There are excellent articles on enotes at the links posted below. The term gastrointestinal refers to both the stomach and the intestines, small and large. The stomach is a muscular J-shaped pouch that churns the food, and is responsible for the beginning of protein digestion due to the protease, an enzyme in the gastric juice secreted by glands in the stomach's lining. The acidic ph provides optimal conditions for the stomach enzymes to function and it also kills germs present on our food. The small intestine is actually over 20 feet in length and contains glands that secrete enzymes needed to hydrolyze fats, proteins and carbohydrates, into endproducts that can then be absorbed. Villi, tiny projections containing capillaries and lymph vessels are present in the small intestine to absorb these endproducts. The large intestine, is stocky but only about five feet in length. It reabsorbs water into the circulating bloodstream and collects undigested materials which become feces, which are eliminated from the anus.