The liver regulates the composition of the blood by receiving the products of digestion directly through the hepatic portal system, permitting it to modify or detoxify materials before they are returned to the heart for pumping throughout the body. In this way, the liver can break down drugs, alcohol, or toxins to which the body has been exposed. Through the secretion of glycogen, the liver can also help control blood glucose levels. The main function of the colon is to concentrate solids and reabsorb water. All of these hormones are stimulated by the presence of food or chyme in the appropriate region of the digestive tract.
Gastrin stimulates secretion of HCl and pepsinogen by the stomach.
GIP retains material in the stomach and stimulates release of insulin. CCK stimulates the gallbladder to contract to release bile for the emulsification of fats. Herbivorous vertebrates need bacteria to digest cellulose. Symbionts have protists instead of bacteria to do the same thing. A vitamin is formally defined as an essential organic substance used in small amounts that must be consumed in the diet because the organism is incapable of synthesizing it.