Stomach Function

The stomach stores ingested foods, breaks them down mechanically, alters them chemically, disinfects them with stomach acid, and passes on the resulting chyme to the duodenum for further digestion in small portions. The proximal stomach (A), also called fundus, develops an active tonus which can adjust to the intragastric pressure. Its primary function is, therefore, storage. Solid food is deposited layer upon layer while liquids run down the stomach wall. The distal stomach sections, corpus, and antrum, contain within their smooth muscle autonomous pacemaker cells, which produce distally advancing waves of potential (slow waves). Their frequency is 3-4/min. That is, however, below the activation threshold. When needed, contractions with the frequency of slow waves are triggered by neuronal and humoral influences. An important stimulus is provided by the stretching of the intestinal wall caused by a full stomach. The peristaltic contractions triggered by the stretching drive the stomach contents towards the pylorus, which closes— controlled by a complex interaction of regulatory mechanisms—squeezing and pushing the contents back into the stomach. Due to this maceration, 90 % of all particles leaving the stomach are less than 0.25 mm in size. The close regulatory monitoring of the pylorus' opening and closing is necessary to prevent overburdening of subsequent digestive and absorptive processes. The 2-3 l of stomach juices are secreted essentially by three types of cells. Parietal cells (B) actively pump protons into the lumen in exchange for potassium. Potassium and chloride are also transported actively, increasing the H+ concentration to a level more than a million times higher than in blood. The protons are produced in a carboanhydrase reaction. The bicarbonate resulting from this reaction is transported into the blood in exchange for chloride. Pepsino-gen, a mixture of protease precursors secreted by the chief cells, is converted into protein-digesting pepsin when exposed to HCl. Pepsin is an autocata-lytic enzyme acting to produce more of itself and needs an acidic environment for its activation. The surface mucous cells produce an alkaline secretion which creates an unstirred layer on the surface of the mucous membrane. This causes the pH at the cell surface to be near neutral even with extremely acidic pH in the lumen—an essential protection for the stomach wall to prevent self-digestion.

Gastric secretion is divided into several distinct phases (C). The cephalic phase is triggered by expectation of food, by seeing, smelling, etc. Pavlov discovered that this secretion, mediated by vagal impulses, can be up to 50 % of maximal total secretory activity. The excitatory phase is dominated by release of gastrin from G cells of the antrum stimulated by partially digested protein fragments. During the inhibitory phase duodenal signals predominate. Signals from stretch-, protein-, fat-, osmo-, and pH-receptors trigger the release of hormones that inhibit the activity of the secretory cells.

|- A. Peristaltic Contractions

Esophagus

|- A. Peristaltic Contractions

Esophagus

Stomach Corpus Antrum Peristalsis

Duodenum

Chyme Peristaltic movement contractions

  1. Parietal Cell (HCl Production)
  2. Phases of Stomach Secretion

Duodenum

Chyme Peristaltic movement contractions

B. Parietal Cell (HCl Production)

J^tocatolys/s^ Pepsinogen •

  • Peptides
  • Proteins —

J^tocatolys/s^ Pepsinogen •

Hcl Production

HCO3

Blood

HCO3

Blood

C. Phases of Stomach Secretion

Excitatory phase of stomach secretion

Excitatory phase of stomach secretion

Stomach Anatomy Secretion
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