Function

Food is mixed thoroughly by the churning action of gastric muscle against a closed pyloric sphincter. The pylorus opens only to allow semiliquid material (chyme) through into the duodenum, preventing the passage of large food particles. Mechanical dysruption increases the surface area for more efficient digestion and prevents damage to the delicate intestinal mucosa from large, hard, irregular food particles.

Rhythmic electric activity in the stomach produces regular peristaltic waves three times a minute, known as the gastric slow wave.

Gastric secretion is stimulated by the anticipation of food, the so-called cephalic phase, and by food reaching the stomach, the gastric phase. Acetylcholine and histamine, acting through M2 muscarinic and H2 receptors stimulate the secretion of HCl.

Parietal cells have an extensive intracellular canalicular system, numerous mitochondria to generate energy, and a highly active K+/H+ adenosine triphosphatase (ATPase) pump (proton pump) that secretes H+ into the lumen. An apical chloride channel transports Cl- into the lumen, to form HCl.

At the basolateral surface, HCO3-, formed intracellularly from CO2 and H2O, is exchanged for Cl-, so that circulating HCO3- levels rise when the stomach secretes acid ('alkali tide'). The basolateral Na+/K+ ATPase pump also replenishes intracellular K+ levels.

Differentiation and secretion of parietal cells is also stimulated by gastrin. Acid secretion is increased by excess gastrin, for example, in the Zollinger-Ellison syndrome (see Chapter 16), and is inhibited by vago-tomy, which removes cholinergic stimulation, by H2 receptor antagonists, such as ranitidine, and by proton pump inhibitors, such as omeprazole, which irreversibly bind to the K+/H+ ATPase.

HCl activates pepsinogen, to produce pepsin, initiating protein digestion. Intrinsic factor binds to vitamin B12, allowing it to escape degradation in the stomach and intestine and to be safely transported to the terminal ileum, where it is absorbed. Gastroferrin binds to Fe2+, facilitating absorption in the duodenum (see Chapter 21).

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Get Rid of Gallstones Naturally

Get Rid of Gallstones Naturally

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