

Increased contraction force and frequency

in smoothmuscles of uterus

Receptors detectincreased stretch

The fetus ispushed againstthe uterine opening,causing the inferioruterus to stretch

+

FIGURE 58.7An example of positive feedback during childbirth. This is oneof the few examples of positive feedback that operate in thevertebrate body.

Osmolality and Osmotic BalanceWater in an animal’s body is distributed between the intra-cellular and extracellular compartments (figure 58.8). Inorder to maintain osmotic balance, the extracellular com-partment of an animal’s body (including its blood plasma)must be able to take water from its environment or to ex-crete excess water into its environment. Inorganic ionsmust also be exchanged between the extracellular body flu-ids and the external environment to maintain homeostasis.Such exchanges of water and electrolytes between the bodyand the external environment occur across specialized ep-ithelial cells and, in most vertebrates, through a filtrationprocess in the kidneys.

Most vertebrates maintain homeostasis in regard to thetotal solute concentration of their extracellular fluids and inregard to the concentration of specific inorganic ions.Sodium (Na+) is the major cation in extracellular fluids, andchloride (Cl–) is the major anion. The divalent cations, cal-cium (Ca++) and magnesium (Mg++), as well as other ions,also have important functions and must be maintained attheir proper concentrations.

Osmolality and Osmotic Pressure

Osmosis is the diffusion of water across a membrane, and italways occurs from a more dilute solution (with a lowersolute concentration) to a less dilute solution (with a highersolute concentration). Because the total solute concentra-tion of a solution determines its osmotic behavior, the totalmoles of solute per kilogram of water is expressed as theosmolality of the solution. Solutions that have the sameosmolality are isosmotic. A solution with a lower or higherosmolality than another is called hypoosmotic or hyperosmotic,respectively.

If one solution is hyperosmotic compared with an-other, and if the two solutions are separated by a semi-permeable membrane, water may move by osmosis fromthe more dilute solution to the hyperosmotic one. In thiscase, the hyperosmotic solution is also hypertonic(“higher strength”) compared with the other solution,and it has a higher osmotic pressure. The osmotic pres-sure of a solution is a measure of its tendency to take inwater by osmosis. A cell placed in a hypertonic solution,which has a higher osmotic pressure than the cell cyto-plasm, will lose water to the surrounding solution andshrink. A cell placed in a hypotonic solution, in contrast,will gain water and expand.

If a cell is placed in an isosmotic solution, there maybe no net water movement. In this case, the isosmotic so-lution can also be said to be isotonic. Isotonic solutionssuch as normal saline and 5% dextrose are used in med-ical care to bathe exposed tissues and to be given as intra-venous fluids.

Osmoconformers and Osmoregulators

Most marine invertebrates are osmoconformers; the os-molality of their body fluids is the same as that of seawater(although the concentrations of particular solutes, such asmagnesium ion, are not equal). Because the extracellularfluids are isotonic to seawater, there is no osmotic gradientand no tendency for water to leave or enter the body.Therefore, osmoconformers are in osmotic equilibriumwith their environment. Among the vertebrates, only theprimitive hagfish are strict osmoconformers. The sharksand their relatives in the class Chondrichthyes (cartilagi-nous fish) are also isotonic to seawater, even though theirblood level of NaCl is lower than that of seawater; the dif-ference in total osmolality is made up by retaining urea at ahigh concentration in their blood plasma.

All other vertebrates are osmoregulators—that is, ani-mals that maintain a relatively constant blood osmolalitydespite the different concentration in the surrounding envi-ronment. The maintenance of a relatively constant bodyfluid osmolality has permitted vertebrates to exploit a widevariety of ecological niches. Achieving this constancy, how-ever, requires continuous regulation.

Freshwater vertebrates have a much higher solute con-centration in their body fluids than that of the surround-ing water. In other words, they are hypertonic to theirenvironment. Because of their higher osmotic pressure,water tends to enter their bodies. Consequently, theymust prevent water from entering their bodies as much aspossible and eliminate the excess water that does enter.In addition, they tend to lose inorganic ions to their envi-ronment and so must actively transport these ions backinto their bodies.

In contrast, most marine vertebrates are hypotonic totheir environment; their body fluids have only about one-third the osmolality of the surrounding seawater. These an-imals are therefore in danger of losing water by osmosisand must retain water to prevent dehydration. They do thisby drinking seawater and eliminating the excess ionsthrough their kidneys and gills.

The body fluids of terrestrial vertebrates have a higherconcentration of water than does the air surrounding them.Therefore, they tend to lose water to the air by evaporationfrom the skin and lungs. All reptiles, birds, and mammals,as well as amphibians during the time when they live onland, face this problem. These vertebrates have evolved ex-cretory systems that help them retain water.

Marine invertebrates are isotonic with theirenvironment, but most vertebrates are either hypertonicor hypotonic to their environment and thus tend to gainor lose water. Physiological mechanisms help mostvertebrates to maintain a constant blood osmolality andconstant concentrations of individual ions.

1178 Part XIV Regulating the Animal Body

58.2 The extracellular fluid concentration is constant in most vertebrates.

Chapter 58 Maintaining the Internal Environment 1179

Extracellular compartment(including blood)

Intracellular compartments

External environment

H2O and solutes

Animal body Integument

Epithelial cell

H2O and solutes

H2O and solutes

Some water and solutes are reabsorbed,but excess water and solutes are excreted.

Water and solutes are transported into and out of the body, depending on concentration gradients.

Connectivetissue

Epithelialtissue

H2O and solutes

Muscletissue

H2O and solutes

Nervetissue

H2O and solutesreabsorbed

Excess H2O and solutes excreted

Filtration in kidneys

FIGURE 58.8The interaction between intracellular and extracellular compartments of the body and the external environment. Water can betaken in from the environment or lost to the environment. Exchanges of water and solutes between the extracellular fluids of the body andthe environment occur across transport epithelia, and water and solutes can be filtered out of the blood by the kidneys. Overall, theamount of water and solutes that enters and leaves the body must be balanced in order to maintain homeostasis.

Osmoregulatory OrgansAnimals have evolved a variety of mechanisms to cope withproblems of water balance. In many animals, the removal ofwater or salts from the body is coupled with the removal ofmetabolic wastes through the excretory system. Protistsemploy contractile vacuoles for this purpose, as do sponges.Other multicellular animals have a system of excretorytubules (little tubes) that expel fluid and wastes from thebody.

In flatworms, these tubules are called protonephridia, andthey branch throughout the body into bulblike flame cells(figure 58.9). While these simple excretory structures opento the outside of the body, they do not open to the inside ofthe body. Rather, cilia within the flame cells must draw influid from the body. Water and metabolites are then reab-sorbed, and the substances to be excreted are expelledthrough excretory pores.

Other invertebrates have a system of tubules that openboth to the inside and to the outside of the body. In theearthworm, these tubules are known as metanephridia(figure 58.10). The metanephridia obtain fluid from thebody cavity through a process of filtration into funnel-shaped structures called nephrostomes. The term filtrationis used because the fluid is formed under pressure andpasses through small openings, so that molecules largerthan a certain size are excluded. This fil-tered fluid is isotonic to the fluid in thecoelom, but as it passes through the tubulesof the metanephridia, NaCl is removed byactive transport processes. A general termfor transport out of the tubule and into thesurrounding body fluids is reabsorption. Be-cause salt is reabsorbed from the filtrate,the urine excreted is more dilute than thebody fluids (is hypotonic). The kidneys ofmollusks and the excretory organs of crus-taceans (called antennal glands) also produceurine by filtration and reclaim certain ionsby reabsorption.

The excretory organs in insects are theMalpighian tubules (figure 58.11), exten-sions of the digestive tract that branch offanterior to the hindgut. Urine is notformed by filtration in these tubules, be-cause there is no pressure difference be-tween the blood in the body cavity and thetubule. Instead, waste molecules and potas-sium (K+) ions are secreted into the tubulesby active transport. Secretion is the oppo-site of reabsorption—ions or molecules aretransported from the body fluid into thetubule. The secretion of K+ creates an os-motic gradient that causes water to enterthe tubules by osmosis from the body’s

1180 Part XIV Regulating the Animal Body

Excretorypores

Cilia

Collectingtubule

Flamecell

FIGURE 58.9The protonephridia of flatworms. A branching system oftubules, bulblike flame cells, and excretory pores make up theprotonephridia of flatworms. Cilia inside the flame cells draw influids from the body by their beating action. Substances are thenexpelled through pores which open to the outside of the body.

Coelomic fluid Pore forurine excretion

Nephrostome

Capillarynetwork

Bladder

FIGURE 58.10The metanephridia of annelids. Most invertebrates, such as the annelid shownhere, have metanephridia. These consist of tubules that receive a filtrate of coelomicfluid, which enters the funnel-like nephrostomes. Salt can be reabsorbed from thesetubules, and the fluid that remains, urine, is released from pores into the externalenvironment.

open circulatory system. Most of the water and K+ is thenreabsorbed into the circulatory system through the ep-ithelium of the hindgut, leaving only small molecules andwaste products to be excreted from the rectum along withfeces. Malpighian tubules thus provide a very efficientmeans of water conservation.

The kidneys of vertebrates, unlike the Malpighiantubules of insects, create a tubular fluid by filtration ofthe blood under pressure. In addition to containing wasteproducts and water, the filtrate contains many small mol-ecules that are of value to the animal, including glucose,amino acids, and vitamins. These molecules and most ofthe water are reabsorbed from the tubules into the blood,while wastes remain in the filtrate. Additional wastes maybe secreted by the tubules and added to the filtrate, andthe final waste product, urine, is eliminated from thebody.

It may seem odd that the vertebrate kidney should filterout almost everything from blood plasma (except proteins,which are too large to be filtered) and then spend energy totake back or reabsorb what the body needs. But selectivereabsorption provides great flexibility, because various ver-tebrate groups have evolved the ability to reabsorb differ-ent molecules that are especially valuable in particular habi-tats. This flexibility is a key factor underlying the successfulcolonization of many diverse environments by the verte-brates.

Many invertebrates filter fluid into a system of tubulesand then reabsorb ions and water, leaving wasteproducts for excretion. Insects create an excretory fluidby secreting K+ into tubules, which draws waterosmotically. The vertebrate kidney produces a filtratethat enters tubules and is modified to become urine.

Chapter 58 Maintaining the Internal Environment 1181

Air sac

Malpighiantubules

Rectum

Rectum

Poison sac

Midgut

Midgut

Anus

Intestine

Hindgut

Malpighiantubules

FIGURE 58.11The Malpighian tubules of insects. (a) The Malpighiantubules of insects are extensions of the digestive tract thatcollect water and wastes from the body’s circulatory system.(b) K+ is secreted into these tubules, drawing water with itosmotically. Much of this water (see arrows) is reabsorbedacross the wall of the hindgut.

Evolution of the Vertebrate KidneyThe kidney is a complex organ made up of thousands of re-peating units called nephrons, each with the structure of abent tube (figure 58.12). Blood pressure forces the fluid inblood past a filter, called the glomerulus, at the top of eachnephron. The glomerulus retains blood cells, proteins, andother useful large molecules in the blood but allows thewater, and the small molecules and wastes dissolved in it, topass through and into the bent tube part of the nephron. Asthe filtered fluid passes through the nephron tube, usefulsugars and ions are recovered from it by active transport,leaving the water and metabolic wastes behind in a fluidurine.

Although the same basic design has been retained in allvertebrate kidneys, there have been a few modifications.Because the original glomerular filtrate is isotonic to blood,all vertebrates can produce a urine that is isotonic to bloodby reabsorbing ions and water in equal proportions or hy-potonic to blood—that is, more dilute than the blood, byreabsorbing relatively less water blood. Only birds andmammals can reabsorb enough water from their glomeru-lar filtrate to produce a urine that is hypertonic to blood—that is, more concentrated than the blood, by reabsorbingrelatively more water.

Freshwater Fish

Kidneys are thought to have evolved first among thefreshwater teleosts, or bony fish. Because the body fluidsof a freshwater fish have a greater osmotic concentrationthan the surrounding water, these animals face two seri-ous problems: (1) water tends to enter the body from theenvironment; and (2) solutes tend to leave the body and

enter the environment. Freshwater fish address the firstproblem by not drinking water and by excreting a largevolume of dilute urine, which is hypotonic to their bodyfluids. They address the second problem by reabsorbingions across the nephron tubules, from the glomerular fil-trate back into the blood. In addition, they actively trans-port ions across their gill surfaces from the surroundingwater into the blood.

Marine Bony Fish

Although most groups of animals seem to have evolved firstin the sea, marine bony fish (teleosts) probably evolvedfrom freshwater ancestors, as was mentioned in chapter 48.They faced significant new problems in making the transi-tion to the sea because their body fluids are hypotonic tothe surrounding seawater. Consequently, water tends toleave their bodies by osmosis across their gills, and theyalso lose water in their urine. To compensate for this con-tinuous water loss, marine fish drink large amounts of sea-water (figure 58.13).

Many of the divalent cations (principally Ca++ and Mg++)in the seawater that a marine fish drinks remain in the di-gestive tract and are eliminated through the anus. Some,however, are absorbed into the blood, as are the monova-lent ions K+, Na+, and Cl–. Most of the monovalent ions areactively transported out of the blood across the gill sur-faces, while the divalent ions that enter the blood are se-creted into the nephron tubules and excreted in the urine.In these two ways, marine bony fish eliminate the ions theyget from the seawater they drink. The urine they excrete isisotonic to their body fluids. It is more concentrated thanthe urine of freshwater fish, but not as concentrated as thatof birds and mammals.

1182 Part XIV Regulating the Animal Body

Proximalarm

Distal armGlomerulus Neck

Collecting duct

Intermediatesegment(Loop of Henle)

Amino acids

Glucose H2OH2O

H2O

NaCl

NaClH2O

H2O

Divalentions

H2O

FIGURE 58.12The basic organization ofthe vertebrate nephron. Thenephron tubule of thefreshwater fish is a basicdesign that has been retainedin the kidneys of marine fishand terrestrial vertebrates thatevolved later. Sugars, aminoacids, and divalent ions such asCa++ are recovered in theproximal arm; monovalentions such as Na+ and Cl– arerecovered in the distal arm;and water is recovered in thecollecting duct.

Cartilaginous Fish

The elasmobranchs, including sharks and rays, are by far themost common subclass in the class Chondrichthyes (carti-laginous fish). Elasmobranchs have solved the osmotic prob-lem posed by their seawater environment in a different waythan have the bony fish. Instead of having body fluids thatare hypotonic to seawater, so that they have to continuouslydrink seawater and actively pump out ions, the elasmo-branchs reabsorb urea from the nephron tubules and main-

tain a blood urea concentration that is 100 times higher thanthat of mammals. This added urea makes their blood ap-proximately isotonic to the surrounding sea. Because there isno net water movement between isotonic solutions, waterloss is prevented. Hence, these fishes do not need to drinkseawater for osmotic balance, and their kidneys and gills donot have to remove large amounts of ions from their bodies.The enzymes and tissues of the cartilaginous fish haveevolved to tolerate the high urea concentrations.

Chapter 58 Maintaining the Internal Environment 1183

Food,fresh water

UrineIntestinalwastes

NaClNaCl

Freshwater fish

Marine fish

Food,seawater

MgSO4

MgSO4

Kidney tubule

Largeglomerulus

Active tubularreabsorptionof NaCl

Kidney: Excretionof dilute urine

Gills:Active absorption ofNaCl, water entersosmotically

Glomerulusreduced orabsent

Stomach:Passive reabsorptionof NaCl and water

Gills:Active secretion ofNaCl, water loss

Intestinal wastes:MgSO4 voidedwith feces

Kidney:Excretion of MgSO4,urea, little water

Active tubularsecretionof MgSO4

FIGURE 58.13Freshwater and marine teleosts (bony fish) face different osmotic problems. Whereas the freshwater teleost is hypertonic to itsenvironment, the marine teleost is hypotonic to seawater. To compensate for its tendency to take in water and lose ions, a freshwater fishexcretes dilute urine, avoids drinking water, and reabsorbs ions across the nephron tubules. To compensate for its osmotic loss of water,the marine teleost drinks seawater and eliminates the excess ions through active transport across epithelia in the gills and kidneys.

Amphibians and Reptiles

The first terrestrial vertebrates were the amphibians, andthe amphibian kidney is identical to that of freshwater fish.This is not surprising, because amphibians spend a signifi-cant portion of their time in fresh water, and when on land,they generally stay in wet places. Amphibians produce avery dilute urine and compensate for their loss of Na+ byactively transporting Na+ across their skin from the sur-rounding water.

Reptiles, on the other hand, live in diverse habitats.Those living mainly in fresh water occupy a habitat simi-

lar to that of the freshwater fish and amphibians andthus have similar kidneys. Marine reptiles, includingsome crocodilians, sea turtles, sea snakes, and one lizard,possess kidneys similar to those of their freshwater rela-tives but face opposite problems; they tend to lose waterand take in salts. Like marine teleosts (bony fish), theydrink the seawater and excrete an isotonic urine. Marineteleosts eliminate the excess salt by transport across theirgills, while marine reptiles eliminate excess salt throughsalt glands located near the nose or the eye (f ig-ure 58.14).

1184 Part XIV Regulating the Animal Body

Skin absorbs Na+ from water

Drinks seawater

Salt gland secretes excess salts

Drinks seawater

Salt gland secretes excess salts

Does not drink seawater

Drinks fresh water

Drinks no water

Obtains water from foodand metabolic processes

Urine concentrationrelative to blood

Vertebrate

Strongly hypotonic

Isotonic

Weakly hypertonic

Strongly hypertonic

Weakly hypertonic

Strongly hypertonic

Amphibian

Marine reptile

Marine bird

Marine mammal

Terrestrial bird

Desert mammal

Excretes weakly hypertonic urine

FIGURE 58.14Osmoregulation by some vertebrates. Only birds and mammals can produce a hypertonic urine and thereby retain water efficiently, butmarine reptiles and birds can drink seawater and excrete the excess salt through salt glands.

The kidneys of terrestrial reptiles also reabsorb muchof the salt and water in their nephron tubules, helpingsomewhat to conserve blood volume in dry environments.Like fish and amphibians, they cannot produce urine thatis more concentrated than the blood plasma. However,when their urine enters their cloaca (the common exit ofthe digestive and urinary tracts), additional water can bereabsorbed.

Mammals and Birds

Mammals and birds are the only vertebrates able to pro-duce urine with a higher osmotic concentration thantheir body fluids. This allows these vertebrates to excretetheir waste products in a small volume of water, so thatmore water can be retained in the body. Human kidneyscan produce urine that is as much as 4.2 times as concen-trated as blood plasma, but the kidneys of some othermammals are even more efficient at conserving water.For example, camels, gerbils, and pocket mice of thegenus Perognathus can excrete urine 8, 14, and 22 times asconcentrated as their blood plasma, respectively. Thekidneys of the kangaroo rat (figure 58.15) are so efficientit never has to drink water; it can obtain all the water itneeds from its food and from water produced in aerobiccell respiration!

The production of hypertonic urine is accomplished bythe loop of Henle portion of the nephron (see figure 58.18),found only in mammals and birds. A nephron with a longloop of Henle extends deeper into the renal medulla, wherethe hypertonic osmotic environment draws out more water,and so can produce more concentrated urine. Most mam-mals have some nephrons with short loops and othernephrons with loops that are much longer (see figure58.17). Birds, however, have relatively few or no nephronswith long loops, so they cannot produce urine that is asconcentrated as that of mammals. At most, they can onlyreabsorb enough water to produce a urine that is abouttwice the concentration of their blood. Marine birds solvethe problem of water loss by drinking salt water and thenexcreting the excess salt from salt glands near the eyes (fig-ure 58.16).

The moderately hypertonic urine of a bird is deliveredto its cloaca, along with the fecal material from its digestivetract. If needed, additional water can be absorbed acrossthe wall of the cloaca to produce a semisolid white paste orpellet, which is excreted.

The kidneys of freshwater fish must excrete copiousamounts of very dilute urine, while marine teleostsdrink seawater and excrete an isotonic urine. The basicdesign and function of the nephron of freshwater fisheshave been retained in the terrestrial vertebrates.Modifications, particularly the presence of a loop ofHenle, allow mammals and birds to reabsorb morewater and produce a hypertonic urine.

Chapter 58 Maintaining the Internal Environment 1185

FIGURE 58.15The kangaroo rat, Dipodomys panamintensis. This mammal hasvery efficient kidneys that can concentrate urine to a high degreeby reabsorbing water, thereby minimizing water loss from thebody. This feature is extremely important to the kangaroo rat’ssurvival in dry or desert habitats.

Salt glands

Salt secretion

FIGURE 58.16Marine birds drink seawater and then excrete the saltthrough salt glands. The extremely salty fluid excreted by theseglands can then dribble down the beak.

The Mammalian KidneyIn humans, the kidneys are fist-sized organs located in theregion of the lower back. Each kidney receives blood froma renal artery, and it is from this blood that urine is pro-duced. Urine drains from each kidney through a ureter,which carries the urine to a urinary bladder. Within thekidney, the mouth of the ureter flares open to form a fun-nel-like structure, the renal pelvis. The renal pelvis, in turn,has cup-shaped extensions that receive urine from the renaltissue. This tissue is divided into an outer renal cortex andan inner renal medulla (figure 58.17). Together, thesestructures perform filtration, reabsorption, secretion, andexcretion.

Nephron Structure and Filtration

On a microscopic level, each kidney contains about onemillion functioning nephrons. Mammalian kidneys contain amixture of juxtamedullary nephrons, which have long loopswhich dip deeply into the medulla, and cortical nephronswith shorter loops (see figure 58.17). The significance ofthe length of the loops will be explained a little later.

Each nephron consists of a long tubule and associatedsmall blood vessels. First, blood is carried by an afferent ar-teriole to a tuft of capillaries in the renal cortex, theglomerulus (figure 58.18). Here the blood is filtered as theblood pressure forces fluid through the porous capillarywalls. Blood cells and plasma proteins are too large to enter

1186 Part XIV Regulating the Animal Body

58.3 The functions of the vertebrate kidney are performed by nephrons.

Renalcortex

Juxtamedullarynephron

Renal medulla

Collecting duct

Corticalnephron

Nephrontubule

Adrenal gland

Inferior vena cava

Renal veinand artery

Aorta

Ureter

Urinary bladder

Urethra

Ureter

Kidney

Renal pelvis

Renal medulla

Renal cortex

(a)

(b)

(c)

FIGURE 58.17The urinary system of a human female. (a)The positions of the organs of the urinarysystem. (b) A sectioned kidney, revealing theinternal structure. (c) The position of nephrons inthe mammalian kidney. Cortical nephrons arelocated predominantly in the renal cortex, whilejuxtamedullary nephrons have long loops thatextend deep into the renal medulla.

this glomerular filtrate, but large amounts of water anddissolved molecules leave the vascular system at this step.The filtrate immediately enters the first region of thenephron tubules. This region, Bowman’s capsule, envelopsthe glomerulus much as a large, soft balloon surroundsyour fist if you press your fist into it. The capsule has slitopenings so that the glomerular filtrate can enter the sys-tem of nephron tubules.

After the filtrate enters Bowman’s capsule it goes intoa portion of the nephron called the proximal convolutedtubule, located in the cortex. The fluid then moves downinto the medulla and back up again into the cortex in aloop of Henle. Only the kidneys of mammals and birdshave loops of Henle, and this is why only birds and mam-mals have the ability to concentrate their urine. Afterleaving the loop, the fluid is delivered to a distal convo-

luted tubule in the cortex that next drains into a collect-ing duct. The collecting duct again descends into themedulla, where it merges with other collecting ducts toempty its contents, now called urine, into the renalpelvis.

Blood components that were not filtered out of theglomerulus drain into an efferent arteriole, which thenempties into a second bed of capillaries called peritubularcapillaries that surround the tubules. This is the only loca-tion in the body where two capillary beds occur in series.The glomerulus is drained by an arteriole and this secondarteriole delivers blood to a second capillary bed, the per-itubular capillaries. As described later, the peritubularcapillaries are needed for the processes of reabsorptionand secretion.

Chapter 58 Maintaining the Internal Environment 1187

Glomerulus

Renal cortex

Renal medulla

Bowman'scapsule

Proximalconvoluted tubule

Descending limbof loop of Henle

Loop of Henle

Distalconvoluted tubule

Ascending limbof loop of Henle

Collecting duct

To ureter

Peritubulecapillaries

FIGURE 58.18A nephron in a mammalian kidney. The nephron tubule is surrounded by peritubular capillaries, which carry away molecules and ionsthat are reabsorbed from the filtrate.

Reabsorption and Secretion

Most of the water and dissolved solutes that enter theglomerular filtrate must be returned to the blood (figure58.19), or the animal would literally urinate to death. In ahuman, for example, approximately 2000 liters of bloodpasses through the kidneys each day, and 180 liters ofwater leaves the blood and enters the glomerular filtrate.Because we only have a total blood volume of about 5liters and only produce 1 to 2 liters of urine per day, it isobvious that each liter of blood is filtered many times perday and most of the filtered water is reabsorbed. The re-absorption of water occurs as a consequence of salt(NaCl) reabsorption through mechanisms that will be de-scribed shortly.

The reabsorption of glucose, amino acids, and manyother molecules needed by the body is driven by activetransport carriers. As in all carrier-mediated transport, amaximum rate of transport is reached whenever the carriersare saturated (see chapter 6). For the renal glucose carriers,saturation occurs when the concentration of glucose in theblood (and thus in the glomerular filtrate) is about 180 mil-ligrams per 100 milliliters of blood. If a person has a bloodglucose concentration in excess of this amount, as happensin untreated diabetes mellitus, the glucose left untrans-ported in the filtrate is expelled in the urine. Indeed, thepresence of glucose in the urine is diagnostic of diabetesmellitus.

The secretion of foreign molecules and particularwaste products of the body involves the transport of thesemolecules across the membranes of the blood capillariesand kidney tubules into the filtrate. This process is similarto reabsorption, but it proceeds in the opposite direction.Some secreted molecules are eliminated in the urine sorapidly that they may be cleared from the blood in a sin-gle pass through the kidneys. This rapid elimination ex-

plains why penicillin, which is secreted by the nephrons,must be administered in very high doses and several timesper day.

Excretion

A major function of the kidney is the elimination of a vari-ety of potentially harmful substances that animals eat anddrink. In addition, urine contains nitrogenous wastes, suchas urea and uric acid, that are products of the catabolism ofamino acids and nucleic acids. Urine may also contain ex-cess K+, H+, and other ions that are removed from theblood. Urine’s generally high H+ concentration (pH 5 to 7)helps maintain the acid-base balance of the blood within anarrow range (pH 7.35 to 7.45). Moreover, the excretion ofwater in urine contributes to the maintenance of blood vol-ume and pressure; the larger the volume of urine excreted,the lower the blood volume.

The purpose of kidney function is therefore homeosta-sis—the kidneys are critically involved in maintaining theconstancy of the internal environment. When disease inter-feres with kidney function, it causes a rise in the blood con-centration of nitrogenous waste products, disturbances inelectrolyte and acid-base balance, and a failure in bloodpressure regulation. Such potentially fatal changes high-light the central importance of the kidneys in normal bodyphysiology.

The mammalian kidney is divided into a cortex andmedulla and contains microscopic functioning unitscalled nephrons. The nephron tubules receive a bloodfiltrate from the glomeruli and modify this filtrate toproduce urine, which empties into the renal pelvis andis expelled from the kidney through the ureter.

1188 Part XIV Regulating the Animal Body

Glomerulus

Renal tubule

Bowman'scapsule

Excretion

Filtration

Reabsorption to blood

Secretion from blood

FIGURE 58.19Four functions of thekidney. Molecules enterthe urine by filtration outof the glomerulus and bysecretion into the tubulesfrom surroundingperitubular capillaries.Molecules that enteredthe filtrate can bereturned to the blood byreabsorption from thetubules into surroundingperitubular capillaries, orthey may be eliminatedfrom the body by excretionthrough the tubule to aureter, then to thebladder.

Transport Processes in theMammalian NephronAs previously described, approximately 180 liters (in ahuman) of isotonic glomerular filtrate enters the Bowman’scapsules each day. After passing through the remainder ofthe nephron tubules, this volume of fluid would be lost asurine if it were not reabsorbed back into the blood. It isclearly impossible to produce this much urine, yet water isonly able to pass through a cell membrane by osmosis, andosmosis is not possible between two isotonic solutions.Therefore, some mechanism is needed to create an osmoticgradient between the glomerular filtrate and the blood, al-lowing reabsorption.

Proximal Tubule

Approximately two-thirds of the NaCl and water filteredinto Bowman’s capsule is immediately reabsorbed acrossthe walls of the proximal convoluted tubule. This reabsorp-tion is driven by the active transport of Na+ out of the fil-trate and into surrounding peritubular capillaries. Cl– fol-lows Na+ passively because of electrical attraction, andwater follows them both because of osmosis. Because NaCland water are removed from the filtrate in proportionateamounts, the filtrate that remains in the tubule is still iso-tonic to the blood plasma.

Although only one-third of the initial volume of filtrateremains in the nephron tubule after the initial reabsorptionof NaCl and water, it still represents a large volume (60 Lout of the original 180 L of filtrate produced per day byboth human kidneys). Obviously, no animal can excretethat much urine, so most of this water must also be reab-sorbed. It is reabsorbed primarily across the wall of the col-lecting duct because the interstitial fluid of the renalmedulla surrounding the collecting ducts is hypertonic.The hypertonic renal medulla draws water out of the col-lecting duct by osmosis, leaving behind a hypertonic urinefor excretion.

Loop of Henle

The reabsorption of much of the water in the tubular fil-trate thus depends on the creation of a hypertonic renalmedulla; the more hypertonic the medulla is, the steeperthe osmotic gradient will be and the more water will leavethe collecting ducts. It is the loops of Henle that createthe hypertonic renal medulla in the following manner(figure 58.20):

1. The ascending limb of the loop actively extrudes Na+,and Cl– follows. The mechanism that extrudes NaClfrom the ascending limb of the loop differs from thatwhich extrudes NaCl from the proximal tubule, butthe most important difference is that the ascendinglimb is not permeable to water. As Na+ exits, the fluidwithin the ascending limb becomes increasingly di-

lute (hypotonic) as it enters thecortex, while the surroundingtissue becomes increasinglyconcentrated (hypertonic).

Chapter 58 Maintaining the Internal Environment 1189

Glomerulus

Inner medulla

Outer medulla

Cortex

Bowman'scapsule

Proximaltubule

Loop of Henle

Distal tubule

Collectingduct

UreaH2O

H2O

H2O

Na+

Cl–

Cl–Na+

H2O

H2O

300

600

1200

Tota

l sol

ute

conc

entr

atio

n (m

Osm

)

FIGURE 58.20The reabsorption of salt andwater in the mammalian kidney.Active transport of Na+ out of theproximal tubules is followed by thepassive movement of Cl– and water.Active extrusion of NaCl from theascending limb of the loop of Henlecreates the osmotic gradient requiredfor the reabsorption of water fromthe collecting duct. The changes inosmolality from the cortex to themedulla is indicated to the left of thefigure.

2. The NaCl pumped out of the ascending limb of theloop is trapped within the surrounding interstitialfluid. This is because the peritubular capillaries inthe medulla also have loops, called vasa recta, so thatNaCl can diffuse from the blood leaving themedulla to the blood entering the medulla. Thus,the vasa recta functions in a countercurrent ex-change, similar to that described for oxygen in thecountercurrent flow of water and blood in the gillsof fish (see chapter 53). In the case of the renalmedulla, the diffusion of NaCl between the bloodvessels keeps much of the NaCl within the intersti-tial fluid, making it hypertonic.

3. The descending limb is permeable to water, so waterleaves by osmosis as the fluid descends into the hy-pertonic renal medulla. This water enters the bloodvessels of the vasa recta and is carried away in thegeneral circulation.

4. The loss of water from the descending limb multi-plies the concentration that can be achieved at eachlevel of the loop through the active extrusion ofNaCl by the ascending limb. The longer the loop ofHenle, the longer the region of interaction betweenthe descending and ascending limbs, and the greaterthe total concentration that can be achieved. In ahuman kidney, the concentration of filtrate enteringthe loop is 300 milliosmolal, and this concentrationis multiplied to more than 1200 milliosmolal at thebottom of the longest loops of Henle in the renalmedulla.

Because fluid flows in opposite directions in the twolimbs of the loop, the action of the loop of Henle in creat-ing a hypertonic renal medulla is known as the countercur-rent multiplier system. The high solute concentration of therenal medulla is primarily the result of NaCl accumulationby the countercurrent multiplier system, but urea also con-tributes to the total osmolality of the medulla. This is be-cause the descending limb of the loop of Henle and thecollecting duct are permeable to urea, which leaves theseregions of the nephron by diffusion.

Distal Tubule and Collecting Duct

Because NaCl was pumped out of the ascending limb, thefiltrate that arrives at the distal convoluted tubule and en-ters the collecting duct in the renal cortex is hypotonic(with a concentration of only 100 mOsm). The collectingduct carrying this dilute fluid now plunges into themedulla. As a result of the hypertonic interstitial fluid ofthe renal medulla, there is a strong osmotic gradient thatpulls water out of the collecting duct and into surroundingblood vessels.

The osmotic gradient is normally constant, but the per-meability of the collecting duct to water is adjusted by ahormone, antidiuretic hormone (ADH, also called vaso-pressin), discussed in chapters 52 and 56. When an animal

needs to conserve water, the posterior pituitary gland se-cretes more ADH, and this hormone increases the numberof water channels in the plasma membranes of the collect-ing duct cells. This increases the permeability of the col-lecting ducts to water so that more water is reabsorbed andless is excreted in the urine. The animal thus excretes a hy-pertonic urine.

In addition to the regulation of water balance, the kid-neys regulate the balance of electrolytes in the blood byreabsorption and secretion. For example, the kidneys re-absorb K+ in the proximal tubule and then secrete anamount of K+ needed to maintain homeostasis into thedistal convoluted tubule (figure 58.21). The kidneys alsomaintain acid-base balance by excreting H+ into the urineand reabsorbing bicarbonate (HCO3

–), as previouslydescribed.

The loop of Henle creates a hypertonic renal medulla asa result of the active extrusion of NaCl from theascending limb and the interaction with the descendinglimb. The hypertonic medulla then draws waterosmotically from the collecting duct, which ispermeable to water under the influence of antidiuretichormone.

1190 Part XIV Regulating the Animal Body

H+

H+

H+

K+

K+

K+

K+

HCO3�

HCO3�

Filtered

Reabsorbed Secreted

Distalconvolutedtubule

FIGURE 58.21The nephron controls the amounts of K+, H+, and HCO3

–

excreted in the urine. K+ is completely reabsorbed in theproximal tubule and then secreted in varying amounts into thedistal tubule. HCO3

– is filtered but normally completelyreabsorbed. H+ is filtered and also secreted into the distal tubule,so that the final urine has an acidic pH.

Ammonia, Urea, and Uric AcidAmino acids and nucleic acids are nitrogen-containingmolecules. When animals catabolize these molecules forenergy or convert them into carbohydrates or lipids, theyproduce nitrogen-containing by-products called nitroge-nous wastes (figure 58.22) that must be eliminated fromthe body.

The first step in the metabolism of amino acids and nu-cleic acids is the removal of the amino (—NH2) group andits combination with H+ to form ammonia (NH3) in theliver. Ammonia is quite toxic to cells and therefore is safeonly in very dilute concentrations. The excretion of am-monia is not a problem for the bony fish and tadpoles,which eliminate most of it by diffusion through the gillsand less by excretion in very dilute urine. In elasmo-branchs, adult amphibians, and mammals, the nitrogenouswastes are eliminated in the far less toxic form of urea.Urea is water-soluble and so can be excreted in largeamounts in the urine. It is carried in the bloodstreamfrom its place of synthesis in the liver to the kidneyswhere it is excreted in the urine.

Reptiles, birds, and insects excrete nitrogenous wastes inthe form of uric acid, which is only slightly soluble inwater. As a result of its low solubility, uric acid precipitates

and thus can be excreted using very little water. Uric acidforms the pasty white material in bird droppings calledguano. The ability to synthesize uric acid in these groups ofanimals is also important because their eggs are encasedwithin shells, and nitrogenous wastes build up as the em-bryo grows within the egg. The formation of uric acid,while a lengthy process that requires considerable energy,produces a compound that crystallizes and precipitates. Asa precipitate, it is unable to affect the embryo’s develop-ment even though it is still inside the egg.

Mammals also produce some uric acid, but it is a wasteproduct of the degradation of purine nucleotides (see chap-ter 3), not of amino acids. Most mammals have an enzymecalled uricase, which converts uric acid into a more solublederivative, allantoin. Only humans, apes, and the dalmat-ian dog lack this enzyme and so must excrete the uric acid.In humans, excessive accumulation of uric acid in the jointsproduces a condition known as gout.

The metabolic breakdown of amino acids and nucleicacids produces ammonia as a by-product. Ammonia isexcreted by bony fish, but other vertebrates convertnitrogenous wastes into urea and uric acid, which areless toxic nitrogenous wastes.

Chapter 58 Maintaining the Internal Environment 1191

HN

NH

O

O

HN

NH

O

Ammonia

Mostfish

Mammals,some others

Reptilesand birds

Urea Uric acid

NH3 O C

NH2

NH2

FIGURE 58.22Nitrogenous wastes. When amino acids and nucleic acids are metabolized, the immediate by-product is ammonia, which is quite toxicbut which can be eliminated through the gills of teleost fish. Mammals convert ammonia into urea, which is less toxic. Birds and terrestrialreptiles convert it instead into uric acid, which is insoluble in water.

Hormones ControlHomeostatic FunctionsIn mammals and birds, the amount ofwater excreted in the urine, and thus theconcentration of the urine, varies ac-cording to the changing needs of thebody. Acting through the mechanismsdescribed next, the kidneys will excrete ahypertonic urine when the body needs toconserve water. If an animal drinks toomuch water, the kidneys will excrete ahypotonic urine. As a result, the volumeof blood, the blood pressure, and the os-molality of blood plasma are maintainedrelatively constant by the kidneys, nomatter how much water you drink. Thekidneys also regulate the plasma K+ andNa+ concentrations and blood pH withinvery narrow limits. These homeostaticfunctions of the kidneys are coordinatedprimarily by hormones (see chapter 56).

Antidiuretic Hormone

Antidiuretic hormone (ADH) is pro-duced by the hypothalamus and secreted by the posteriorpituitary gland. The primary stimulus for ADH secretionis an increase in the osmolality of the blood plasma. Theosmolality of plasma increases when a person is dehy-drated or when a person eats salty food. Osmoreceptors inthe hypothalamus respond to the elevated blood osmolal-ity by sending more nerve signals to the integration cen-ter (also in the hypothalamus). This, in turn, triggers asensation of thirst and an increase in the secretion ofADH (figure 58.23).

ADH causes the walls of the collecting ducts in the kid-ney to become more permeable to water. This occurs be-cause water channels are contained within the membranesof intracellular vesicles in the epithelium of the collectingducts, and ADH stimulates the fusion of the vesicle mem-brane with the plasma membrane, similar to the process ofexocytosis. When the secretion of ADH is reduced, theplasma membrane pinches in to form new vesicles that con-tain the water channels, so that the plasma membrane be-comes less permeable to water.

Because the extracellular fluid in the renal medulla is hy-pertonic to the filtrate in the collecting ducts, water leavesthe filtrate by osmosis and is reabsorbed into the blood.Under conditions of maximal ADH secretion, a person ex-cretes only 600 milliliters of highly concentrated urine perday. A person who lacks ADH due to pituitary damage hasthe disorder known as diabetes insipidus and constantly ex-

cretes a large volume of dilute urine. Such a person is indanger of becoming severely dehydrated and succumbingto dangerously low blood pressure.

Aldosterone and Atrial Natriuretic Hormone

Sodium ion is the major solute in the blood plasma. Whenthe blood concentration of Na+ falls, therefore, the bloodosmolality also falls. This drop in osmolality inhibits ADHsecretion, causing more water to remain in the collectingduct for excretion in the urine. As a result, the blood vol-ume and blood pressure decrease. A decrease in extracellu-lar Na+ also causes more water to be drawn into cells byosmosis, partially offsetting the drop in plasma osmolaritybut further decreasing blood volume and blood pressure. IfNa+ deprivation is severe, the blood volume may fall solow that there is insufficient blood pressure to sustain life.For this reason, salt is necessary for life. Many animalshave a “salt hunger” and actively seek salt, such as the deerat “salt licks.”

A drop in blood Na+ concentration is normally compen-sated by the kidneys under the influence of the hormone al-dosterone, which is secreted by the adrenal cortex. Aldos-terone stimulates the distal convoluted tubules to reabsorbNa+, decreasing the excretion of Na+ in the urine. Indeed,under conditions of maximal aldosterone secretion, Na+

may be completely absent from the urine. The reabsorp-

1192 Part XIV Regulating the Animal Body

58.4 The kidney is regulated by hormones.

—

Dehydration

Increased osmolalityof plasma

Posteriorpituitary gland

IncreasedADH secretion

Increased reabsorptionof water

Increasedwater intake

ThirstOsmoreceptorsin hypothalamus

Negative feedback

FIGURE 58.23Antidiuretic hormone stimulates the reabsorption of water by the kidneys. Thisaction completes a negative feedback loop and helps to maintain homeostasis of bloodvolume and osmolality.

tion of Na+ is followed by Cl– and by water, so aldosteronehas the net effect of promoting the retention of both saltand water. It thereby helps to maintain blood volume andpressure.

The secretion of aldosterone in response to a de-creased blood level of Na+ is indirect. Because a fall inblood Na+ is accompanied by a decreased blood volume,there is a reduced flow of blood past a group of cellscalled the juxtaglomerular apparatus, located in the re-gion of the kidney between the distal convoluted tubuleand the afferent arteriole (figure 58.24). The juxta-glomerular apparatus responds by secreting the enzymerenin into the blood, which catalyzes the production ofthe polypeptide angiotensin I from the protein an-giotensinogen (see chapter 52). Angiotensin I is thenconverted by another enzyme into angiotensin II, whichstimulates blood vessels to constrict and the adrenal cor-tex to secrete aldosterone. Thus, homeostasis of bloodvolume and pressure can be maintained by the activationof this renin-angiotensin-aldosterone system.

In addition to stimulating Na+ reabsorption, aldosteronealso promotes the secretion of K+ into the distal convolutedtubules. Consequently, aldosterone lowers the blood K+

concentration, helping to maintain constant blood K+ levelsin the face of changing amounts of K+ in the diet. Peoplewho lack the ability to produce aldosterone will die if un-treated because of the excessive loss of salt and water in theurine and the buildup of K+ in the blood.

The action of aldosterone in promoting salt and waterretention is opposed by another hormone, atrial natriuretichormone (ANH, see chapter 52). This hormone is secretedby the right atrium of the heart in response to an increasedblood volume, which stretches the atrium. Under theseconditions, aldosterone secretion from the adrenal cortexwill decrease and atrial natriuretic hormone secretion willincrease, thus promoting the excretion of salt and water inthe urine and lowering the blood volume.

ADH stimulates the insertion of water channels into thecells of the collecting duct, making the collecting ductmore permeable to water. Thus, ADH stimulates thereabsorption of water and the excretion of a hypertonicurine. Aldosterone promotes the reabsorption of NaCland water across the distal convoluted tubule, as well asthe secretion of K+ into the tubule. ANH decreasesNaCl reabsorption.

Chapter 58 Maintaining the Internal Environment 1193

Low bloodvolume

Low bloodflow

Bowman'scapsule

Distalconvolutedtubule

Proximalconvolutedtubule

Glomerulus

Afferentarteriole

Efferentarteriole

Loopof Henle

Renin

Adrenalcortex

Kidney

Angiotensinogen

Angiotensin II

Aldosterone

Increased NaCland H2O

reabsorption

Negativefeedback

1

2

3

4

5

6

78

9

Juxtaglomerularapparatus

FIGURE 58.24A lowering of blood volumeactivates the renin-angiotensin-aldosterone system. (1) Lowblood volume accompanies adecrease in blood Na+ levels.(2) Reduced blood flow past thejuxtaglomerular apparatus triggers(3) the release of renin into theblood, which catalyzes theproduction of angiotensin I fromangiotensinogen. (4) AngiotensinI converts into an active form,angiotensin II. (5) Angiotensin IIstimulates blood vesselconstriction and (6) the release ofaldosterone from the adrenalcortex. (7) Aldosterone stimulatesthe reabsorption of Na+ in thedistal convoluted tubules.(8) Increased Na+ reabsorption isfollowed by the reabsorption ofCl- and water. (9) This increasesblood volume. An increase inblood volume may also trigger therelease of atrial natriuretichormone that inhibits the releaseof aldosterone. These twosystems work together tomaintain homeostasis.

1194 Part XIV Regulating the Animal Body

Chapter 58 Summary Questions Media Resources

58.1 The regulatory systems of the body maintain homeostasis.

• Negative feedback loops maintain nearly constantextracellular conditions in the internal environmentof the body, a condition called homeostasis.

• Antagonistic effectors afford an even finer degree ofcontrol.

1. What is homeostasis? Whatis a negative feedback loop? Givean example of how homeostasisis maintained by a negativefeedback loop.

www.mhhe.com/raven6e www.biocourse.com

• Osmoconformers maintain a tissue fluid osmolalityequal to that of their environment, whereasosmoregulators maintain a constant blood osmolalitythat is different from that of their environment.

• Insects eliminate water by secreting K+ intoMalpighian tubules and the water follows the K+ byosmosis.

• The kidneys of most vertebrates eliminate water byfiltering blood into nephron tubules.

• Freshwater bony fish are hypertonic to theirenvironment, and saltwater bony fish are hypotonicto their environment; these conditions place differentdemands upon their kidneys and other regulatorysystems.

• Birds and mammals are the only vertebrates that haveloops of Henle and thus are capable of producing ahypertonic urine.

2. What is the differencebetween an osmoconformer andan osmoregulator? What areexamples of each?3. How does the body fluidosmolality of a freshwatervertebrate compare with that ofits environment? Does watertend to enter or exit its body?What must it do to maintainproper body water levels?4. In what type of animal areMalpighian tubules found? Bywhat mechanism is fluid causedto flow into these tubules? Howis this fluid further modifiedbefore it is excreted?

58.2 The extracellular fluid concentration is constant in most vertebrates.

• The primary function of the kidneys is homeostasis ofblood volume, pressure, and composition, includingthe concentration of particular solutes in the bloodand the blood pH.

• Bony fish remove the amine portions of amino acidsand excrete them as ammonia across the gills.

• Elasmobranchs, adult amphibians, and mammalsproduce and excrete urea, which is quite soluble butmuch less toxic than ammonia.

• Insects, reptiles, and birds produce uric acid from theamino groups in amino acids; this precipitates, so thatlittle water is required for its excretion.

5. What drives the movement offluid from the blood to theinside of the nephron tubule atBowman’s capsule? 6. In what portion of thenephron is most of the NaCl andwater reabsorbed from thefiltrate? 7. What causes waterreabsorption from the collectingduct? How is this influenced byantidiuretic hormone?

58.3 The functions of the vertebrate kidney are performed by nephrons.

• Antidiuretic hormone is secreted by the posteriorpituitary gland in response to an increase in bloodosmolality, and acts to increase the number of waterchannels in the walls of the collecting ducts.

8. What effects does aldosteronehave on kidney function? How isthe secretion of aldosteronestimulated?

58.4 The kidney is regulated by hormones.

• Osmoregulation

• Body fluid distribution• Water balance

• Bioethics case study:Kidney transplant

• Art activitiesUrinary systemAnatomy of kidneyand lobeNephron anatomy

• Kidney function

• Kidney function