The kidneys are responsible for the excretion of nitrogenous waste products and the elimination of the non-volatile acids produced by metabolism. Through selective reabsorption of the bulk of the glomerular filtrate, they regulate the volume and composition of the body fluids. Because of the part they play in the control of sodium balance and in the production of the enzyme renin, they are intimately concerned in the regulation of blood pressure. As the main source of the hormone erythropoetin, they are essential to normal red blood cell production. In view of the variety and importance of the tasks performed by the kidneys, it is not surprising that when renal failure supervenes the effects are manifest in practically every system of the body.