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23.4: The Excretory System

Difficulty Level: At Grade Created by: CK-12
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Lesson Objectives

  • 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.


  • bladder
  • dialysis
  • excretion
  • excretory system
  • kidney failure
  • nephron
  • ureter
  • urethra
  • urinary system
  • urination
  • urine


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 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.
  • 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 below. The main function of the urinary system is to filter waste products and excess water from the blood and excrete them from the body.

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 below. 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!

Each kidney is supplied by a renal artery and renal vein.

The kidney and nephron are discussed at http://www.youtube.com/watch?v=cc8sUv2SuaY (18:38).

Additional information about the nephron is shown at http://www.youtube.com/watch?v=czY5nyvZ7cU.

Filtering Blood and Forming Urine

As shown in Figure below, 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.

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 above). 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.
  • 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 below).

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.

Lesson Review Questions


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?

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