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.

2. 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.

3. 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.