In vertebrates and some invertebrates, organ that maintains water balance and expels metabolic wastes. Primitive and embryonic kidneys consist of two series of specialized tubules that empty into two collecting ducts, the Wolffian ducts. The more advanced kidney (metanephros) of adult reptiles, birds, and mammals is a paired compact organ whose functional units, called nephrons, filter initial urine from the blood, reabsorb water and nutrients, and secrete wastes, producing the final urine, which is expelled.

Mammalian kidneys have a somewhat granular outer section (the cortex), containing the glomeruli and convoluted tubules, and a smooth, somewhat striated inner section (the medulla), containing the loops of Henle and the collecting tubules. As the ureter enters the kidney it enlarges into a cavity, the renal pelvis; urine passes into this pelvis from the collecting tubules. Nephrons are numerous.

In humans the kidneys are about 10 centimetres long and are located beneath the diaphragm and behind the peritoneum. Each kidney contains 1,000,000.1,250,000 nephrons that filter the entire five-quart water content of the blood every 45 minutes.an equivalent of 160 quarts a day. Of this, only 1 1/2 quarts are excreted; the remainder is reabsorbed by the nephrons.