The Stomach

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The stomach, located in the upper abdomen just below the diaphragm, is a saclike structure with strong, muscular walls. The stomach can expand significantly to store all the food from a meal for both mechanical and chemical processing. The stomach contracts about three times per minute, churning the food and mixing it with gastric juice. This fluid, secreted by thousands of gastric glands in the lining of the stomach, consists of water, hydrochloric acid, an enzyme called pepsin, and mucin (the main component of mucus). Hydrochloric acid creates the acidic environment that pepsin needs to begin breaking down proteins. It also kills microorganisms that may have been ingested in the food. Mucin coats the stomach, protecting it from the effects of the acid and pepsin. About four hours or less after a meal, food processed by the stomach, called chyme, begins passing a little at a time through the pyloric sphincter into the duodenum, the first portion of the small intestine.

Stomach

Stomach, organ of the digestive system. Most animals, like humans, have a single stomach, but birds and ruminants have digestive organs composed of two or more chambers. The outer surface of the stomach is smooth; the inner surface is folded into numerous complex ridges, which assist in the mixing of food with digestive juices and channel this material through the stomach into the intestines. Only water, alcohol, and certain drugs seem to be absorbed from the stomach; most food absorption takes place in the small intestine.

In humans the stomach is situated in the upper part of the abdominal cavity (see Abdomen), mostly to the left of the midline. The large, domed end of the stomach, the fundus, lies in the left vault of the diaphragm; the esophagus enters the upper side, or lesser curvature, a short distance below the fundus. The region immediately below the fundus is called the body. The upper part of the stomach, spoken of as the cardiac portion, includes the fundus and body. The lower, or pyloric, portion curves downward, forward, and to the right and includes the antrum and pyloric canal. The latter is continuous with the upper part of the small intestine, the duodenum.

The tissues of the stomach include an outer fibrous coat derived from the peritoneum and, beneath this, a coat of smooth muscle fibers arranged in diagonal, longitudinal, and circular layers. At the junction of the esophagus and stomach the circular muscles are much enlarged, forming the esophageal sphincter. Contraction of this muscle prevents the regurgitation of gastric contents into the esophagus. A similar structure, the pyloric sphincter, is found at the junction of the pylorus and the duodenum. Another layer of the stomach, the submucosa, is made up of loose connective tissue in which are found numerous blood and lymph vessels (see Circulatory System) and nerves of the autonomic nervous system. The innermost layer, the mucosa, contains secretory cells.