Excretion and Water Balance
In the body, water is found in three areas, or compartments:
6 Plasma, the liquid portion of the blood without the blood cells, makes up about 7 percent of
body fluid.
6 The intercellular fluid and lymph make up about 28 percent of body fluid.
6 The intracellular compartment, the fluid located inside cells, makes up about 65 percent of
body fluid.
In terrestrial vertebrates, including ourselves, the organ primarily responsible for regulating the
chemical composition of plasma is the kidney. Approximately 20 percent of the total blood flow in the
body passes through the kidneys at any moment, even though the kidneys make up less than 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.
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. Water diffuses across a cell membrane
toward an area of higher salt concentration.
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.
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. Other wastes produced during metabolic activities include water from dehydration
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). Ammonia is a toxic compound that can kill living cells. All animals produce ammonia in their
cells as they metabolize proteins. For an ameba, and some aquatic animals, ammonia diffuses into the
water through the cell membrane. Other animals must change ammonia into a less harmful substance.
Birds, terrestrial reptiles, and insects change ammonia into solid crystals of uric acid and pass it with
digestive waste.
Mammals have evolved a different solution for dealing with ammonia wastes. In mammals, ammonia
is mostly changed into urea. To be removed from the body, urea is dissolved in water, is diluted and
removed from the body in urine.
We have seen that single-celled organisms, such as an ameba and a paramecium, and some
multicellular animals, such as sponges rid themselves of wastes by simple diffusion across their cell
membrane.
Most other animals, from earthworms on up, have some type of internal organ that contains tubelike
structures to carry out chemical regulation.
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. Large blood vessels bring blood to the
kidneys. As blood passes through the kidneys, metabolic wastes are removed, and the correct balance
of salt, water and many other materials 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 passed from the body. Urine leaves the body
through the urethra.
If you examined 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 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, where urine is collected, is hollow. The outer portion of the kidney
consists of the medulla and the cortex. In the cortex are the beginnings of many tiny, individual
tubular structures that extend into the medulla. These structures, the functional unit of the kidneys, are
called nephrons. There are about a million nephrons in each kidney. Nephrons are tubes a single cell
layer thick. Complex networks of capillaries wrapped around each nephron. Blood from the renal
artery, under high pressure, enters a tight bundle of capillaries called the glomerulus. Much of the
liquid of the blood, including the chemicals in the fluid, is forced out of the glomerular capillaries into
a cuplike structure, the Bowman’s capsule.
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 a result, by osmosis, water automatically moves out of the filtrate. And 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. Finally 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 by the removal of
the proper amount of hydrogen ions.
The urine contains wastes, as well as any excess salts and water. 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.
The skin is the largest organ of the body. It is made up of a variety of different types of cells and
tissues. There are two separate layers in the skin: the outer layer, or epidermis, and the inner layer, or
dermis. The skin forms a protective layer that prevents bacteria and harmful chemicals from entering,
and water from leaving, our body. There are sensory nerve endings that can detect temperature and
touch; hairs; and capillaries that help regulate body temperature. There are also 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.
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. 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. 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. Continued heavy drinking can, in time, cause liver disease.
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 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 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.
1. The organ in vertebrates that is primarily responsible for regulating the chemical composition of
plasma is ___________________________.
2. 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.
3. 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.
4. The functional units of the human kidneys are the a. nephridia b. nephrons c. glomeruli
d. renal medulla.
5. 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.
6. Identify the structures labeled A
through L in the diagram.
7. Our kidneys need to filter nitrogenous
wastes from our blood. Where do these
nitrogenous wastes come from?
8. What do the ureters do?
9. What is the function of the urinary
bladder?
10. What is the function of the urethra?
11. What are nephrons? How many of
these microscopic structures are found
in each kidney?
12. Why are sweat glands considered to be excretory structures?
13. What is the chief function of sweat glands?
14. List the major functions of the liver?
15. What is nephritis and what usually causes this?
16. What causes gout? What is a symptom of this disorder?
17. What is a common cause of cirrhosis of the liver?
Match the organ with the function in the table below. Kidney, nephron, ureter, urinary bladder,
renal artery, nephrons, glomerulus, Bowman’s capsule, sweat glands, urethra, liver
Excretory Organ
Function
6 Tube that drains urine from kidney to bladder
6 Carries approximately 20% of the body’s blood flow to the kidney
6 Tight bundle of capillaries that carries blood to the nephron under high
pressure
6 One of two organs located in the back of the abdominal wall
6 Contains approximately 1, 000, 000 nephrons
6 Collects liquid filtered out of glomerulus
6 Structure in the skin with excretory function
6 Chief function is temperature control
6 Functional unit of the kidney
6 Converts wastes from amino acids to urea
6 Functional unit of kidneys
6 Filters blood, removes wastes, produces urine
6 Collects and temporarily stores urine until passed from the body
6 Carries urine from the bladder to the outside of the body
Add the following labels to the diagram of a kidney below. If you like you can also color in the
diagram as indicated. :capsule- turquoise; renal artery – red; renal vein – blue; cortex - brown;
medulla - pink; pelvis - yellow; ureter – green
Add the following labels to the diagram below of a kidney tubule or nephron.
collecting duct; branch of renal artery; loop of Henle; distal convoluted tubule; glomerulus; proximal convoluted tubule; Bowman’s capsule 1. Circle the substances in the list below that are NOT found in the fluid that has filtered through into
the Bowman’s capsule of a healthy animal?
water; sodium chloride; red blood cells; glucose; amino acids; proteins; urea; white blood cells 2. Circle the substances in the list below that are NOT found in the urine of a healthy animal?
water; urea; red blood cells; mucus; glucose; proteins; sodium chloride; white blood cells 3. Normally all the glucose filtered into the kidney tubule is absorbed further down the tubule. If
glucose is found in the urine, what might one suspect to be the cause?
Biology: Excretory System
Name:
Date:
Instructions: Complete the crossword puzzle. Use the clues to help you solve the puzzle.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
Across
1.
4.
7.
10.
11.
13.
urinary ______________ temporarily stores urine before it is passed from the body
tube that transports urine from each kidney to the urinary bladder.
______________ glands, have excretory function but main function is temperature control
_______________ wastes result when amino acids, the building blocks of proteins, are broken
down
_____ bladder: smooth muscle bag that stores urine until it is expelled from the body.
ammonia is mostly changed into __________ for elimination from the body
Biology: Excretory System
15.
16.
tube through which urine is passed from the urinary bladder to the outside of the body.
Kidney __________ formed from mineral buildup in kidneys. Painful when passing out of
body.
Down
2.
3.
5.
6.
8.
9.
12.
14.
.
the process of ridding the body of metabolic wastes
inflammation of the kidney
Scarring of the liver cells. Frequently caused by chronic alcohol use.
organs of the vertebrate urinary system; remove wastes, control sodium levels of the blood,
and regulate blood pH levels.
Bowman's ______________ collects filtrate from the glomerulus to be processed in nephron
high pressure, tight bundle of capillaries that filters blood to the nephron
individual filtering unit of the kidneys.
liquid composed of wastes that is filtered from the blood by the kidneys, stored in the urinary
bladder, and eliminated through the urethra.
2