Stomach

The stomach lies just below the dome of the diaphragm, where it is protected by the rib cage. It is an expanded portion of the GI tract, like a large muscular, J-shaped bag that has a greater curvature towards the left side of the body and a lesser curvature towards the right side. The food bolus passes from the oesophagus through the lower oesophageal sphincter (LOS) into the stomach, where it is digested further.

The stomach has four principal functions:

• It stores ingested nutrients until they can be released into the duodenum of the small intestine
• It secretes gastric juice from gastric glands, which contain enzymes to initiate protein degradation
• Its three muscle layers (oblique, circular, and longitudinal) contract every 15–25 seconds to combine the food with gastric juices, thus forming a semi-liquid acid and food mass (chyme), which passes into the duodenum
• It secretes hydrochloric acid (HCl) to activate digestive pro-enzymes and kill unwanted bacteria that enter the stomach with the food

The stomach can be divided into four principal regions; a small cardiac region immediately below the LOS, the top part of the stomach or fundus, a large central body section and the antrum, which contains the pyloric canal leading to the pyloric sphincter, through which food passes to enter the duodenum (Figure 1).

The stomach is well suited for storage purposes as the muscles have little tone, and the extensive folds (rugae) allow for expansion up to a capacity of four litres, without any significant increase in intra-abdominal pressure.

Gastric glands

The inner mucosa of the body and fundic sections of the stomach contains millions of deep gastric pits, narrow channels that lead into gastric glands.

Specialised secretory cells within the gastric glands produce different components of gastric juice (Table 1). This clear, highly acidic (pH 1–2), colourless fluid is composed of mucus, digestive enzymes, hormones and HCl, and is produced in response to the stomach stretching on receipt of food, and to other stimuli.

TABLE 1 – Cells of the gastric glands and the functions they perform

Type of cell	Functions
Surface mucous cells	Secrete alkaline mucus to protect the stomach mucosa from ulceration caused by pepsin and the highly acidic stomach contents
Neck mucous cells	Located in the upper region of the glands. Structurally distinct and produce considerably less and more neutral mucus than the surface mucous cells, and do so only when the stomach contains food to digest. Replace lost surface cells.
Parietal cells	Large, sometimes binucleate cells, located in the upper region of the gastric glands, interspersed among the neck mucous cells. Secrete HCl into the gastric lumen via the proton pump (Figure 2). The proton pump (an enzyme – H+/K+-ATPase) in the parietal cell membrane uses adenosine triphosphate (ATP) to transport H+ across the cell membrane into the gastric lumen and potassium ions (K+) in the opposite direction. The parietal cell is stimulated to produce HCl by the combined action of acetylcholine, histamine and gastrin, which act on the muscarinic (M), histamine (H) and gastrin (G) receptors, respectively, located on the parietal cell membrane. Parietal cells also produce intrinsic factor, required for the absorption of vitamin B12 used in the production of erythrocytes.
Chief cells	Located principally in the basal region of the gastric glands. Produce pepsinogen, an inactive zymogen, which is converted into active pepsin by HCl produced by the parietal cells. Active pepsin enzymatically degrades complex proteins into smaller peptides and single amino acids by hydrolysis, as well as converting other pepsinogen molecules into active pepsin in a positive feedback mechanism.
Enteroendocrine cells	Also known as amine precursor uptake and decarboxylation (APUD) cells. Found throughout the gastric glands and release a variety of compounds, such as serotonin, gastrin, histamine, endorphins, cholecystokinin (CCK) and somatostatin, some of which regulate stomach secretions and gastric motility. These cells are thought to sample the contents of the gland lumen through endocytosis and release their products into the lamina propria accordingly.
Undifferentiated cells	Located mainly in the upper neck region of the gland, and, when required, differentiate to produce the mature, functional cells described above.

Protection of the stomach mucosa

The delicate stomach mucosa is protected from the damaging effects of acid and pepsin by two principal mechanisms; the mucus-bicarbonate barrier and rapid cell turnover.

The mucus-bicarbonate barrier

Mucus is continuously secreted by the surface mucous cells, and forms a 5 µm thick layer, which covers the entire stomach mucosa. The bicarbonate ions contained in the mucus diffuse from the cell surface to neutralise the HCl and prevent corrosion of the delicate stomach mucosa (Figure 3).

Although the mucus layer is thin, there is a significant pH gradient across it – from 7.4 at the epithelial cell surface to 1–2 at the surface of the mucus.

The mucus-bicarbonate barrier lines the gastric pits, but does not extend down into the gastric glands. The mechanisms by which the glands protect themselves from their own secretions are not fully understood. It is known that the enzyme pepsin is secreted in an inactive form known as pepsinogen, which is converted to active pepsin by the stomach acid.

Bicarbonate secretion, and perhaps mucus production by epithelial cells, is influenced by prostaglandins, especially prostaglandins E and F, produced by the activity of cyclo-oxygenase (COX-1). Non-steroidal anti-inflammatory drugs (NSAIDs, such as aspirin) can inhibit prostaglandin synthesis by blocking the COX-1 enzyme, and may cause failure of the mucus-bicarbonate barrier. This is why such drugs can cause gastritis, gastric blood loss and, in some cases, gastric ulcer. Prostaglandin derivatives have been introduced to help improve mucosal protection and are often co-prescribed with NSAIDs.

Rapid cell turnover

Protection of the stomach mucosa is also achieved through the rapid turnover of epithelial cells. Cells from the base of the gland migrate to the mucosal surface within 2 days, and are then shed into the stomach lumen. Approximately 500,000 cells are shed each minute from the healthy mucosa, which are digested along with ingested food and their nutrients reabsorbed into the body. Thus, the rapid cycling of cells means that damaged areas of epithelium are quickly repaired.

Cells migrating downward into the glands differentiate into the various kinds of secretory cell and are probably eventually destroyed in the gland base.

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