The stomach is part of the digestion system and essential for the nutrient supply to the body. Its acidic gastric juice acts as a barrier to bacteria which could otherwise infiltrate the intestines and other abdominal organs.
The stomach may be subdivided into four sections:
- Cardiac part - Below the diaphragm, the abdominal part of the esophagus becomes the cardiac part of the stomach. Here the ingested food reaches the stomach through the cardiac orifice.
- Fundus - Further cranially lies the dome-like fundus filled with gas (in the upright body position), which is sharply separated from the cardia by the cardiac incisure
- Corpus - The largest part of the stomach is the corpus (or body), which forms the lesser curvature (cranial) and the greater curvature (caudal).
- Pyloric part - The corpus is followed by the pyloric part, which is perpendicular to the long axis of the stomach. The pylorus sphincter marks the lower end of the stomach and the entrance to the duodenum.
Histologically the stomach wall is divided into five layers. From superficial to deep there is the:
- Serosa (peritoneum)
- Muscularis externa - It consists of an external longitudinal, a middle circular and an inner oblique layer.
- Submucosa - This layer consists of loose connective tissue with large blood vessels.
Mucosa - It
lines the stomach cavity and consists of an about 1 mm high single-layer columnar epithelium which is covered by a 100 to 200 µm thick mucus carpet. Further gastric foveolae (at which ends the gastric glands sit) extend up to the muscularis mucosa. The tubular glands are formed by different exocrine cells which secrete various substances – depending on the part of the stomach.
In the corpus and fundus there are mainly mucous neck cells, parietal cells, chief cells, but also enteroendocrine cells. The mucous neck cells produce mucous for the alkaline mucus carpet which protects the epithelium from self-digestion through gastric juice. On the other hand, the parietal cells secrete hydrogen and chloride ions which bond together to hydrochloric acid and keep the pH level of the stomach at around 1 to 1,5. The chief cells release an inactive precursor of the pepsin enzyme, the pepsinogen. In the gastric juice it is activated through the acidic milieu and starts it proteolysis functions.
The three main arteries supplying the stomach are the:
- Common hepatic artery - Its branches supply the greater and lesser curvature.
- Splenic artery - It supplies the posterior stomach wall, fundus and the greater curvature.
- Left gastric artery - Its branches participate in the supply of the upper part of the lesser curvature.
They all arise from the coeliac trunk, a branch of the abdominal aorta.
The stomach is innervated sympathetically and parasympathetically. The parasympathetic supply comes from the vagus nerve (cranial nerve X). Both of its branches (anterior and posterior vagal trunk) stimulate the gastric glands and musculature, which leads to slow peristaltic contractures during the passage of chime.
Sympathetic fibers from the splanchnic nerves are responsible for the motor innervation of the pylorus and the sensory innervation of the stomach mucosa.