How Your Kidneys Work

  • The two kidneys are vital organs, performing many functions to keep your blood clean and chemically balanced.
  • The kidneys are bean-shaped organs, each about the size of a fist. They are located near the middle of the back, just below the rib cage.
  • Kidneys are sophisticated trash collectors. Every day, the kidneys process about 200 quarts of blood to sift out about 2 quarts of waste products and extra water. The waste and extra water become urine, which flows to the bladder through tubes called ureters. The bladder stores the urine until you go to the bathroom.
  • The wastes in your blood come from the normal breakdown of active muscle and from food eaten. The body uses the food for energy and self-repair. After the body takes what it needs from the food, waste is sent to the blood. If the kidneys did not remove these wastes, the wastes would build up in the blood and damage the body.
  • The actual filtering occurs in tiny units inside the kidneys called nephrons. Every kidney has about a million nephrons. In the nephron, tiny blood vessels called capillaries intertwine with tiny urine-carrying tubes called tubules.
  • The tubules first receive a combination of waste materials and chemicals that the body can still use. The kidneys measure out chemicals like sodium, phosphorus, and potassium and release them back to the blood to return to the body. In this way, the kidneys regulate the body's level of these substances. The right balance is necessary for life, but excess levels can be harmful.
  • In addition to removing wastes, the kidneys release three important hormones:
    o Erythropoietin (EPO), which stimulates the bones to make red blood cells;
    o Renin, which regulates blood pressure; and
    o The active form of vitamin D, which helps maintain calcium for bones and for normal chemical balance in the body.

Excerpted and Adapted from Your Kidneys and How They Work. NIH Publication No. 98-4241, March 1998. National Institute of Diabetes and Digestive and Kidney Diseases, National Institutes of Health, U.S. Department of Health and Human Services.

Reviewed by Francine Kaufman, M.D., 04/08

Last Modified Date: November 28, 2012

All content on is created and reviewed in compliance with our editorial policy.

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