STOMACH

saclike expansion of the animal digestive system, between the esophagus and the small intestine; it is located in the anterior portion of the abdominal cavity in most vertebrates. The stomach serves as a temporary receptacle for storage and mechanical distribution of food before it is passed into the intestines. In animals whose stomachs contain digestive glands, some of the chemical processes of digestion also occur in the stomach. Humans. The human stomach is subdivided into four regions: the cardia (so named because it is nearest to the heart), an opening leading down from the esophagus; the fundus, an expanded area curving up above the cardiac opening; the body, or intermediate, region, the central and largest portion; and the pylorus (q.v.), a narrowing where the stomach joins the small intestine. Each of the openings, the cardiac and the pyloric, has a sphincter muscle that keeps the neighbouring region closed, except when food is passing through. In this manner, food is enclosed by the stomach until ready for digestion. The stomach has the ability to expand or contract depending upon the amount of food contained within it. When contracted, the interior walls form numerous folds (rugae), which disappear when the walls are distended. The thick mucous-membrane lining of the walls is densely packed with small gastric glands (q.v.) that secrete a mixture of enzymes and hydrochloric acid to partly digest proteins and fats. Human stomach muscles are rarely inactive. Upon entry of food they relax briefly, then begin to contract. Periodic contractions churn and knead food into a semifluid mixture called chyme; rhythmical pumping (peristaltic) waves move food toward the pylorus and small intestine. Peristaltic contractions persist after the stomach empties and, increasing with time, may become painful. Such hunger pangs may also be related to the amount of sugar in the blood. If the level of sugar decreases significantly, hunger can be experienced without the stomach's intervention. The absorption of food, water, and electrolytes by the stomach is practically negligible, but iron and highly fat-soluble substances such as alcohol and some drugs are absorbed directly. Secretions and movements of the stomach are controlled by the vagus nerve and the sympathetic nervous system; emotional stress can alter normal stomach functions. Common stomach disorders include peptic ulcer, cancer, and gastritis.