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In anatomy, the stomach (in ancient Greek στόμαχος) is an organ of the gastrointestinal tract involved in the second phase of digestion, following mastication. The stomach's primary function is to break down large molecules (such as from food) into smaller ones using hydrochloric acid so that they can be absorbed from the small intestine. Secondary functions include protein digestion using pepsin enzymes, and absorbing water, some ions, and some lipid soluble compounds such as alcohol, aspirin, and caffeine. The Latin names Ventriculus and Gaster became the basis for the medical terms "gastro-" and "gastric" meaning related to the stomach. The stomach is generally a highly acidic environment with a pH range of 1 to 4 depending on food intake and other factors. Pepsinogen is secreted by chief cells and turns into pepsin in the presence of hydrochloric acid. The stomach's interior can secrete 2 to 3 litres of gastric fluid per day.
Anatomy of the human stomach The stomach lies between the esophagus and the duodenum (the first part of the small intestine). It is on the left side of the abdominal cavity, the fundus of the stomach lying against the diaphragm. Lying beneath the stomach is the pancreas, and the greater omentum hangs from the greater curvature. The stomach is divided into four sections, each of which has different cells and functions. The sections are: 1) Cardiac region, where the contents of the esophagus empty into the stomach, 2) Fundus, formed by the upper curvature of the organ, 3) Body, the main central region, and 4) Pylorus or antrum, the lower section of the organ that facilitates emptying the contents into the small intestine. Two smooth muscle valves, or sphincters, keep the contents of the stomach contained. They are the: 1) Cardiac or esophageal sphincter, dividing the tract above, and 2) Pyloric sphincter, dividing the stomach from the small intestine. In humans, the stomach has a volume of about 50 mL when empty. After a meal, it can expand to hold about 1 liter of food, but it can actually expand to hold as much as 4 L. Vessels and nerves
Histology of the human stomach Like the other parts of the gastrointestinal tract, the stomach walls are made of a number of layers. From inside to outside, the first main layer is the mucosa. This consists of an epithelium, the lamina propria underneath, and a thin bit of smooth muscle called the muscularis mucosae. The submucosa lies under this and consists of fibrous connective tissue, separating the mucosa from the next layer, the muscularis externa. The muscularis in the stomach differs from that of other GI organs in that it has three layers of muscle instead of two. Under these muscle layers is the adventitia, layers of connective tissue continuous with the omenta. The epithelium of the stomach forms deep pits, called fundic or oxyntic glands. Different types of cells are at different locations down the pits. The cells at the base of these pits are chief cells, responsible for production of pepsinogen, an inactive precursor of pepsin, which degrades proteins. The secretion of pepsinogen prevents self-digestion of the stomach cells. Further up the pits, parietal cells produce gastric acid and intrinsic factor. Near the top of the pits, closest to the contents of the stomach are goblet cells which form the mucus gel layer above the mucosa which help protect the stomach from self-digestion. The muscularis externa is made up of three layers of smooth muscle. The innermost layer is obliquely-oriented; this is not seen in other parts of the digestive system; this layer is responsible for creating the motion that churns and physically breaks down the food. The next layers are the square and then the longituditinal, which are present as in other parts of the GI tract. Theoso antrum which has thicker skin cells in its walls and performs more forceful contractions than the fundus. The pylorus is surrounded by a thick circular muscular wall which is normally tonically constricted forming a functional (if not anatomically discrete) pyloric sphincter, which controls the movement of chyme into the duodenum. Control of secretion and motility
Ruminants In ruminants, such as bovines, the stomach is a large multichamber organ which hosts symbiotic bacteria that produce enzymes required for the digestion of cellulose from plant matter. The partially digested plant matter passes through each of the intestine chambers in sequence, being regurgitated and rechewed at least once in the process. Diseases of the stomach For a long time, many people believed that the highly acidic environment of the stomach would keep the stomach immune from infection. However, a large number of studies have indicated that most cases of stomach ulcers, gastritis and stomach cancer are caused by Helicobacter pylori infection. It is believed that excess hydrochloric acid can cause stomach ulcers in the presence of holes in the mucus gel layer. See also | |||||||||||||||||||||||||||||||||||||
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