To maintain blood volume within a normal range, the kidneys regulate the amount of water and sodium lost into the urine. For example, if excessive water and sodium are ingested, the kidneys normally respond by excreting more water and sodium into the urine.
Enzyme secretion is stimulated by endocrine factors (cholecystokinin, secretin, and, much more weakly, gastrin) and neuronal factors [acetylcholine and vasoactive intestinal polypeptide (VIP)]. The pancreas produces enzymes that act on starch, lipids, and proteins introduced with the diet.
Supportive connective tissue—bone and cartilage—provide structure and strength to the body and protect soft tissues.