The stomach is a pouch-like organ in the abdomen into which ingested material descends from the esophagus. The stomach empties into the duodenum. The cells on the mucosa of the stomach secrete about 3000 ml of gastric juice each day to aid the digestion of food, particularly proteins (the stomach secretes protein-digesting enzymes such as pepsin and gastrin. The intrinsic factor, a substance necessary for the absorption of vitamin B12 from the small intestine, is also secreted in the stomach.