Kidney

Either of the 2 organs that filter the blood and excrete waste products and excess water as urine. The kidneys are situated at the back of the abdominal cavity, on either side of the spine. Each kidney is surrounded by a fibrous capsule and is made up of an outer cortex and an inner medulla. The cortex contains specialized capillaries called glomeruli, which, together with a series of tubules, make up the nephrons, the filtering units of the kidney. The nephrons filter blood under pressure and then selectively reabsorb water and certain other substances back into the blood. Urine is formed from substances that are not reabsorbed. The urine is conducted through tubules to the renal pelvis (the central collecting area of the kidney) and then through tubes called ureters to the bladder. The kidneys also regulate the body’s fluid balance. To do this, the kidneys excrete excess water, and when water is lost from the body (for example as a result of sweating), they conserve it (see ADH). In addition, the kidneys control the body’s acid–base balance by adjusting urine acidity. The kidneys are also involved in hormonal regulation of red blood cell production and blood pressure.