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Stomach

The stomach is the hollow organ that helps along digestion after mastication (chewing). It is the next step after the esophagus and before the small intestines.

Formation and Orientation

The stomach is composed of four parts. The cardia is the first part of the stomach in the digestive tract. It is the part of the stomach that allows the food to empty from the esophagus. The most northern part of the stomach is the Fundus. This section is the part that creates the curved part of the stomach. Next, the corpus or body is the piece of the stomach that holds most food as the stomach works to break it down. The last part is the Pylorus which is the passage that leads from the lower portion of the stomach to the small intestine.

The stomach is also made of many layers. The mucosa is comprised of the epithelium, loose connective tissues called the lamina propria, and the muscularis mucosae which is a thin layer of smooth muscle. The submucosa is beneath the mucosa. The Meissner’s plexus is in the submucosa which is accompanied by fibrous connective tissues. Next, the muscularis externa consists of four layers of its own. The inner oblique layer is the layer that causes stomach contractions. The middle circular layer controls the movement of chime into the duodenum. The Auerbach’s plexus layer also helps with the churning of the stomach. The final layer of the muscularis externa is the outer longitudinal layer. The last layer of the stomach is the  serosa which contains the peritoneum as well as the layers of connective tissues.

There are three main sections for glands in the stomach; the cardiac, pyloric and fundic areas. In all three types, the glands contain mucous neck cells and enteroendocrine (APUD) cells at the isthmus and bases respectively. The fundic glands have two extra types of cells, the pariental (oxyntic) and chief (zymogenic) cells in the body and base of the glands.

Functions

The stomach functions as a major part of the digestive system. It works as a stopping point in the digestive tract to begin the process of turning food to fuel and waste.

To assist with the breakdown of foods, the number of glands secrete different gastrointestinal hormones; Gastrin, Cholecystokinin, Secretin, Motilin, Vasoactive Intestinal Peptide (VIP), Gastric Inhibitory Polypeptide (GIP) and Enteroglucagon. Somatostatin is released by the fundic, cardiac and pyloric glands to suppress the release of these gastrointestinal hormones.

Gastrin is the hormone that stimulates the release of gastric acid and aids in motility. Cholecystokinin helps to cause contractions in the gall bladder. Secretin regulates the secretion in the stomach and pancreas all while regulating water homeostasis. Motilin is a basic amino acid hormone that influences gastric motility. Vasoactive Intestinal Peptide (VIP) is the hormone that stimulates the relaxation of smooth muscle in the stomach and gall bladder. Gastric Inhibitory Polypeptide (GIP) neutralizes stomach acid to protect the small intestine from acid damage. Enteroglucagon is released after ingestion to delay gastric emptying.

Image Caption: Stomach diagram in Inkscape. Credit: Henry W. Schmitt/Wikipedia

Stomach


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