The kidneys are two bean-shaped organs at the back of the abdomen that filter and clean the blood, removing chemical wastes and excess water. Wastes drain out of the kidneys as urine.
Each kidney contains about a million tiny filtering units called nephrons. Blood flows through a kind of filter at the top of each nephron, and water and small molecules pass through it into a long tube. In the tube useful substances such as glucose and salt are then reabsorbed, and what is left over forms urine.
If the kidneys did not filter the blood, chemical wastes would build up in the body and become poisonous. The kidneys also control the level of water in the blood by varying how much water is reabsorbed in the nephrons. If we drink a lot, the kidneys can excrete the excess to stop the blood from becoming diluted.
Urine from each kidney drains through a tube called a ureter and collects in the bladder. As the bladder fills, its muscular wall stretches. When it is full, receptors in the wall send a signal to the brain and trigger the urge to urinate. During urination the muscles that close the bladder exit relax to release the urine.