The kidneys remove excess organic molecules from the blood, which are the waste products of metabolism. They are also essential in controlling homeostatic functions, such as the regulation of electrolytes, maintenance of acid-base balance, and regulation of blood pressure (via maintaining salt and water balance). They serve the body as a natural filter of the blood, and remove water soluble wastes. The kidneys also produce hormones, including calcitriol, erythropoietin, and the enzyme renin.