Kidney

Kidney, one of a pair of organs of the urinary system, located in the back part of the abdomen, on each side of the vertebral column; the left lies slightly higher than the right. A high concentration of blood vessels gives the kidney a dark, reddish-brown color, and each is bean-shaped and slightly tilted. In adults, the kidney is about 4 in (10 cm) long and 2.5 in (6.5 cm) wide. At least one kidney must function for life to be maintained.

Through the activity of millions of nephrons, filtering units that are the organ's basic functional components, the kidneys perform a number of tasks: the removal from the blood of nitrogenous waste, mostly in the form of urea (a main constituent of the urine that the kidneys form); the maintenance of the body's electrolyte balance; the maintenance of the body's water balance; the regulation of the body's acid-base balance. In addition, the kidneys release a number of chemical substances (acting as hormones) into the blood. Urine is formed in the kidneys as an aqueous solution (95% water) containing metabolic waste products, foreign substances, and water-soluble constituents of the body. The quantity of urine produced by the kidneys depends on the balance of water and other essential substances in the body.

Among the most common kidney diseases are pyelonephritis, or kidney infection, and a type of inflammation called glomerulonephritis (formerly Bright's disease). If a person loses or suffers damage to both kidneys, he or she may be kept alive by a dialysis machine. The machine is attached to an artery in the patient's arm; blood flows through one tube into the machine, which removes wastes, and then flows back into the patient through a second tube. The patient must undergo this procedure for several hours, three days a week.