Excretion and
Water Balance
13
After you have finished reading this chapter, you should be able to:
Discuss
the importance of the kidneys, skin, and liver to excretion and
to homeostasis.
Compare and contrast the process of excretion in different organisms.
Describe
the structures and functions of kidneys in humans.
Day after day, day after day,
We struck, nor breath nor motion;
As idle as a painted ship
Upon a painted ocean.
Water, water, everywhere,
And all the boards did shrink;
Water, water, everywhere
Nor any drop to drink.
Samuel Taylor Coleridge,
The Rime of the Ancient Mariner
Introduction
Imagine yourself in the middle of a calm sea. No winds blow. Your sailing ship is unable to move. You have been at this place in the ocean for
weeks. Food and water have been used up. You and your fellow sailors
are becoming very thirsty. In every direction, stretching as far as the horizon, all you can see is water. And yet, as the Ancient Mariner said in the
poem by Coleridge, there is not a “drop to drink.” How could this be?
(See Figure 13-1 on page 276.)
Ocean water is salty. When dried, ocean salt is almost chemically identical to table salt. Salt, a solute, is only one of many substances that can
be dissolved in water. This is true of the water in the human body. The
275
Figure 13-1 You
can be sailing in
the middle of an
ocean and still not
have a “drop to
drink,” since
seawater is salty,
and we can drink
only freshwater.
human body is about 70 percent water. This water contains many types
of solutes, including table salt. The amounts of these solutes in our bodies must remain constant day by day, and hour by hour. We have seen
how regulating our internal body temperature is an important example of
homeostasis. An even more important function of homeostasis is maintaining a constant internal chemical environment. Drinking salt water
disrupts the careful balance that our bodies need internally. The results
could be very dangerous, perhaps even fatal. We cannot drink salt water,
only freshwater.
■■ WATER COMPARTMENTS
In the body, water is found in three areas, or compartments:
◆ Plasma, the liquid portion of the blood without the blood cells, makes
up about 7 percent of body fluid.
◆ The intercellular fluid and lymph make up about 28 percent of body
fluid.
◆ The intracellular compartment, the fluid located inside cells, makes
up about 65 percent of body fluid.
The real meaning of the cell theory is that we are only as healthy as
our cells are. As you might expect, the chemical contents of our cells are
therefore regulated carefully. We have seen that all body cells are surrounded by pools of intercellular fluid, which is mostly water. Because
the level of chemicals in cells must remain within very narrow limits, it
is essential that the composition of the intercellular fluid remains within
Chapter 13 / Excretion and Water Balance 277
Chemical composition of body fluids
narrow limits, too. How is the chemical composition of the intercellular
fluid regulated? As you know, capillaries deliver blood to every cell in the
body. Capillaries exchange materials with the intercellular fluids that surround each cell. It is the job of the blood, and more specifically the blood
plasma, to control the levels of substances in the fluid that surrounds each
cell. So, one of the most important jobs of homeostasis is regulating the
chemical composition of blood plasma. (See Figure 13-2.)
+
Healthy changes
Unhealthy changes
Normal
–
Figure 13-2 If we are to
remain healthy, the
composition of our body
fluids must remain within
very narrow limits.
In terrestrial vertebrates, including ourselves, the organ primarily
LIVINGfor
ENVIRONMENT
2e/fig. 13-2
s/s
responsible
regulating BIOLOGY,
the chemical
composition
of plasma is the
kidney. Approximately 25 percent of the total blood flow in the body
passes through the kidneys at any moment, even though the kidneys
make up only 1 percent of the body’s total weight. When kidneys do not
work properly, it results in blood that does not have the proper balance
of chemicals. In time, this can cause death, unless the kidneys’ functions
are taken over by mechanical means. Let us now examine what the kidneys do and learn how they function.
■■ THE KIDNEYS
A single bag of potato chips contains enough salt to kill you. Of course it
doesn’t, because your body does not allow this salt to be added directly
to your body’s fluids. One of the jobs of the kidneys is to maintain the
proper levels of both salt and water in the body. Improper levels of salt
cause a number of potential problems. The sodium and chloride ions of
salt can disrupt normal chemical activity. For example, if for some reason
your body did increase the level of salt in its fluids, your heart might no
longer beat properly. Salt also affects the rate at which osmosis occurs.
Water diffuses across a cell membrane toward an area of higher salt concentration. Therefore, the job of regulating both salt and water levels is
278 Maintaining a Dynamic Equilibrium
often called maintaining a water balance in the body. This is one of the
functions of the kidneys.
The kidneys are also part of the sanitation system of the body. All
metabolic activities in the body produce wastes. These wastes are toxic
and must not be allowed to accumulate in the body. So another important function of the kidneys is to carefully select the chemical wastes for
removal while keeping useful nutrients. This process of ridding the body
of metabolic wastes is called excretion.
■■ WASTES THAT NEED TO BE REMOVED
You have already learned that cellular respiration produces carbon dioxide and water. The waste, carbon dioxide (CO2), is released from the body
primarily through gas exchange that occurs in the respiratory system.
Carbon dioxide leaves the blood and enters the air in the alveoli of the
lungs. Other wastes produced during metabolic activities include water
from condensation synthesis and a variety of mineral salts from other
metabolic processes.
However, by far the most important and potentially dangerous metabolic wastes produced in the body contain the element nitrogen. These
nitrogenous wastes result when amino acids, the building blocks of proteins, are broken down. The main nitrogenous waste produced during
this process is ammonia (NH3). (See Figure 13-3.) Ammonia is a toxic
compound that can kill living cells. Open a bottle of household cleanser
that contains ammonia and you will smell its unpleasant characteristic
odor.
All animals produce ammonia in their cells as they metabolize proteins. How do they get rid of this toxic waste? For an ameba, and some
aquatic animals, ammonia diffuses into the water through the cell membrane. Other animals cannot get rid of ammonia so easily. These animals
must change ammonia into a less harmful substance. Birds, terrestrial reptiles, and insects change ammonia into solid crystals of uric acid. This
uric acid is mixed with solid, undigested wastes from the digestive system and passed out of the body. Any person trying to avoid being hit by
bird droppings is trying to steer clear of these wastes. Nitrogen is a good
fertilizer for plants, however. In some places, where large numbers of birds
roost, these nitrogen-rich wastes accumulate in vast amounts. In the past,
these droppings were shoveled into bags and sold for high prices as a
potent plant fertilizer. Today, these natural sources of nitrogen have been
replaced with nitrogen from ores that have been processed in factories.
Mammals have evolved a different solution for dealing with ammonia
Chapter 13 / Excretion and Water Balance 279
Tanks-a-Lot
Ammonia
Out!
TLY
INSTAN ES
FI
DETOXI A!
AMMONI
WORKS INSTANTLY TO:
Detoxify Ammonia Formed
by Fish Waste
Eliminate Ammonia Stress;
Protect Healthy Gill Function
Remove Chlorine and
Chloramine from Tap Water
Treats 60 Gallons (227 L) of
Freshwater or Salt Water
HAPPY FISH PRODUCTS, INC.
NET 1 FL OZ. (30 ml)
PRODUCT NO. 13
Figure 13-3 All animals produce ammonia—a
toxic compound—when they metabolize amino
acids. Ammonia that is formed by fish waste in
an aquarium must be chemically treated to
prevent damage.
wastes. In mammals, ammonia is mostly changed into urea, not uric acid.
Unlike uric acid, which does not dissolve in water, urea is very soluble in
water. To be removed from the body, urea must be dissolved in water. The
ammonia, now in the form of urea, is diluted and removed from the body
in urine.
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 13-3 s/s (rev. 10/13/03)
■■ EVOLUTION AND EXCRETION
A variety of solutions to the problem of maintaining a constant internal
chemical environment can be seen in different organisms. We have seen
that single-celled organisms, such as an ameba and a paramecium, rid
themselves of wastes by simple diffusion across their cell membrane.
However, these protists have evolved a structure that handles the problem
of maintaining water balance. These protists live in freshwater, where the
level of solutes inside them is higher than in the external freshwater. As
a result, water tends to diffuse into these organisms. A unique organelle,
the contractile vacuole, takes in this excess water in the cell and pumps
it out of the organism and back into the environment. In this way, the
proper level of water is maintained in the cell. Because the diffusion of
280 Maintaining a Dynamic Equilibrium
Figure 13-4 The paramecium rids itself
of wastes by diffusion across its cell
membrane. The contractile vacuole aids
it in osmoregulation (see star-shaped
organelle in upper half of paramecium).
water is called osmosis, the process of maintaining water balance is called
osmoregulation. (See Figure 13-4.)
Some multicellular animals, such as sponges, sea anemones, and sea
stars, have no special organs for osmoregulation and excretion. Instead,
the exchange of water and wastes between their cells and the environment occurs by diffusion through the membranes of the cells on their
body surfaces.
Most other animals, from earthworms to crabs to polar bears, have
some type of internal organ that contains tubelike structures to carry out
chemical regulation. By examining this structure in earthworms, we can
learn about the kidneys in humans.
One thing you notice when you examine an earthworm is its segments
—often, more than 100 of them. Each segment contains a pair of twisting tubes, each called a nephridium. The tube has an opening into the
previous segment. The earthworm’s body fluids pass into the opening. As
these fluids move through the nephridia (tubes), useful materials are
absorbed out of the tubes and into the capillaries that surround the tubes.
The useful materials stay in the earthworm’s blood. The wastes remain in
the nephridia and pass out to the external environment through openings
in the earthworm’s body wall. (See Figure 13-5.)
The waste gas CO2 diffuses directly from capillaries just below the
earthworm’s moist skin cells into the air between particles of soil. This
reminds us of the process of excretion that occurs in the alveoli in the
human lungs. During and just after a soaking rain, earthworms crawl out
of their burrows in the ground onto the soil’s surface. When the soil is saturated with water, an earthworm cannot get rid of waste gases. It cannot
take in oxygen either. If it remains in wet soil, it might suffocate.
Chapter 13 / Excretion and Water Balance 281
Blood vessel
Liquid wastes
Body
cavity
Capillaries
Septum
Opening
to outside
Ciliated
funnel
Excretory tube
Skin
Figure 13-5
Nephridia tubes
remove wastes
from the body
fluids of the
earthworm.
Having seen the structure of the nephridia tubes in earthworms, we
can now examine how the human kidneys work.
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 13-5 s/s
Check Your Understanding
What are some ways in which an excretory system helps organisms
maintain homeostasis?
■■ THE HUMAN KIDNEY: STRUCTURE AND FUNCTION
The kidneys are the most important part of the excretory system in
humans. There are two kidneys, located in the lower, rear portion of the
abdominal cavity. (See Figure 13-6 on page 282.) Every boxer knows
which punches are illegal—a punch to the lower back area, on either side
of the spinal column, can damage a kidney and is therefore banned.
Large blood vessels bring blood to the kidneys. Other blood vessels
leave the kidneys with the newly cleansed blood. As blood passes through
the kidneys, metabolic wastes are removed, and the correct balance of
salt and water in the blood is maintained. As a result, the kidneys produce
urine. Urine leaves each kidney through a tube, the ureter. The two
ureters constantly drip urine into the urinary bladder. Here the urine is
stored until it is convenient to pass it from the body. Urine leaves the
body through the urethra when instructions from the brain are given.
A close look at the kidney’s structure gives some idea of how this
important organ works. If you examined a section of the kidney, you
would see three large tubes at the center of the concave surface. One of
these tubes is the renal artery, which brings blood to the kidney from the
282 Maintaining a Dynamic Equilibrium
Hollow region
Artery
Cortex
Solid
region
Medulla
Kidney
(cut)
Figure 13-6 Human kidneys are
located in the lower, rear portion of
the abdominal cavity.
Vein
Kidney
(uncut)
Ureter
Urethra
Urinary bladder
body; another is the renal vein, which returns blood to the body; and the
third tube is the ureter, which connects each kidney to the urinary bladder. The inner portion of the kidney
hollow. ThisBIOLOGY,
is where2e/fig.
the urine
is
LIVINGisENVIRONMENT
13-6 s/s
collected. The outer portion of the kidney consists of the medulla and
the cortex. (See Figure 13-7.)
If you examined some tissue from the kidney under a microscope, you
would see that the blood vessels go through the medulla and into the cortex. In the cortex are the beginnings of many tiny, individual tubular
structures that extend into the medulla. These structures are called
nephrons. It is in the nephrons that the real work of the kidney occurs.
There are about a million nephrons in each kidney. So you have to jump
in even closer for a microscopic view of an individual nephron.
Medulla
Cortex
Renal artery
Renal vein
Figure 13-7 The outer portion
of the kidney consists of the
medulla and cortex. The inner
portion, where urine collects, is
hollow.
Renal pelvis
Ureter
Chapter 13 / Excretion and Water Balance 283
Under a microscope, you can observe the structure of a nephron. It is
a tube whose walls are a single cell layer thick. You can see the complex
network of capillaries wrapped around each nephron. Blood is cleansed,
water balance is maintained, and urine is produced through the exchange
of materials between the capillaries and the nephron.
The processes of excretion and chemical regulation begin in the
nephron as blood from the renal artery enters a tight bundle of capillaries called the glomerulus. Inside these capillaries the blood is under high
pressure. Filtration of the blood occurs here. Twenty percent of the liquid
material of the blood, including the chemicals in the fluid, is forced out
of the glomerular capillaries into a cuplike structure of the nephron that
surrounds them. Almost all that remains behind in the capillaries are the
relatively large blood cells and protein molecules. The cuplike structure is
called Bowman’s capsule. It is named after Sir William Bowman (1816–
1892), an English physician who gained fame because of his work on how
the kidneys function. (See Figure 13-8.)
Is this filtration sufficient to clean the blood and return it to the body?
Definitely not, for two important reasons. First, so much fluid is forced out
of the blood here that we would have to drink 7.5 liters of liquid every
hour—180 liters per day—to replace the lost liquid. We would do little
else than drink and urinate all day! Second, the process of filtration does
not sort materials. Harmful wastes and valuable nutrients alike are forced
out. Amino acids, sugar, salts, vitamins, and other nutrients are removed
from the blood at this point. Therefore, the process of filtration alone is
not enough to maintain homeostasis.
Tubule
Glomerulus Capsule
Solid outer
region of
kidney
Urine
flow
Artery
Capillary
Collecting
tubule
Vein
Hollow inner
region of kdney
Figure 13-8 The structure of
the nephron.
284 Maintaining a Dynamic Equilibrium
Living with Kidney Failure
Several years ago, a young woman named Jeanette fell and ruptured
her patella, the knee bone. Surgery was planned to repair it, but this
was abruptly cancelled when blood tests showed that Jeanette was in
kidney failure. A few decades ago, this would have meant certain
death. Now, however, Jeanette had options. The one she chose was
at-home hemodialysis.
Kidney failure, now called ESRD (end-stage renal disease), leads to a
buildup of wastes in the blood, higher concentrations of salt and water,
and unbalanced pH levels. Homeostasis is seriously disrupted. Without
kidney function, life cannot continue. Fortunately, however, treatments
have been developed to artificially cleanse the blood. The treatments
involve the exchange of substances across membranes in a process
called dialysis. In the 1960s, when the first artificial kidney was
developed, a patient had to be connected to a dialysis machine for 8 to
10 hours at a time, several times a week. There were also serious side
effects from the treatment, such as severe infections and inflammation
around the heart.
The many options that Jeanette and other ESRD patients today can
choose from are remarkable. The two basic choices are hemodialysis
and peritoneal dialysis. With hemodialysis, the patient’s blood passes
through tubes into a machine next to him/her, where materials are
exchanged between the blood and fluids in the machine. The patient
has the option to undergo hemodialysis at home, like Jeanette did, or in
the hospital, and at different times during the week. Some people have
dialysis treatment every day, others two or three times a week.
The other kind of treatment, peritoneal dialysis, amazingly uses the
lining of the patient’s own abdomen to filter the blood. A cleansing
solution is transported into the abdomen, where it stays for a while. As
the person’s blood passes in capillaries through the solution, the blood
gets cleansed. Then the solution is drained out, and a fresh solution is
put into the patient’s abdomen to start the process again. Peritoneal
dialysis can be done in several ways. One method uses no machine; it
just goes on continuously, even as the patient walks around. Another
way uses a machine to move the cleansing solution through a tube, in
and out of the person’s abdomen. The machine does this at night while
the person sleeps. Another method also uses a machine, but only at
certain times, and usually in the hospital.
So there are choices for people with kidney failure, choices that allow
those who no longer have the use of an essential internal organ—the
kidney—to go on living.
Chapter 13 / Excretion and Water Balance 285
The next stage, reabsorption, occurs as the filtrate passes through the
nephron tubule. By the process of active transport, glucose, salts, and
other valuable nutrients are moved back from the filtrate into the blood
in the capillaries. As these solutes move out of the filtrate, the concentration of water inside the nephron tubule becomes higher than outside
it. As a result, by osmosis, water automatically moves out of the filtrate.
In fact, almost 99 percent of the liquid in the filtrate is reabsorbed back
into the blood. As a result, the quantity of urine we release is usually
between one and two liters a day.
The final stage of kidney function occurs in the last portion of the
nephron tubule. Secretion of additional wastes from the blood, including
urea, occurs here. This process of secretion is one of the final steps in
maintaining homeostasis of the fluids in the body. Regulating the pH of
the blood is one more way the kidneys maintain homeostasis. In this case,
if the blood becomes too acidic, hydrogen ions are removed from the
blood. The reverse occurs if the blood is too basic.
The result of all this activity in the kidneys is that the blood returns
through the renal vein to the body, newly cleansed of wastes and properly
balanced with salts and water. In addition, urine that contains wastes, as
well as any excess salts and water, is collected and removed from the body.
■■ REGULATION OF THE EXCRETORY SYSTEM
The purpose of the excretory system is to maintain homeostasis by regulating the chemical composition of the blood and in turn of all the body’s
cells. At times, both the endocrine system and the nervous system help
to carry out this extremely important job.
Perhaps, on occasion, you have eaten an entire bag of salty potato
chips. As we have said, the salt in that one bag, if it all got into your blood,
could be dangerous. However, all the salt in the bag of potato chips does
not get into your blood. Why? As soon as the sodium level in your blood
starts to rise, the adrenal glands—the endocrine gland located on top of
each kidney—respond by secreting less of the hormone aldosterone. This
hormone, when it reaches the kidney, causes nephrons to reabsorb salt
from the urine into the blood. Smaller amounts of the hormone result in
less reabsorption of salt. Therefore, after you have eaten the salty chips,
more salt is excreted in your urine. (See Figure 13-9 on page 286.)
Regulation of the level of water in the body is very important. Consider when you have been exercising a great deal, sweating a lot, and
drinking little to replace your water loss. Your body has been losing water.
The result is that the volume of your blood decreases, since it consists
286 Maintaining a Dynamic Equilibrium
Figure 13-9 The adrenal glands
help regulate the amount of salt
in our blood: Sodium level rises;
adrenal glands secrete less
aldosterone; kidneys reabsorb less
salt from urine into blood; more
salt is excreted in urine;
homeostasis is maintained.
Adrenal glands
Kidney
Kidney
mostly of water. It is the blood and not your cells that loses the water.
Our bodies “know” that it is absolutely
necessary
to keep
the13-9
right
LIVING ENVIRONMENT
BIOLOGY,
2e/fig.
s/s
amount of water inside our cells in the intracellular compartment.
However, once again the lower volume of the blood acts as a stimulus
to the endocrine and nervous systems. This time the hypothalamus in
the brain detects the lower blood volume. It directs the posterior pituitary gland to secrete antidiuretic hormone (ADH). When ADH reaches
the kidneys, it increases the reabsorption of water from the urine back
into the blood. The blood volume returns to normal. Meanwhile, solutes,
including urea and urobilin—a yellow-colored waste—become more concentrated in the urine. The urine becomes darker. The darker color of the
urine shows that the body is automatically readjusting itself by not releasing as much water.
Why do we feel thirsty? To be aware of this feeling, the cerebral cortex of the brain must be involved. It is the hypothalamus once again that
does this job. In addition to releasing ADH when blood volume is low, the
hypothalamus sends a message to the cerebral cortex. This message is the
feeling you know as thirst. So you drink more water and, once again,
homeostasis is maintained.
■■ HOMEOSTASIS AND THE SKIN
The skin is the largest organ of the body. It is made up of a variety of different types of cells and tissues. (See Figure 13-10.) There are two separate
layers in the skin: the outer layer, or epidermis, and the inner layer, or
dermis. If we examine the structure of these layers, we can see how they
function on behalf of homeostasis. It should be no surprise that the organ
that surrounds the body has important roles in keeping the conditions
that exist inside us fairly constant.
The epidermis is a very thin layer. New skin cells are formed at the
bottom of this layer by cell division. These cells eventually push their way
toward the surface, replacing the dead skin cells there. This process occurs
Chapter 13 / Excretion and Water Balance 287
Hair
Muscle
Pore
Dead
cells
Region of
dividing cells
Epidermis
Sebaceous
gland
Nerve
ending
Hair
pocket
Capillary
Hair
root
Tubule
of sweat
gland
Dermis
Fat
Sweat
gland
Figure 13-10 The skin, the largest organ of the body, helps us maintain
homeostasis.
LIVING ENVIRONMENT BIOLOGY, 2e/fig. 13-10 s/s
all the time. We constantly leave behind a trail of dead skin cells wherever
we have been. These epidermal cells are very tough. They form a protective layer that prevents bacteria and harmful chemicals from entering,
and water from leaving, our body. Beneath this layer are cells that produce
the pigments that give skin its color.
The dermis is a thick, complex layer beneath the epidermis that contains a variety of structures. There are sensory nerve endings that can
detect temperature and touch; hairs; and capillaries that help regulate
body temperature (as discussed in Chapter 10). There also are muscles
attached at the bottom ends of hairs. These muscles are able to cause the
hairs to straighten, a response to fear in some animals. Some animals also
conserve heat by causing their hairs to stand up. This increases the
amount of air between the hairs, which insulates the body against heat
loss. Early humans evolved in warm, tropical climates, where it was a disadvantage to be covered with hair. Thus, less body hair was a trait that was
selected for, and humans lost the fur that most other mammals have.
When these little muscles in our skin contract, the result is simply what
we call goose bumps. In the dermis, there also are sebaceous glands,
which keep skin and hair soft by releasing oils. Finally, there are sweat
glands. Perspiration from sweat glands contains some urea, so they can
be considered excretory structures. The main role of perspiration, however, is to assist in regulating temperature by cooling the body through
evaporation.
288 Maintaining a Dynamic Equilibrium
Beneath the dermis is a layer of fatty tissue that many people wish
were missing. However, fatty tissue contributes to homeostasis by acting
as an insulator. It prevents the loss of body heat to the environment.
■■ HOMEOSTASIS AND THE LIVER
No theme on homeostasis and excretion would be complete without talking about the liver. The liver is the largest organ in the abdominal cavity.
It is involved in homeostasis by assisting most of the important systems
of the body. (See Figure 13-11.)
The liver helps in excretion by removing nitrogen from waste amino
acids and turning it into urea. The liver adds this urea to the blood. When
the blood reaches the kidneys, this urea is removed in the nephrons. Red
blood cells live for about 90 days. The liver breaks down old red blood
cells. The components of these old cells are recycled to make new red
blood cells. Chemical poisons are also made harmless by the liver. This is
called detoxification. For example, alcohol—considered a poison by the
body—is detoxified in the liver. Drinking excess alcohol makes the liver
work very hard. Continued heavy drinking can, in time, cause liver
disease.
A clear example of homeostasis is the need to keep blood sugar levels
constant. The liver stores excess glucose in the form of the polysaccharide
glycogen. The liver releases stored glycogen into the blood when glucose
is needed.
Liver
Figure 13-11 The liver
turns amino acid wastes
into urea, breaks down old
red blood cells, and
detoxifies chemical poisons
such as alcohol.
Small intestine
Stomach
Large intestine
Chapter 13 / Excretion and Water Balance 289
The list of jobs of this 1.4-kilogram chemical factory, the liver, goes
on. In Chapter 8, we saw how the liver helps the digestive process by making bile, the substance used by the body to digest fats. The liver packages
fats for transport in the blood, controls the level of cholesterol in the
blood, and stores vitamins A, D, and E. Finally, the liver helps maintain
water balance between the blood and the intracellular fluid by making
plasma proteins, important solutes in the blood.
■■ WHEN THINGS GO WRONG:
DISEASES OF THE EXCRETORY SYSTEM
One of the most common diseases of the kidney is nephritis, inflammation of the kidney. One way this disease can be detected is by a microscope examination of the urine. Cells and chemicals that normally should
not be in the urine are indicators of the disease. Nephritis occurs most
often in children and adolescents. Nephritis is caused by a bacterial infection and can usually be treated with antibiotics. Occasionally, however,
nephritis lasts a long time and can lead to kidney failure.
Kidney stones occur when a chemical compound that contains the
mineral calcium builds up in the kidney. In some cases, an improper diet
leads to kidney stones. However, in most cases, the cause of kidney stones
is unknown. A kidney stone can produce a great deal of pain when it travels through the ureter. Once the kidney stone is in the urinary bladder, it
often leaves the body unnoticed. If the stone is too large to be passed,
surgery to remove it, or the use of shock waves to break it into smaller
pieces, may be necessary.
High levels of the nitrogenous waste uric acid in the blood causes gout,
which occurs more often in men. The uric acid forms sharp crystals that
produce severe pain in joints, usually in the big toe and sometimes in the
anklebones. Treatment of gout includes drinking a lot of water, eating a
special diet, and sometimes taking medication to lower the level of uric
acid in the blood.
As was mentioned, liver disease can occur when too much alcohol is
consumed. The liver needs to work extra hard to metabolize alcohol.
Some by-products of this breakdown of alcohol, such as fats, accumulate
in the liver. This causes scarring of the liver cells, called cirrhosis. Cirrhosis causes more of the liver cells to stop working. As the liver shuts
down, homeostasis can no longer be maintained. Cirrhosis of the liver is
the ninth leading cause of death in the United States.
LABORATORY INVESTIGATION 13
How Can We Test for the Presence
of Certain Substances in “Urine”?
INTRODUCTION
The wastes produced by our cells are filtered from the blood in the kidneys. Then these wastes are eliminated from the body in urine. The kidneys also reabsorb many materials that must be returned to the blood.
The kidneys are excretory organs that carry out the process of homeostasis in the body. During medical examinations, physicians often test urine
for the presence of substances that may indicate a disease. Urine is also
tested to determine if a person has used an illegal or controlled substance.
The chemical testing of urine is called urinalysis. In this investigation,
you will analyze mystery substances to determine what they contain.
MATERIALS
Artificial “urine” samples, test tubes, test-tube holder, test-tube rack, heatresistant 500-mL beaker, tap water, hot plate, graduated cylinder, 10%
acetic acid, Benedict solution, silver nitrate, biuret solution, safety goggles
PROCEDURE
CAUTION: Wear safety goggles during this laboratory investigation.
1. Your teacher will give you eight “urine” samples. Label eight clean test
tubes the same way these samples are labeled. Place the labeled test
tubes in your rack.
2. Prepare a hot-water bath by filling the beaker half full with tap water
and placing it on the hot plate. Turn on the hot plate.
3. Test for phosphates:
Place 10 mL of each of the two phosphate-urine samples into your two
appropriately labeled test tubes. Look at the top of the liquid in the
tube as you add five drops of acetic acid. Repeat for the other sample.
Record your observations.
4. Test for glucose:
Place 5 mL of each of the two glucose-urine samples into your appropriately labeled test tubes. Add five drops of Benedict solution to each
test tube. Place both tubes in the hot-water bath for two minutes.
Observe and record any color changes.
290 Maintaining a Dynamic Equilibrium
5. Test for chlorides:
Place 5 mL of each of the two chloride-urine samples into your appropriately labeled test tubes. Carefully add three drops of silver nitrate to
each test tube. Observe the surface of the solution in each tube and
record any changes you observe.
6. Test for albumin (protein):
Place 10 mL of each of the two albumin-urine samples into your appropriately labeled test tubes. Add five drops of biuret solution to each
test tube. Observe and record the results.
7. Make a data table of your results.
8. Obtain an unknown “urine” sample from your teacher. The unknown
samples will vary; some may test positive for one or more substances,
some may test negative for one or more substances. Design and carry
out an investigation to determine the substances present in your
sample.
INTERPRETIVE QUESTIONS
1. Human blood contains molecules of glucose and albumin, along with
phosphates and chlorides. Urine normally contains phosphate and
chloride ions. What does this information indicate about the functions of the kidneys?
2. Why may the presence of glucose in the urine be a cause of concern?
3. What may be indicated by the presence of albumin in the urine?
Chapter 13 / Excretion and Water Balance 291
292 Maintaining a Dynamic Equilibrium
■■ CHAPTER 13 REVIEW
Answer these questions on a separate sheet of paper.
VOCABULARY
The following list contains all of the boldfaced terms in this chapter. Define
each of these terms in your own words.
aldosterone, ammonia, antidiuretic hormone, Bowman’s capsule,
cirrhosis, contractile vacuole, dermis, detoxification, epidermis,
excretion, filtrate, glomerulus, kidney, nephridium, nephrons,
nitrogenous wastes, osmoregulation, sebaceous glands, sweat glands,
urea, ureter, urethra, uric acid, urinary bladder, urine
PART A—MULTIPLE CHOICE
Choose the response that best completes the sentence or answers the question.
1. The organ in vertebrates that is primarily responsible for regulating
the chemical composition of plasma is the a. lymphatic system
b. liver c. kidney d. hypothalamus.
2. Which of these is not a solute in blood? a. water b. salt
c. urea d. carbon dioxide
3. Which of these is not one of the water compartments in the body?
a. lymph b. intracellular fluid c. plasma d. urinary bladder
4. The process of maintaining water balance is called
a. thermoregulation b. osmoregulation c. excretion
d. metabolism.
5. Excretion is best described as the process of a. perspiring
b. ridding the body of metabolic wastes c. regulating the water
balance in the body d. producing urine.
6. An ameba produces nitrogenous wastes in the form of
a. uric acid b. urea c. ammonia d. carbon dioxide.
7. The excretory structures in earthworms are a. nephridia
b. kidneys c. urinary bladders d. contractile vacuoles.
8. The nitrogenous waste that is least soluble in water is a. uric acid
b. urea c. ammonia d. carbon dioxide.
9. Gout occurs when a. too much alcohol is consumed, causing
liver damage b. uric acid crystals form in the joints c. calcium
compounds build up in the kidneys d. bacteria infect the
kidneys, causing inflammation.
10. When a paramecium (a one-celled organism) is put into distilled
water, structures at its “front” and “tail” ends start to pulsate
Chapter 13 / Excretion and Water Balance 293
11.
12.
13.
14.
15.
rapidly. These structures are probably a. lysosomes b. hearts
c. sodium pumps d. contractile vacuoles.
The functional units of the human kidneys are the a. nephridia
b. nephrons c. glomeruli d. renal medulla.
The thin, outermost layer of the skin is the a. endoderm
b. dermis c. epidermis d. cortex.
Overactive sebaceous glands cause a. acne b. gout
c. kidney failure d. gallstones.
Nephritis is a. an inflammation of the kidneys b. a scarring of
liver cells c. an excretory organ in a crayfish d. a type of
nitrogenous waste.
The dermis is involved with a. detecting temperature and
pressure b. causing goose bumps in response to cold or fear
c. regulating body temperature d. all of these.
PART B—CONSTRUCTED RESPONSE
Use the information in the chapter to respond to these items.
16. Identify the
structures
C
labeled A
D
E
through L in
B
J
the diagram.
K
17. Where does
L
the process of
A
F
reabsorption
take place? Why
is this process
important to
G
maintaining
homeostasis?
H
18. How do
aldosterone and
ADH contribute
to homeostasis?
19. Compare the three
typesENVIRONMENT
of nitrogenous
wastes
in animals.
LIVING
BIOLOGY,
2e/fig.
13-Q16 s/sHow
(rev. 10/13/03)
might the excretion of uric acid be an adaptation to life on dry
land?
20. Explain how the liver helps to maintain homeostasis.
294 Maintaining a Dynamic Equilibrium
PART C—READING COMPREHENSION
Base your answers to questions 21 through 23 on the information below and
on your knowledge of biology. Source: Science News (March 1, 2003): vol.
163, p. 142.
Good Taste in Men Linked to Colon Risks
Men with exceptionally good taste may pay for it in health risks.
About 25 percent of people have extra taste buds on their tongues.
They live in “a neon taste world” instead of a “pastel” one, as Yale University researcher Linda Bartoshuk puts it (SN: 7/5/97, p. 24).
There may be a nasty consequence to the benefit. Among men over
65, intense tasters have significantly more colon polyps than other
tasters, according to Bartoshuk and Marc Basson of Wayne State University of Detroit. Extra polyps suggest an extra risk of colon cancer.
Bartoshuk speculates that sensitive tongues lead these men astray in
food choices. Supertasters often cringe at intense vegetable flavors, and
the supertasting seniors eat fewer vegetables than do their counterparts
with normal taste sensitivity. The supertasters also tended to weigh
more. Low-vegetable diets and extra weight both raise the risk of colon
cancer.
Ear infections may exacerbate this cancer risk. The nerves from the
tongue pass through the ear, and ear infections distort neural mechanisms so that the tongue increases its sensitivity to fat. Bartoshuk has
found that among men with a history of ear infections, supertasters are
especially likely to be very overweight.
21. State the difference that has been discovered among men in their
ability to taste foods.
22. State the health risk associated with the men that have increased
taste sensitivity.
23. Describe two hypotheses that might explain the health risk faced
by the men with sensitive tongues.