25.1 Digestion in the stomach
25.2 Digestion in the small intestine
25.3 Digestion in the colon
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25.1. Digestion in the stomach
In the stomach, secretory, hydrolytic, motor functions are well marked, and the absorption function is less pronounced.
Secretory function of the stomach. The gastric mucosa performs the functions of external and internal secretion (Fig. 25.1).
The function of external secretion is realized by its own, pyloric, and cardiac glands and leads to formation of gastric juice in an amount of 1.5-2.5 l/day.
Composition of the gastric juice. Gastric juice, in addition to water, contains hydrochloric acid, digestive enzymes, mucus and bicarbonate, transport proteins.
Hydrochloric acid is secreted by parietal exocrino-cytes of their own glands. The concentration of HCl in gastric juice is about 120 mmol/l; pH ≈1; 40-60 T.E. The main zones of hydrochloric acid secretion are the fundus and body of the stomach (especially the lesser curvature).
Mechanism of hydrochloric acid secretion (Fig. 25.2). The source of H+ is dissociation of H2CO3. H+ ions are transferred to the lumen of the intracellular tubule of the parietal cell using a К+/H+-pump, Cl- ions are transferred from the blood plasma to the parietal cell as a re-
sult of ion exchange to HCO3- and enter the intracellular tubule via the chlorine channel. From the intracellular tubule, H+ and Cl- enter the stomach cavity.
Regulation of hydrochloric acid secretion (Fig. 25.3).
Three main factors stimulate HCl secretion: parasympathetic effects: acetylcholine through M2-cholinergic receptors, histamine through H2 receptors, gastrin through G-receptors. The stimulating effect is also exerted by mechanical and chemical irritation with chymus, protein digestion products.