Located just below your ribcage on either side of your spine are your kidneys. These two bean-shaped organs, though no larger than your fist, constitute a vital part of maintaining the proper functioning of your body. The primary role of the kidneys is the removal of harmful substances, waste products and excess fluids from the body through the urine (production of which involves excretion and re-absorption). This process is necessary to maintain a stable balance of body chemicals and fluids. The kidneys also perform the critical function of regulating the body's salt, potassium and acid levels. In addition, the kidneys produce hormones that affect the function of other organs. For instance, erythropoietin – a hormone produced by the kidneys – determines the rate of red blood cell production. The kidneys also use calcitriol, an active form of vitamin D for the absorption of calcium, thereby promoting strong, healthy bones. Another hormone produced by the kidneys is renin which helps regulate blood pressure.