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The kidney is a complex filtering organ that collects waste from the blood and delivers it to the bladder.

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Why is a bean-shaped organ so important?

Shown above are the isolated kidneys from many little mice. OK, they're really just kidney beans. But this is what the important kidney looks like. Why is it so important? Your kidneys filter and remove wastes from your blood.

The Kidneys

The kidneys are a pair of bean-shaped organs just above the waist. They are important organs with many functions in the body, including producing hormones, absorbing minerals, and filtering blood and producing urine.

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. Wastes in the blood come from the normal breakdown of tissues, such as muscles, and from food. The body uses food for energy. After the body has taken the nutrients it needs from food, some of the wastes are absorbed into the blood. If the kidneys did not remove them, these wastes would build up in the blood and damage the body.

Kidneys and Nephrons

The actual removal of wastes from the blood occurs in tiny units inside the kidneys called nephrons. Nephrons are the structural and functional units of the kidneys. A single kidney may have more than a million nephrons! This is further discussed in the Urinary System concept.

Kidney cutaway

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

Kidneys and Homeostasis

The kidneys play many vital roles in homeostasis. They work with many other organ systems to do this. For example, they work with the circulatory system to filter blood, and with the urinary system to remove wastes.

The kidneys 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. They also secrete renin, which regulates blood pressure, and calcitriol, the active form of vitamin D, which helps maintain calcium for bones. 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.

Other Functions

In addition to filtering blood and producing urine, the kidneys are also involved in maintaining the water level in the body, and regulating red blood cell levels and blood pressure.

  • As the kidneys are mainly involved in the production of urine, they react to changes in the body’s water level throughout the day. As water intake decreases, the kidneys adjust accordingly and leave water in the body instead of helping remove it through the urine, maintaining the water level in the body.
  • The kidneys also need constant pressure to filter the blood. When the blood pressure drops too low, the kidneys increase the pressure. One way is by producing angiotensin, a blood vessel-constricting protein. This protein also signals the body to retain sodium and water. Together, the constriction of blood vessels and retention of sodium and water help restore normal blood pressure.
  • When the kidneys don’t get enough oxygen, they send out a signal in the form of the hormone erythropoietin, which stimulates the bone marrow to produce more oxygen-carrying red blood cells.


  • The kidneys maintain homeostasis by controlling the amount of water, ions, and other substances in the blood.
  • Kidneys also secrete hormones that have other homeostatic functions.


  1. What is the nephron? How many nephrons are in each kidney?
  2. Explain how the kidneys maintain homeostasis.
  3. What is the role of antidiuretic hormone?

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    antidiuretic hormone Hormone that stimulates the kidneys to conserve water by producing more concentrated urine.
    erythropoietin A kidney hormone that stimulates bone marrow to produce red blood cells when more are needed.
    homeostasis Process of maintaining a stable environment inside a cell or an entire organism.
    urine Liquid waste product of the body; formed by the kidneys and excreted by the other organs of the urinary system.

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