The chemistry of the human body is a marvelous thing. An

intricate system of checks and balances controls everything

from your blood pressure to your body temperature. Nutrients

are absorbed, stored, and carefully released when they are

needed. However, every living system, including the human

body, produces chemical waste products that are not useful to

the body. In fact, some waste products are so toxic that they will

cause death if they are not eliminated.

Functions of the Excretory SystemYou might think that homeostasis involves the body’s efforts

to respond only to changes in the external environment. However,

homeostasis also requires the body to deal with internal processes

that might upset the internal cellular environment. For example,

as a normal consequence of being alive, every cell in the body

produces metabolic wastes, such as excess salts, carbon dioxide,

and urea. Urea is a toxic compound that is produced when amino

acids are used for energy. The process by which these metabolic

wastes are eliminated is called excretion. Excretion is one part of

the many processes that maintain homeostasis.

You have already learned about two organs of excretion—

the skin and the lungs. The skin excretes excess water and salts,

as well as a small amount of urea, in the form of sweat. The

lungs excrete carbon dioxide, a gas produced when energy is

captured from compounds in foods.

The liver, which we normally think of as a digestive organ,

also plays a number of important roles in excretion.

When cells of the body break down proteins, excess amino

acids are released into the bloodstream. The liver takes

up these amino acids and converts them into other useful

compounds, producing nitrogen wastes in the process.

The liver quickly converts these potentially poisonous

nitrogen compounds into urea. Urea, in turn, is removed

from the bloodstream along with other metabolic wastes

by the body ’s principal organs of excretion, the kidneys.

The kidneys play an important role in main-

taining homeostasis. They remove waste products

from the blood; maintain blood pH; and regulate

the water content of the blood and, therefore,

blood volume.

Key Concepts• What are the functions of the

kidneys?• How is blood filtered?

Vocabularykidneyureterurinary bladdernephronfiltrationglomerulusBowman’s capsulereabsorptionloop of Henleurethra

Reading Strategy: Building VocabularyBefore you read, preview Figure 38–17 to identifyvocabulary with which you areunfamiliar. Look for the mean-ings of these terms as you read.

� Figure 38–16 As part of theexcretory system, the skin excreteswater, salts, and urea in sweat.

38–3 The Excretory System

1 FOCUSObjectives38.3.1 Identify the functions of the

kidneys.38.3.2 Explain how blood is filtered.

Vocabulary PreviewPoint out that all but two of theVocabulary terms are parts of theexcretory system. The other twoterms refer to processes of the excre-tory system. Ask: Which two termsrefer to processes? (Filtration andreabsorption) Have students predictwhat these two terms meanand check to see if they were correctafter they read the section.

Reading StrategySuggest that students outline the sec-tion by first writing the headings andsubheadings on a separate sheet ofpaper and then filling in importantdetails as they read.

2 INSTRUCT

Functions of theExcretory SystemDemonstrationBefore chemical wastes are excretedfrom the body, they must beremoved from individual cells.Demonstrate the importance of thisprocess by modeling a cell with aballoon. Attach the balloon to afaucet, and gradually add water. Asthe balloon fills up, explain that thisis what would happen to a cell if itcould not eliminate its waste prod-ucts. Ask: What would happen tothe cell if the amount of wasteproducts continued to increase?(The cell would swell until it burst.)

Digestive and Excretory Systems 985

Section 38–3

SECTION RESOURCES

Print:

• Laboratory Manual A, Chapter 38 Lab• Teaching Resources, Lesson Plan 38–3,

Adapted Section Summary 38–3, SectionSummary 38–3, Worksheets 38–3, SectionReview 38–3

• Reading and Study Workbook A, Section 38–3• Adapted Reading and Study Workbook B,

Section 38–3

• Issues and Decision Making, Issues andDecisions 40

Technology:• iText, Section 38–3• Animated Biological Concepts DVD,

41 Kidney Function• Transparencies Plus, Section 38–3

Tim

eSaver

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The KidneysThe are located on either side of the spinal column near

the lower back. A tube, called the (yoo-REET-ur), leaves

each kidney, carrying urine to the urinary bladder. The

is a saclike organ where urine is stored before being

excreted. The structures of the kidney are shown in Figure 38–17.What does a kidney do? As waste-laden blood enters the

kidney through the renal artery, the kidney removes urea,

excess water, and other waste products and passes them to the

ureter. The clean, filtered blood leaves the kidney through the

renal vein and returns to circulation.

Kidney Structure If a kidney is cut in half, two distinct

regions can be seen. The inner part is called the renal medulla.

The outer part is called the renal cortex. The functional units of

the kidney are called (NEF-rahnz). Each nephron is

a small, independent processing unit. Nephrons are located in

the renal cortex, except for their loops of Henle, which descend

into the renal medulla.

nephrons

bladderurinary

ureterkidneys

Kidneys are made up of nephrons. Blood enters the nephron, where impurities arefiltered out and emptied into the collecting duct. The purified blood leaves the nephronthrough the renal vein.

NephronKidney

Cortex

Medulla

Capillaries

Vein

Renal�artery

Renal vein

Ureter

To the bladder

Bowman’s�capsule

Glomerulus

Collecting�duct

To the ureter

Loop of Henle

Artery

FIGURE 38–17 STRUCTURE OF THE KIDNEYS

986 Chapter 38

The KidneysUse VisualsFigure 38–17 Explain that eachnephron is about 3 cm long and only0.03 mm in diameter, so it is too thinto be seen with the unaided eye.Also, explain that each kidney isabout 10 cm long and 6 cm in diam-eter and contains about a millionnephrons. Ask: How do the sizes of the kidney and nephron in thefigure compare with their actualsizes? (The kidney in the drawing issomewhat smaller than its actual size.The nephron in the drawing is muchlarger than its actual size.) Suggestthat students locate each part of thekidney and nephron in the figure asthey read about it in the text.

Build Science SkillsUsing Models Explain that thefunction of the kidney is to filter outwastes and other substances fromthe blood. Model how the kidneyworks by pouring water mixed withfood coloring, sand, and silt througha paper coffee filter. Invite studentsto examine the contents of the coffeefilter and the colored water thatemerges from it. Ask: How are kid-neys and coffee filters similar?(Both filter out substances from afluid.) How are they different? (Thekidneys filter out water and dissolvedmaterials, whereas the coffee filter fil-ters out only solids. The kidneys alsoreturn some of the filtered materials tothe fluid.)

38–3 (continued)

Inclusion/Special NeedsHave students construct a three-dimensionalmodel of the excretory system, including thekidneys, renal blood vessels, ureter, and urinarybladder. They can use dry pasta, legumes, cereal, clay, string, or other suitable materials to represent the different parts of the system.Ask students to explain what each part of theircompleted model represents.

English Language LearnersHelp students interpret Figure 38–17 so thatthey will not need to rely on the text as much tounderstand the kidney. Point out the parts ofthe kidney. Then, have students trace the paththrough the nephron as you describe howblood enters through the artery; impurities arefiltered out in the glomerulus, pass through theloop of Henle, and empty into the collectingduct; purified blood exits through the vein.

UNIVERSAL ACCESS

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SE page reduction 80%

Each nephron has its own blood supply: an arteriole, a

venule, and a network of capillaries connecting them. In addi-

tion, each nephron releases fluids to a collecting duct, which

leads to the ureter. As blood enters a nephron through

the arteriole, impurities are filtered out and emptied into

the collecting duct. The purified blood exits the nephron

through the venule. The mechanism of blood purification

involves two distinct processes: filtration and reabsorption.

What are the two parts of a kidney?

Filtration Passing a liquid or gas through a filter to remove

wastes is called The filtration of blood mainly takes

place in the glomerulus (gloh-MUR-yoo-lus). The is

a small network of capillaries encased in the upper end of the

nephron by a hollow, cup-shaped structure called

A glomerulus is shown in Figure 38–18.Because the blood is under pressure and the walls of the

capillaries and Bowman’s capsule are permeable, much of the fluid

from the blood flows into Bowman’s capsule. The materials that

are filtered from the blood are collectively called the filtrate. The

filtrate contains water, urea, glucose, salts, amino acids, and some

vitamins. Because plasma proteins, cells, and platelets are too large

to pass through the capillary walls, they remain in the blood.

Reabsorption The kidneys filter all the blood in the body

approximately every 45 minutes. Needless to say, not all of the

filtrate is excreted. Most of the material removed from the blood

at Bowman’s capsule makes its way back into the blood. The

process in which liquid is taken back into a vessel is called

A number of materials, including amino acids, fats, and

glucose, are removed from the filtrate by active transport and

reabsorbed by the capillaries. Because water follows these

materials by osmosis, almost 99 percent of the water that enters

Bowman’s capsule is reabsorbed into the blood. When the

filtrate drains in the collecting ducts, most of the water and

nutrients have been reabsorbed into the blood.

Urine Formation The material that remains,

called urine, is emptied into a collecting duct. Urine,

which contains urea, excess salts, and water, among

other substances, is primarily concentrated in the loop

of Henle. The is a section of the

nephron tubule in which water is conserved and the

volume of urine minimized.

As the kidney works, purified blood is returned to

circulation while urine is collected in the urinary

bladder. Urine is stored in the urinary bladder until it

can be released from the body through a tube called

the (yoo-REE-thruh).urethra

loop of Henle

reabsorption.

capsule.

Bowman’s

glomerulus

filtration.

� Figure 38–18 Blood enterseach nephron through a ball ofcapillaries called a glomerulus.

The glomerulus is wherefiltration takes place.

(magnification: 185�)

DemonstrationObtain a beef or lamb kidney from abutcher and display it to the class.Ask: How might this kidney be dif-ferent from a human kidney? (Itmight be different in size, dependingon the animal it came from, but itsstructure and function should be similar to those of a human kidney.)Point out the renal artery, renal vein,and ureter. Ask: What roles do thesestructures play? (The renal artery carries blood with impurities to the kid-ney, the renal vein carries purified bloodaway from the kidney, and the uretercarries urine from the kidney to thebladder.) Cut the kidney lengthwiseand point out the renal medulla andrenal cortex. Ask: Where are thenephrons located? (In the renal cortex,except for their loops of Henle)

Build Science SkillsInferring Ask students: Why is somuch of the material that is fil-tered from blood returned to theblood? (Because it consists of sub-stances such as nutrients and waterthat the body needs)

Digestive and Excretory Systems 987

Maple syrup urine and other diseasesSome people have urine that smells like maplesyrup. Others have urine that turns black when itis exposed to air. The reason? They have disordersof protein metabolism—maple syrup urine diseaseor alkaptonuria, respectively—that result in thekidneys excreting substances that are not nor-mally found in the urine. Diagnosing diseases byanalyzing urine is called urinalysis. It involves

examination of the urine chemically, physically,and microscopically. Urinalysis is one of the mostcommonly performed medical laboratory proce-dures because the composition of the urinereflects the status of many different body func-tions. For example, in diabetes mellitus thereoften is sugar in the urine because the excesssugar in the blood is filtered out by the kidneysand excreted in the urine.

FACTS AND FIGURES

Answer to . . . The renal medulla, which

is the inner part, and the renal cortex,which is the outer part

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Urine Testing When blood is filtered through the kidneys,

small molecules, including salts, amino acids, sugars, and many

drugs, are removed from the circulation. Although many of these

are reabsorbed into the bloodstream, drugs generally remain in

the filtrate and are eliminated in the urine. This is one of the

principal reasons why the effects of many drugs, including

antibiotics, wear off over time. This also means that drugs, legal

and illegal, become concentrated in the urine, providing a quick

and easy way to test for their presence. Urine testing is now

done routinely to check for the presence of prohibited drugs in

athletes. It has also become common for employers to use such

tests to screen job applicants for illegal drug use.

Kidney Stones Sometimes substances such as calcium,

magnesium, or uric acid salts in the urine crystallize and form

kidney stones. When kidney stones block the ureter, they cause

great pain. Kidney stones are often treated using ultrasound

waves. The sound waves pulverize the stones into smaller

fragments, which are eliminated with the urine.

Control of Kidney FunctionTo a large extent, the activity of the kidneys is controlled by the

composition of blood itself. In addition, regulatory hormones are

released in response to the composition of blood. These mecha-

nisms combine to ensure that the kidneys will maintain the

proper composition of blood.

When you drink glass after glass of liquid, the liquid is

quickly absorbed into the blood through the digestive system. As

a result, the concentration of water in the blood increases. If it

were not for your kidneys, this increased concentration of water

in the blood would force water into cells and tissues by osmosis,

causing your body to swell.

As the amount of water in the blood increases, the rate of

water reabsorption in the kidneys decreases. Less water is

returned to the blood, and the excess water is sent to the uri-

nary bladder to be excreted as urine.

If you eat salty food, your kidneys will respond to the

increased level of salt in your blood. When your kidneys detect

an increase in salt, they respond by returning less salt to the

blood by reabsorption. The excess salt the kidneys retain is

excreted in urine, thus maintaining the composition of the blood.

Homeostasis by MachineThe kidneys are the master chemists of the blood supply. If

anything goes wrong with the kidneys, serious medical prob-

lems soon follow. Fortunately, humans have two kidneys and can

survive with only one. If both kidneys are damaged by disease

or injury, however, there are only two ways to keep an individual

alive. The first way is to transplant a healthy kidney from a

compatible donor to the person in need of the kidney.

For: Links on the excretory system

Visit: www.SciLinks.orgWeb Code: cbn-0383

NSTA

988 Chapter 38

Bright’s disease and Bowman’s capsuleThe first person to use urinalysis to diagnose dis-ease was an English physician named RichardBright. In 1829, Bright discovered that peoplewho suffer from “dropsy,” or what we now calledema, have a substance in their urine that coag-ulates, just as egg white does, when the urine isboiled. The substance is now known to be serumalbumin, and it does not normally appear in the

urine. When it does, it is a sign that the patienthas nephritis, or inflammation of the kidneys.Because of Bright’s hallmark work, nephritis is stillsometimes referred to as Bright’s disease. Anothernineteenth-century English physician, Sir WilliamBowman, discovered the capsules in the kidneythat were subsequently named after him.Bowman also discovered how urine is producedby filtration.

HISTORY OF SCIENCE

Control of KidneyFunctionBuild Science SkillsInferring Help students build infer-ring skills and increase theirunderstanding of how the kidneysmaintain homeostasis. Ask: Why isyour urine lighter in color after youdrink a lot of fluids? (The kidneysremove the excess water from theblood and excrete it in the urine, mak-ing the urine less concentrated andlighter in color.) Why might yoururine be darker in color after youhave been sweating a lot? (The kid-neys remove less water from the bloodto compensate for the water lost insweat, making the urine more concen-trated and darker in color.)

Make ConnectionsHealth Science Relate the kidneys’ability to maintain homeostasis tohealth issues. Tell students that therole of the kidneys is essential to life,yet people can live long, healthy liveswith just one kidney. Ask: Why cansomeone function normally withjust one kidney? (Students shouldinfer that the one kidney can accom-modate the greater workload placed onit by filtering out more substances fromthe blood.)

38–3 (continued)

NSTA

Download a worksheet on the excretory system for studentsto complete, and find additionalteacher support from NSTA SciLinks.

0970_0989_bi_c06_te 3/9/06 1:47 PM Page 988

A second way is used when a donor is not available or

surgery is not advisable. In these instances, a kidney dialysis

machine becomes a lifesaver. In a common form of dialysis,

blood is removed from the body through a tube inserted in the

arm and pumped through special tubing that acts like

nephrons. Tiny pores in the tubing allow salts and small mol-

ecules, including nitrogen wastes, to pass through. Wastes—

urea and excess salts—diffuse out of the blood into the

fluid-filled chamber, allowing purified blood to be returned to

the body. This process of dialysis is shown in Figure 38–19.Dialysis is not only expensive, but it also is time-consuming,

occupying several hours a day as often as three times a week.

The ideal solution, assuming a kidney transplant is not possible,

would be the implantation of an artificial kidney. Medical

science is working toward developing such an artificial kidney.

Vein

Artery

Shunt

Blood pump

Blood in tubing flowsthrough dialysis fluid

Used dialysis fluid

Dialysismachine

Compressedair

Freshdialysisfluid

Air detector

� Figure 38–19 For people withdamaged kidneys, dialysis machinescan perform many of the functionsof the kidneys. ApplyingConcepts Why is dialysis such animportant lifesaving technique?

1. Key Concept What arethe functions of the kidneys?

2. Key Concept Describehow blood is purified.

3. Describe the structures of akidney.

4. What is the role of the skin inexcretion?

5. Critical Thinking InferringWhen there is too much fluid inthe blood, the heart must pumpharder. Diuretics are substancesthat stimulate the kidneys toremove more fluid from thebody. Why do you think diureticsare often prescribed as a treat-ment for high blood pressure?

38–3 Section AssessmentConstructing a FlowchartConstruct a flowchart thatillustrates how wastes areremoved by the kidneys. Besure to include the termsBowman’s capsule, loop ofHenle, capillaries, collectingduct, and ureter.

Homeostasis by MachineUse Community ResourcesInvite a dialysis technician or nurse tospeak to the class about dialysis.Suggest that the speaker addresssuch issues as how effective dialysis iscompared with normal kidney func-tion, who needs dialysis and why,and what it is like to receive dialysistreatments. Have students take notesduring the presentation and later usethem to write a summary of whatthey learned.

3 ASSESSEvaluate UnderstandingCall on students at random to statethe functions of the different parts ofthe kidney and nephron that arehighlighted in the text. Call on otherstudents to correct any errors.

ReteachHave students use Figure 38–17 totrace the path of blood into and outof the kidney and to trace the path ofurine from the kidney to the urethra.

Answer to . . .

Figure 38–19 Without dialysis,wastes in the blood would soon reachtoxic levels, and the person could die.

Provide students with large sheetsof paper and colored pens ormarkers. Flowcharts shoulddemonstrate an understanding ofhow the kidney removes wastessuch as urea from blood.

If your class subscribes to the iText,use it to review the Key Concepts inSection 38–3.

38–3 Section Assessment1. Kidneys remove waste products from blood,

maintain blood pH, and regulate water con-tent and volume of blood.

2. Blood enters a nephron through the arteriole;impurities are filtered out and emptied intothe collecting duct; purified blood exitsthrough the venule.

3. Renal medulla: inner part of kidney; renal cor-tex: outer part of kidney; nephron: functionalunit of kidney; renal artery and vein: blood

vessels entering and leaving kidney; glomeru-lus: small network of capillaries in nephron;Bowman’s capsule: hollow, cup-shaped struc-ture in nephron; loop of Henle: section ofnephron tubule; collecting duct: tube fortransferring waste

4. The skin excretes water, salts, and urea insweat.

5. Because diuretics reduce the volume of bloodand, thereby, lower blood pressure

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