The main job of the kidneys is to remove wastes from the blood and return the cleaned blood back to the body.

Every minute, about one litre of blood (one fifth of all the blood pumped by the heart) enters the kidneys through the renal arteries. The blood then runs through tiny filtering units of the kidney, called nephrons.

Each kidney is made up of approximately one million nephrons. The nephrons either save substances the body needs (to be carried by the renal veins back to the body's cells) or they eliminate excess nutrients, water and waste products, as urine. The ureters carry the urine from the kidneys to the bladder where it is stored until you urinate. Urine passes out of the body through a tube called the urethra.