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[Gr. stomachos, mouth, gullet, opening]
A muscular, distensible saclike portion of the alimentary tube between the esophagus and duodenum.
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It is below the diaphragm to the right of the spleen, partly under the liver. It is composed of an upper fundus, a central body, and a distal pylorus. It has two openings: the upper cardiac orifice opens from the esophagus and is surrounded by the lower esophageal (cardiac) sphincter. The lower pyloric orifice opens into the duodenum and is surrounded by the pyloric sphincter. The wall of the stomach has four layers. The outer serous layer (visceral peritoneum) covers almost all of the organ. The muscular layer just beneath it has three layers of smooth muscle: an outer longitudinal layer, a medial circular layer, and an inner oblique layer. The submucosa is made of connective tissue that contains blood vessels. The mucosa is the lining that contains the gastric glands, simple tubular glands of columnar epithelium that secrete gastric juice. Chief cells secrete pepsinogen; parietal cells secrete hydrochloric acid and the intrinsic factor; mucous cells secrete mucus; G cells secrete gastrin.

The stomach is a reservoir that permits digestion to take place gradually; emptying of the stomach is under both hormonal and nervous control. Secretions and motility are increased by parasympathetic impulses (vagus nerves) and decreased by sympathetic impulses. The presence of food stimulates the production of the hormone gastrin, which increases the secretion of gastric juice. Protein digestion begins in the stomach; pepsin digests proteins to peptones. Hydrochloric acid converts pepsinogen to active pepsin and has little effect on unemulsified fats except those of cream. The intrinsic factor in gastric juice combines with vitamin B12 (extrinsic factor) to prevent its digestion and promote its absorption in the small intestine. Little absorption takes place in the stomach because digestion has hardly begun, but water and alcohol are absorbed.

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