kidney [ME, kidnere] , one of a pair of bean-shaped, purplish brown urinary organs in the retroperitoneal abdominal cavity, located on each side of the vertebral column between the twelfth thoracic and third lumbar vertebrae. In most individuals the right kidney is slightly lower than the left. Each kidney is about 11 cm long, 6 cm wide, and 2.5 cm thick. In the newborn the kidneys are about three times as large in proportion to the body weight as in the adult. The kidneys filter the blood and eliminate wastes in the urine through a complex filtration network and resorption system comprising more than 2 million nephrons. The nephrons are composed of glomeruli and renal tubules that filter blood under high pressure, removing urea, salts, and other soluble wastes from blood plasma and returning the purified filtrate to the blood. The average blood flow through the kidney is 1200 mL/min. Blood enters the kidneys through the renal arteries and leaves through the renal veins. The kidneys remove water as urine and return water that has been filtered to the blood plasma, thus helping to maintain the water balance of the body. Hormones produced by the pituitary gland, especially the antidiuretic hormone, control the function of the kidneys in regulating the water-electrolyte balance of the body.
