The inner lining of the stomach is a thick, wrinkled mucous membrane dotted with gastric pits. These pits are the open ends of gastric glands , some of which secrete mucus, while others secrete digestive enzymes or hydrochloric acid into the stomach. The stomach has a band of circular muscle at each end regulating the flow into and out of the stomach. When the cardiac sphincter is closed, food cannot reenter the esophagus. For food to pass out of the lower end of the stomach, the pyloric sphincter must open.
Food usually remains in the stomach for three to four hours.
Secretions from the liver and pancreas, as well as enzymes from the intestine lining break down peptides into amino acids, disaccharides into monosaccharides, and fats into glycerol and fatty acids. Minerals, vitamins, and the remainder of the water are absorbed into the blood stream from the large intestine. The first half of the colon absorbs fluids and recycles them into the blood stream. The second half compacts the wastes and secretes mucus which binds the wastes into feces. The liver produces bile to be stored in the gallbladder.