The task of gastric acid
In the fundus and corpus area of the stomach, the cells of the stomach mucosa secrete hydrochloric acid (HCl), which is the main component of gastric juice. Here, the hydrochloric acid reaches a concentration of up to 150 mM, which allows the pH value to drop locally to values below 1.0. This low pH value prevents the growth of bacteria and other pathogens.
In addition, proteins contained in the food pulp denature (= the structure is destroyed) in an acidic environment and can thus be more easily split by peptidases. Another important function of gastric acid is the activation of the inactive pepsinogen, which is produced by the main cells of the stomach mucosa, to pepsin, a peptidase that cleaves proteins taken in with food. Parietal cells in the mucosa produce HCl by secreting hydrogen protons into the gastric lumen via H+K+-ATPases (“proton pumps”) in the apical (upper) membrane of the activated parietal cells.
The proton concentration in the gastric juice can be up to 150 mmol/l and is thus 106 times higher than in blood. Chloride ions follow the protons via apical chloride channels into the stomach lumen and HCl is formed. The speed-determining step of the hydrochloric acid secretion is the incorporation of the proton pumps into the apical membrane of the document cells: in the resting state, the H+K+-ATPases are stored in tubulovesicles, after activation they fuse with the cell membrane.
Renunciation of the gastric juice
In the glands of the stomach mucosa there are different types of cells, the secondary cells, parietal cells, main cells and endocrine cells. Together they produce 2-3 l of gastric juice a day, an isotonic liquid whose main components are hydrochloric acid, pepsinogens, mucus, bicarbonate and intrinsic factor. The pH value of the gastric juice is largely determined by the gastric acid and varies between 1 and 7 depending on the acid production. The secretion is adjusted to the need and thus a small amount of gastric juice is continuously secreted during the interdigestive phases (phases between meals), while maximum secretion occurs after food intake. The production of gastric juice is subject to a complex endocrine regulation, which is controlled by a multitude of gastrointestinal hormones and neurotransmitters: Gastrin, histamine and acetylcholine promote the secretion of gastric juice, whereas somatostatin, GIP (gastric inhibitory protein), secretin, CCK (cholecystokinin) and prostaglandin E2 have an inhibitory effect.
Task of the stomach gatekeeper
The stomach gate (pylorus) consists of ring-shaped smooth muscles that form a strong sphincter muscle (M. sphinkter pylori) at the exit of the stomach, thus separating the stomach from the duodenum. The task of the pylorus is to transport the homogenised food pulp in the stomach in portions into the duodenum by rhythmic contractions. It also prevents the intestinal contents from flowing back into the stomach. The opening of the pylorus is controlled by the nervus vagus by a reflex (pyloric reflex) which triggers peristaltic contraction waves, allowing small portions (bolus) of the stomach contents (chyme) to enter the duodenum. In addition, the pyloric area contains glands that secrete a basic secretion that serves to neutralize the acidic food pulp.