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The Excretory System
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C HAPTER
Chapter 1. The Excretory System
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The Excretory System
Lesson Objectives
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Define excretion, and identify organs of the excretory system.
Explain how the urinary system filters blood and excretes wastes.
Describe the roles of the kidneys in homeostasis.
Identify kidney diseases, and describe dialysis.
Vocabulary
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bladder
dialysis
excretion
excretory system
kidney failure
nephron
ureter
urethra
urinary system
urination
urine
Introduction
If you exercise on a hot day, you are likely to lose a lot of water in sweat. Then, for the next several hours, you may
notice that you do not pass urine as often as normal and that your urine is darker than usual. Do you know why this
happens? Your body is low on water and trying to reduce the amount of water lost in urine. The amount of water
lost in urine is controlled by the kidneys, the main organs of the excretory system.
Excretion
Excretion is the process of removing wastes and excess water from the body. It is one of the major ways the body
maintains homeostasis. Although the kidneys are the main organs of excretion, several other organs also excrete
wastes. They include the large intestine, liver, skin, and lungs. All of these organs of excretion, along with the
kidneys, make up the excretory system. This lesson focuses on the role of the kidneys in excretion. The roles of the
other excretory organs are summarized below:
• The large intestine eliminates solid wastes that remain after the digestion of food.
• The liver breaks down excess amino acids and toxins in the blood.
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• The skin eliminates excess water and salts in sweat.
• The lungs exhale water vapor and carbon dioxide.
Urinary System
The kidneys are part of the urinary system, which is shown in Figure 1.1. The main function of the urinary system
is to filter waste products and excess water from the blood and excrete them from the body.
FIGURE 1.1
The kidneys are the chief organs of the
urinary system.
Kidneys and Nephrons
The kidneys are a pair of bean-shaped organs just above the waist. A cross-section of a kidney is shown in Figure
1.2. The function of the kidney is to filter blood and form urine. Urine is the liquid waste product of the body that
is excreted by the urinary system. Nephrons are the structural and functional units of the kidneys. A single kidney
may have more than a million nephrons!
The kidney and nephron are discussed at http://www.youtube.com/watch?v=cc8sUv2SuaY (18:38).
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Chapter 1. The Excretory System
FIGURE 1.2
Each kidney is supplied by a renal artery
and renal vein.
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/233
Additional information about the nephron is shown at http://www.youtube.com/watch?v=czY5nyvZ7cU .
MEDIA
Click image to the left or use the URL below.
URL: http://www.ck12.org/flx/render/embeddedobject/234
Filtering Blood and Forming Urine
As shown in Figure 1.3, each nephron is like a tiny filtering plant. It filters blood and forms urine in the following
steps:
1. Blood enters the kidney through the renal artery, which branches into capillaries. When blood passes through
capillaries of the glomerulus of a nephron, blood pressure forces some of the water and dissolved substances
in the blood to cross the capillary walls into Bowman’s capsule.
2. The filtered substances pass to the renal tubule of the nephron. In the renal tubule, some of the filtered
substances are reabsorbed and returned to the bloodstream. Other substances are secreted into the fluid.
3. The fluid passes to a collecting duct, which reabsorbs some of the water and returns it to the bloodstream. The
fluid that remains in the collecting duct is urine.
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FIGURE 1.3
The parts of a nephron and their functions
are shown in this diagram.
Excretion of Urine
From the collecting ducts of the kidneys, urine enters the ureters, two muscular tubes that move the urine by
peristalsis to the bladder (see Figure 1.1). The bladder is a hollow, sac-like organ that stores urine. When the
bladder is about half full, it sends a nerve impulse to a sphincter to relax and let urine flow out of the bladder
and into the urethra. The urethra is a muscular tube that carries urine out of the body. Urine leaves the body
through another sphincter in the process of urination. This sphincter and the process of urination are normally
under conscious control.
Kidneys and Homeostasis
The kidneys play many vital roles in homeostasis. They filter all the blood in the body many times each day and
produce a total of about 1.5 liters of urine. The kidneys control the amount of water, ions, and other substances in the
blood by excreting more or less of them in urine. The kidneys also secrete hormones that help maintain homeostasis.
Erythropoietin, for example, is a kidney hormone that stimulates bone marrow to produce red blood cells when more
are needed. The kidneys themselves are also regulated by hormones. For example, antidiuretic hormone from the
hypothalamus stimulates the kidneys to produce more concentrated urine when the body is low on water.
Kidney Disease and Dialysis
A person can live a normal, healthy life with just one kidney. However, at least one kidney must function properly
to maintain life. Diseases that threaten the health and functioning of the kidneys include kidney stones, infections,
and diabetes.
• Kidney stones are mineral crystals that form in urine inside the kidney. They may be extremely painful. If
they block a ureter, they must be removed so urine can leave the kidney and be excreted.
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Chapter 1. The Excretory System
• Bacterial infections of the urinary tract, especially the bladder, are very common. Bladder infections can be
treated with antibiotics prescribed by a doctor. If untreated, they may lead to kidney damage.
• Uncontrolled diabetes may damage capillaries of nephrons. As a result, the kidneys lose much of their ability
to filter blood. This is called kidney failure. The only cure for kidney failure is a kidney transplant, but it
can be treated with dialysis. Dialysis is a medical procedure in which blood is filtered through a machine (see
Figure 1.4).
FIGURE 1.4
A dialysis machine filters a patient’s
blood.
Lesson Summary
• Excretion is the process of removing wastes and excess water from the body. It is one of the major ways the
body maintains homeostasis. Organs of excretion make up the excretory system. They include the kidneys,
large intestine, liver, skin, and lungs.
• The kidneys filter blood and form urine. They are part of the urinary system, which also includes the ureters,
bladder, and urethra.
• Each kidney has more than a million nephrons, which are the structural and functional units of the kidney.
Each nephron is like a tiny filtering plant.
• The kidneys maintain homeostasis by controlling the amount of water, ions, and other substances in the blood.
They also secrete hormones that have other homeostatic functions.
• Kidney diseases include kidney stones, infections, and kidney failure due to diabetes. Kidney failure may be
treated with dialysis.
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Lesson Review Questions
Recall
1. What is excretion?
2. List organs of the excretory system and their functions.
3. Describe how nephrons filter blood and form urine.
4. State the functions of the ureters, bladder, and urethra.
Apply Concepts
5. Tom was seriously injured in a car crash. As a result, he had to have one of his kidneys removed. Does Tom need
dialysis? Why or why not?
Think Critically
6. Explain how the kidneys maintain homeostasis.
Points to Consider
Infections caused by microorganisms may affect any of the organ systems described in this chapter. For example,
you have just read that bacterial infections of the bladder are common.
• What defenses do you think the body has to keep out microorganisms?
• Do you know if there are other defenses against microorganisms if they manage to get inside the body?
References
1. Courtesy of National Cancer Institute/SEER Training Modules. http://commons.wikimedia.org/wiki/File:I
llu_urinary_system.svg . Public Domain
2. Laura Guerin. CK-12 Foundation . CC BY-NC 3.0
3. Hana Zavadska. CK-12 Foundation . CC BY-NC 3.0
4. Jodi So. CK-12 Foundation . CC BY-NC 3.0
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