Your kidneys are each about the size of a fist; they're located in the centre of your back, just below your rib cage. The kidneys act as filters for the blood; they remove cell waste products and extra water. This watery waste (called urine) flows to your bladder and is eliminated. If your kidneys didn't remove these wastes, they would build up in the blood and damage your body. The kidneys also control the levels of sodium, phosphorus and potassium in your blood. These chemicals are necessary for the body to function, but too much of any of them can be harmful. In addition to removing wastes, your kidneys release three important hormones: erythropoietin, or EPO, which tells the body's bone marrow to make new red blood cells renin, which controls blood pressure calcitriol, or vitamin D, which helps keep calcium in your bones BACK