Cross section of a kidney. The kidney is made up of an outermost cortex, a middle medulla, and an …—© Merriam-Webster Inc.

One of a pair of organs that maintain water balance and expel metabolic wastes. Human kidneys are bean-shaped organs about 4 in. (10 cm) long, in the small of the back. They filter the entire 5-quart (about 4.5-liter) water content of the blood every 45 minutes. Glucose, minerals, and needed water are returned to the blood by reabsorption. The remaining fluid and wastes pass into collecting ducts, flowing to the ureter and bladder as urine. Each kidney has over 1 million functional units (nephrons) involved in the process of filtration and reabsorption. The kidneys also secrete renin, an enzyme involved in blood pressure regulation. Disorders include kidney failure, kidney stones, and nephritis. See also urinary system.

This entry comes from Encyclopædia Britannica Concise.
For the full entry on kidney, visit

Seen & Heard

What made you look up kidney? Please tell us what you were reading, watching or discussing that led you here.