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Renin Angiotensin Mechanism.

Acid Secretion in Stomach.

 

Acid Secretion in Stomach:

  • Overview:

    • Gastric acid (HCl) is secreted by parietal cells located within oxyntic glands of the stomach. 

    • The main purpose of stomach acid is to make the pH of the gastric contents sufficiently acidic to allow activation of pepsinogen to pepsin. 

    • The activated pepsin is a strong proteolytic enzyme.

    • Stomach walls have protection against pepsin and HCl action while other parts of GIT are not protected.

    • Stomach is the only part of GIT which has a lower pH.

  • Mechanism of Gastric Acid formation and Secretion:

    • Gastric Acid is formed by combining carbon dioxide and water to form carbonic acid. 

    • The hydrogen ion of carbonic acid is transported into the stomach lumen while the resultant bicarbonate ion is transported into the bloodstream. 

    • This bicarbonate ion is later transported into the lumen of the duodenum where it is used to neutralize the previously generated gastric acid.

  • Molecular Mechanism:

    • CO2 within parietal cells is converted, by carbonic anhydrase, to H2CO3, which immediately splits into H+ and HCO3-. 

    • The generated H+ ion is actively transported into the lumen of the oxyntic gland using an HK ATPase on the cell membrane. 

    • The generated HCO3- ion is passively transported into the blood in exchange for a chloride ion.

  • Regulation

    • Three stimuli directly affect stomach acid secretion by parietal cells.

    • Acetylcholine: Acetylcholine is released by parasympathetic fibers in the vagus nerve through “Muscarinic Receptors” present on the parietal cell membrane.

    • Histamine: Histamine is released by enterochromaffin-like cells (ECLs) which lie directly adjacent to parietal cells in the oxyntic gland

    • Gastrin: Gastrin is released by G Cells present in the pyloric glands of the stomach.

  • Potentiation of Acid Secretion: 

    • The vagus nerve helps coordinate these three stimuli so that they act in group. 

    • These three chemical stimuli acting on the parietal cells have synergistic effects on stomach acid production. 

    • This means that more stomach acid is secreted by parietal cells when stimulated by all three molecules simultaneously. 

    • Hence, pharmacological inhibition of just one stimulus can help reduce stomach acid secretion.

  • Phases of Gastric Acid Secretion:

    • Secretion of gastric acid takes place in three stages,

      1. Cephalic Phase.

      2. Gastric Phase.

      3. Intestinal Phase.

  1. Cephalic Phase:

  • This phase is initiated by the sensory experience of seeing and eating food and contributes about 20% of total acid secretion. 

  • stomach acid secretion in this phase is mainly induced by central activation of the vagus nerve.

  1. Gastric Phase:

    • This phase is initiated by entry of food into the stomach and contributes about 70% of total acid secretion. 

    • During the gastric phase a vagal reflex initiated by distension of the stomach further increases vagus nerve stimulation. 

    • In addition, local production of food metabolites, especially proteins and amino acids, directly induce gastrin release from G Cells.

  2. Intestinal Phase:

    • This phase is initiated by entry of food into the duodenum and contributes about 10% of total acid secretion. 

    • It is not fully clear how this phase is regulated but may be due to small amounts of gastrin secretion by the duodenal small intestine mucosa.

  • Inhibition of Gastric Acid Secretion:

    • Several stimuli act to prevent over-secretion of stomach acid which can be damaging both to the stomach as well as the duodenum.. 

    • Many of the inhibitory stimuli originate in duodenum and jejunum and act to inhibit secretion of stomach acid. 

    • They include, 

      1. Presence of nutrients.

      2. Acid-mediated irritation of the small intestine mucosa. 

      3. Gastrin Inhibitory Peptide.

      4. Somatostatin.

Commonly Asked Questions:

  1. Write a short note on “Acid Secretion in Stomach”.

  2. Discuss different phases of Gastric Acid Secretion.

  3. How gastric acid secretion in the stomach is regulated.
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