The kidneys are bean-shaped organs, each about the size of a closed fist. They are located near the middle of the back. The kidneys are sophisticated reprocessing machines. Every day, a person’s kidneys process about 200 liters of blood to sift out about 2 liters of waste products and extra water. The wastes and extra water become urine, which flows to the bladder through tubes called ureters. The bladder stores urine until releasing it through urination.
Wastes in the blood come from the normal breakdown of active tissues, such as muscles, and from food. The body uses food for energy and self-repairs. After the body has taken what it needs from food, wastes are sent to the blood. If the kidneys did not remove them, these wastes would build up in the blood and damage the body.
In addition to removing wastes, the kidneys release three important hormones:
- erythropoietin, or EPO, which stimulates the bone marrow to make red blood cells
- renin, which regulates blood pressure
- calcitriol, the active form of vitamin D, which helps maintain calcium for bones and for normal chemical balance in the body