Chapter 38. The Stomach

The stomach lies in the epigastrium and is composed of mucosa, submucosa, a thick muscle layer, and serosa. The mucosa of the stomach is thrown into regular folds, or rugae. The serosa on the lesser and greater curvatures is continuous with the lesser and greater omentum.

The Gastric Mucosa

(Figure 38-1)

The epithelial lining of the mucosa is composed of uniform mucous cells without goblet cells. Simple tubular glands open on the surface at epithelial pits. The glands vary in structure in different parts of the stomach: (1) In the cardiac region near the gastroesophageal junction, the glands are composed mainly of mucous cells; (2) in the body and fundus, the glands contain parietal (oxyntic) cells that secrete acid and chief (zymogen, or peptic) cells that secrete pepsin. The parietal cells also probably secrete intrinsic factor; (3) in the pyloric antrum, the glands contain mainly mucous cells.

Neuroendocrine cells are present in the mucosa of the stomach just as they are throughout the remainder of the intestinal tract. These cells are present throughout the mucosa and produce a variety of biogenic amines and peptide hormones. Neuroendocrine cells in the pyloric antral region (G cells) represent the source of gastrin and may be stained by immunohistologic methods using antigastrin antibodies.

Gastric Mucosal Resistance to Acid

The secretion of acid by the stomach is a continuous process, occurring at a basal rate during fasting periods and increasing markedly in response to a meal.

The gastric mucosa is protected by a variety of mechanisms from the erosive effect of gastric acid:

1. The anatomic integrity of the mucosa: The mucosal cells have a specialized apical surface membrane that resists the diffusion of acid into the cell. Back-diffused hydrogen ions are actively extruded by ionic carrier mechanisms.

2.  Gastric mucus: Mucin and HCO3− secreted by surface epithelial cells create a mucous layer that has a pH gradient which is very acid in the lumen to nearly neutral near the cell surface.

3. Prostaglandins (E series), which are synthesized and secreted by gastric mucosal cells, have a cytoprotective effect on the gastroduodenal mucosa. They act to increase bicarbonate secretion, gastric mucus production, mucosal blood flow, and the rate of mucosal cell regeneration.

4. Mucosal blood flow: Ischemia of the mucosa decreases mucosal resistance.

Functions of the Stomach

The main function of the stomach is to serve as a reservoir for meals, presenting food to the duodenum in small regulated amounts. The acid gastric juice contains the proteolytic enzyme pepsin and initiates digestion. The acidity also has an antibacterial action. Simple molecules such as iron, alcohol, and glucose may be absorbed from the stomach.