Stomach

Synonyms: Ventriculus, Gaster
German: Magen

1 General

The stomach is the hollow organ interposed between the esophagus and the duodenum in which the first steps of digestion take place. In its most primitive form, it already is present in the Cnidarians (corals, jellyfish, sea anemones and freshwater polyps).

2 Anatomy

2.1 Form

The stomach is a tube to sack-shaped organ that can be found in the upper left quadrant of the abdomen slightly below the diaphragm. Due to its curved shape, it has a convex stomach curvature turned to the left, and a concave curvature turned to the right and cranial side. The supplying vessels of the stomach run along their side. Their Latin names are:

• Curvatura gastrica major (greater curvature of the stomach)
• Curvatura gastrica minor (lesser curvature of the stomach)

2.2 Division

The stomach is divided into four parts that all have a different histology and function. One can differentiate between:

• Pars cardiaca ventriculi or cardia: the short section of the stomach pointing to the oral and, with the junction of the esophagus
• Fundus ventriculi or fundus: the section arched upwards next to the cardia on the left
• Corpus ventriculi or body: the central body of the stomach which makes the main part of the organ
• Pars pylorica ventriculi: the end portion of the stomach with the pyloric antrum and the pylorus.

The superior stomach aperture is called "upper mouth of the stomach" or Ostium cardiacum, the inferior stomach aperture is the "lower mouth of the stomach" or Ostium pyloricum.

2.3 Position

The stomach has an intraperitoneal position; as evolutionary relict, it has two mesogastriums mesenteries) that develop as greater omentum or lesser omentum as well as the connections between the stomach and the neighboring organs, and that include the omental bursa.

2.3.1 Peritoneal covers of the stomach

• Gastrosplenic ligament: it represents the peritoneal connection between the greater curvature of the stomach and the splenic hilus.
• Gastrophrenic ligament: it is the superior continuation of the gastrosplenic ligament between the stomach and the diaphragm.
• Gastrocolic ligament: it is the inferior continuation of the gastrosplenic ligament between the greater curvature of the stomach and the omental taenia of the transverse colon.
• Hepatogastric ligament: it is a part of the lesser omentum between the lesser gastric curvature and the hepatic portal.
• Grater omentum: peritoneal skirt at the greater curvature of the stomach.

2.4 Arterial supply

The arterial supply of the stomach is ensured by various blood vessels that all originate in the celiac trunk and form numerous anastomoses among each other:

• Right gastric artery, out of the proper hepatic artery. Located at the lesser curvature.
• Left gastric artery, directly out of the celiac trunk. Located at the lesser curvature.
• Short gastric arteries, out of the splenic artery, for the supply of the fundus.
• Right gastroepiploic artery, out of the gastroduodenal artery. Located at the greater curvature.
• Left gastroepiploic artery, out of the splenic artery. Located at the greater curvature.
• Posterior gastric artery, out of the splenic artery. Supplies the rear wall of the stomach.

2.5 Venous drainage

The venous drainage of the stomach

• on the side of the lesser curvature goes to the right gastric vein and left, and
• in the area of the greater curvature over the right omental veins under the left under interpolation of the splenic vein and the superior mesenteric vein into the portal vein.

2.6 Innervation

As other parts of the digestive tract, also the stomach has an autonomously working nervous system (enteric nervous system). The autonomous activity of the stomach is regulated by the following efferences:

2.6.1 Parasympathetic fibers

The parasympathetic fibers come from both vagus nerves. Since there are two vagal trunks on the esophagus, both the anterior and posterior gastric plexus, which supply the front or rear wall (Paries anterior and Paries posterior) of the stomach, originate there. Frequently, some small branches (Rami pylorici) leave the vagal trunks directly after passing the diaphragm (esophageal hiatus) to draw towards the pylorus. On the way there, they draw towards the hepatic portal together with the hepatic branches at first, while afterwards reaching the pylorus continuing in caudal direction in the lesser omentum. An activation of the parasympathetic nervous system leads to an increased of gastric motility and secretion, as well as to vascular dilatation (vasodilatation).

2.6.2 Sympathetic innervation

The sympathetic innervation is provided by postganglionic nerve fibers from the celiac ganglia which draw towards the stomach together with the arteries. The greater splanchnic nerve is the main nerve that gives off the preganglionic fibers and thereby makes an essential contribution to the sympathetic innervation. An activation of the sympathetic nervous system leads to an inhibition of gastric motility and reduces gastric secretion. Additionally, it causes vasoconstriction.

3 Histology

The stomach is schematically characterized by the same wall structure as all other sections of the gastrointestinal tract. Underneath the gastric mucosa you can find a submucosal layer of connective tissue; a three-layered, winding muscular coat (Tunica muscularis) ensures the onward transport of the chyme and its mixing via coordinated contractions of the smooth muscles (peristalsis).

The stomach mucosa has additional glands, the so-called gastric glands. One can differentiate between:

• Cardiac glands
• Fundus glands
• Pyloric glands

In these glands, you can find various types of cellls: Especially in the cardiac area, you can find mucous cells secreting mucus, while the chief cells of the corpus area mainly produce pepsinogen. Additionally, there are parietal cells that secrete protones via a luminal H⁺-K⁺-Adenosintriphosphatase and therewith are responsible for the acidic gastric pH value. Moreover, they secrete intrinsic factor.

4 Physiology

The stomach has manifold tasks in the digestion process that go far beyond the simple uptake of the comminuted food. Stomach motility and the continuous secretion of gastric juice, whose most aggressive component contains gastric acid, play a central role in this process.

4.1 Pre-digestion

The stomach takes over the first part of digestion and is subject to a complex endocrine regulation in its secretory and mechanical function. Proteins are initially degraded by an acidification of the chyme with gastric acid and addition with pepsin and cathepsin. This leads to a denaturation or moisture expansion of the proteins, and their splitting into smaller polypeptide fragments. Moreover, the myogenic gastric peristalsis ensures thorough mixing of the chyme, which effects an emulsification of the fats contained in the food and optimizes the fat digestion. Carbohydrate digestion in the stomach is suspended since the activity of the alpha-amylaseis stopped by the acidic gastric pH value.

4.2 Storage function

The stomach is able to store food due to its elasticity and sphincter muscles. Thus, it ensures an uniform "smoothed" onward transport of the chyme to the distal sections of the intestines.