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Explains the functions of the kidneys and how they form urine.

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Why are the kidneys important?

These kidney beans are named after a very important organ in your body. Though you probably can live without these beans, you can't live without at least one kidney. The kidneys have several essential functions. For example, kidneys filter your blood, removing wastes and regulating the amount of water in your body.

The Kidneys

The kidneys (Figure below) are important organs in maintaining homeostasis, the ability of the body to maintain a stable internal environment despite a changing environment. Kidneys perform a number of homeostatic functions.

  • They maintain the volume of body fluids.
  • They maintain the balance of salt ions in body fluids.
  • They excrete harmful nitrogen-containing molecules, such as urea, ammonia, and uric acid.

This illustrations shows the parts of a kidney

Structures of the kidney; fluid leaks from the capillaries and into the nephrons where the fluid forms urine then moves to the ureter and on to the bladder.

There are many blood vessels in the kidneys (Figure above). The kidneys remove urea and other wastes from the blood through tiny filtering units called nephrons. Nephrons (Figure below) are tiny, tube-shaped structures found inside each kidney. Each kidney has up to a million nephrons. Each nephron collects a small amount of fluid and waste from a small group of capillaries.

Nitrogen-containing wastes, together with water and other wastes, form the urine as it passes through the nephrons and the kidney. The fluid within nephrons is carried out into a larger tube in the kidney called a ureter, which carries it to the bladder (Figure below).

The kidneys never stop filtering waste products from the blood, so they are always producing urine. The amount of urine your kidneys produce is dependent on the amount of fluid in your body. Your body loses water through sweating, breathing, and urination. The water and other fluids you drink every day help to replace the lost water. This water ends up circulating in the blood because blood plasma is mostly water.

Many nephrons make up your kidney

The location of nephrons in the kidney. The fluid collects in the nephron tubules and moves to the bladder through the ureter.

Formation of Urine

The process of urine formation is as follows:

  1. Blood flows into the kidney through the renal artery. The renal artery connects to capillaries inside the kidney. Capillaries and nephrons lie very close to each other in the kidney.
  2. The blood pressure within the capillaries causes water, salts, sugars, and urea to leave the capillaries and move into the nephron.
  3. The water and salts move along through the tube-shaped nephron to a lower part of the nephron.
  4. The fluid that remains in the nephron at this point is called urine.
  5. The blood that leaves the kidney in the renal vein has much less waste than the blood that entered the kidney.
  6. The urine is collected in the ureters and is moved to the urinary bladder, where it is stored.

Nephrons filter about ¼ cup of body fluid per minute. In a 24-hour period, nephrons filter 180 liters of fluid, and 1.5 liters of the fluid is released as urine. Urine enters the bladder through the ureters. Similar to a balloon, the walls of the bladder are stretchy. The stretchy walls allow the bladder to hold a large amount of urine. The bladder can hold about 1½ to 2½ cups of urine but may also hold more if the urine cannot be released immediately.

How do you know when you have to urinate? Urination is the process of releasing urine from the body. Urine leaves the body through the urethra. Nerves in the bladder tell you when it is time to urinate. As the bladder first fills with urine, you may notice a feeling that you need to urinate. The urge to urinate becomes stronger as the bladder continues to fill up.

Brain Control of Urination

The filtering action of the kidneys is controlled by the pituitary gland. The pituitary gland is about the size of a pea and is found below the brain (Figure below). The pituitary gland releases hormones that help the kidneys to filter water from the blood.

The movement of water back into blood is controlled by a hormone called antidiuretic hormone (ADH). ADH is one of the hormones released from the pituitary gland in the brain. One of the most important roles of ADH is to control the body’s ability to hold onto water. If a person does not drink enough water, ADH is released. It causes the blood to reabsorb water from the kidneys. If the kidneys remove less water from the blood, what will the urine look like? It will look darker, because there is less water in it.

When a person drinks a lot of water, then there will be a lot of water in the blood. The pituitary gland will then release a lower amount of ADH into the blood. This means less water will be reabsorbed by the blood. The kidneys then produce a large volume of urine. What color will this urine be?

The pituitary gland helps regulate the production of urine

The pituitary gland (green) is found directly below the brain and releases hormones that control how urine is produced.


  • Water and waste molecules move out of the blood capillaries and into the nephrons of the kidney to form the urine.
  • ADH is the hormone released by the pituitary gland and controls how water is reabsorbed by the blood from the kidneys.

Explore More

Use the resources below to answer the questions that follow.

Explore More I

  1. What is a nephron? What is its function in the kidney?
  2. What happens in the coiled tubules in the kidney?
  3. Where is the cortex of the kidney? What structures are located there?
  4. Where is the medulla in the kidney? What structures are located there?

Explore More II

  1. Where does the ureter lead to?
  2. What exits the tubules in the outer medulla? What effect does this have on the fluid moving through the tubules?
  3. What exits the tubules on the ascending limb? What effect does this have on the fluid moving through the tubules?
  4. What controls the permeability of the collecting ducts?


  1. What is the nephron?
  2. Through what blood vessel does blood enter the kidney?
  3. What causes wastes to leave the blood?
  4. What is the difference between the blood that enters and leaves the kidney?
  5. What does antidiuretic hormone (ADH) do?

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antidiuretic hormone

Hormone that stimulates the kidneys to conserve water by producing more concentrated urine.


Ability of the body to maintain a stable internal environment despite a changing environment.


Tiny, tube-shaped structure found inside each kidney that filters the blood and produces urine.

pituitary gland

Small structure at the base of the brain that secretes hormones.


Tube-like organ of the urinary system that moves urine from the kidneys to the bladder.


Process in which urine is released from the body.


Liquid waste that is formed by the kidneys when they filter wastes from the blood.

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