The kidneys are organs that serve several essential regulatory roles in most animals, including vertebrates and some invertebrates.The process of regulating the levels of water and dissolved salts in the blood, is termed as osmoregulation. The kidneys also serve homeostatic functions by regulating the amount of water in the body, and the concentration of dissolved ions in the blood. Homeostasis involves regulating various internal conditions such as the regulation of electrolytes, maintenance of acid–base balance, and regulation of blood pressure (via maintaining salt and water balance) in order to maintain a constant internal environment. The kidneys serve the body as a natural filter of the blood, and remove wastes which are diverted to the urinary bladder. In producing urine, the kidneys excrete wastes such as urea and ammonium, and they are also responsible for the reabsorption of water, glucose, and amino acids. The kidneys also produce hormones including calcitriol, erythropoietin, and the enzyme renin. Located at the rear of the abdominal cavity in the retroperitoneum, the kidneys receive blood from the paired renal arteries, and drain into the paired renal veins. Each kidney excretes urine.