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In human anatomy, the stomach is a J-shaped hollow muscular organ of the gastrointestinal tract involved in the second phase of digestion, following mastication. The word stomach is derived from the Latin stomachus, which derives from the Greek word stomachos (στόμαχος). The words gastro- and gastric (meaning related to the stomach) are both derived from the Greek word gaster (γαστήρ).
The stomach is a highly acidic environment due to hydrochloric acid production and secretion which produces a luminal pH range usually between 1 and 2 depending on the species, food intake, time of the day, drug use, and other factors. Combined with digestive enzymes, such an environment is able to break down large molecules (such as from food) to smaller ones so that they can eventually be absorbed from the small intestine. A zymogen called pepsinogen is secreted by chief cells and turns into pepsin under low pH conditions and is a necessity in protein digestion.
The human stomach can produce and secrete about 2.2 to 3 liters of gastric acid per day with basal secretion levels being typically highest in the evening. The stomach can expand to hold between 2-4 liters of food. It is a temporary food storage area, and in the process of digestion, the food goes into the stomach first.
Absorption of vitamin B12 from the small intestine is dependent on conjugation to a glycoprotein called intrinsic factor which is produced by parietal cells of the stomach.
Other functions include absorbing some ions, water, and some lipid soluble compounds such as alcohol, aspirin, and caffeine.
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 top of the stomach lies against the diaphragm. Lying beneath the stomach is the pancreas, and the greater omentum which hangs from the greater curvature.
Two smooth muscle valves, or sphincters, keep the contents of the stomach contained. They are the esophageal sphincter (found in the cardiac region) dividing the tract above, and the Pyloric sphincter dividing the stomach from the small intestine.
The stomach is surrounded by parasympathetic (stimulant) and orthosympathetic (inhibitor) plexuses (anterior gastric, posterior, superior and inferior, celiac and myenteric), which regulate both the secretory activity and the motor activity of the muscles.
In humans, the stomach has a relaxed volume of about 45 ml, it generally expands to hold about 1 liter of food, but can hold as much as 4 liters.
The stomach is divided into four sections, each of which has different cells and functions. The sections are:
| Cardia | Where the contents of the esophagus empty into the stomach. |
| Fundus | Formed by the upper curvature of the organ. |
| Body or corpus | The main, central region. |
| Pylorus or antrum | The lower section of the organ that facilitates emptying the contents into the small intestine. |
The lesser curvature of the stomach is supplied by the right gastric artery inferiorly, and the left gastric artery superiorly, which also supplies the cardiac region. The greater curvature is supplied by the right gastroepiploic artery inferiorly and the left gastroepiploic artery superiorly. The fundus of the stomach, and also the upper portion of the greater curvature, are supplied by the short gastric artery.
Like the other parts of the gastrointestinal tract, the stomach walls are made of the following layers, from inside to outside:
| mucosa | The first main layer. This consists of an epithelium, the lamina propria underneath, and a thin layer of smooth muscle called the muscularis mucosae. |
| submucosa | This layer lies under the mucosa and consists of fibrous connective tissue, separating the mucosa from the next layer. The Meissner's plexus is in this layer. |
| muscularis externa | Under the submucosa, the muscularis externa in the stomach differs from that of other GI organs in that it has three layers of smooth muscle instead of two.
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| serosa | This layer is under the muscularis externa, consisting of layers of connective tissue continuous with the peritoneum. |
The epithelium of the stomach forms deep pits. The glands at these locations are named for the corresponding part of the stomach:
| Cardiac glands (at cardia) | Pyloric glands (at pylorus) | Fundic glands (at fundus) |
Different types of cells are found at the different layers of these glands:
| Layer of stomach | Name | Secretion | Region of stomach | Staining |
| Isthmus of gland | mucous cells | mucus gel layer | Fundic, cardiac, pyloric | Clear |
| Neck of gland | parietal (oxyntic) cells | gastric acid and intrinsic factor | Fundic, cardiac, pyloric | Acidophilic |
| Base of gland | chief (zymogenic) cells | pepsinogen, rennin | Fundic only | Basophilic |
| Base of gland | enteroendocrine (APUD) cells | hormones gastrin, histamine, endorphins, serotonin, cholecystokinin and somatostatin | Fundic, cardiac, pyloric | - |
The movement and the flow of chemicals into the stomach are controlled by both the autonomic nervous system and by the various digestive system hormones:
| Gastrin | The hormone gastrin causes an increase in the secretion of HCl, pepsinogen and intrinsic factor from parietal cells in the stomach. It also causes increased motility in the stomach. Gastrin is released by G-cells in the stomach to distenstion of the antrum, and digestive products. It is inhibited by a pH normally less than 4 (high acid), as well as the hormone somatostatin. |
| Cholecystokinin | Cholecystokinin (CCK) has most effect on the gall bladder, but it also decreases gastric emptying and increases release of pancreatic juice which is alkaline and neutralizes the chyme. |
| Secretin | In a different and rare manner, secretin, produced in the small intestine, has most effects on the pancreas, but will also diminish acid secretion in the stomach. |
| Gastric inhibitory peptide | Gastric inhibitory peptide (GIP) decreases both gastric acid and motility. |
| Enteroglucagon | enteroglucagon decreases both gastric acid and motility. |
Other than gastrin, these hormones all act to turn off the stomach action. This is in response to food products in the liver and gall bladder, which have not yet been absorbed. The stomach needs only to push food into the small intestine when the intestine is not busy. While the intestine is full and still digesting food, the stomach acts as storage for food.
Historically, it was widely 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.
