Explain the mechanism of aciditification of stomach

The process of acidification of the stomach is very important for digestion. It mainly involves the secretion of hydrochloric acid (HCl) into the lumen of the stomach. The acid helps in several ways such as killing harmful bacteria, breaking down food into simpler forms and activating digestive enzymes like pepsinogen into pepsin. The acid is secreted by specialised cells called parietal cells located mainly in the fundus and body regions of the stomach. These cells are specially designed to produce and secrete the strong acid without damaging themselves.

The mechanism of acidification of the stomach happens through a series of properly organised steps inside the parietal cells:

Step 1: Formation of Carbonic Acid inside Parietal Cells

Inside the parietal cells, carbon dioxide (CO₂) produced by cellular metabolism or absorbed from the blood combines with water (H₂O). This reaction is catalysed by the enzyme called carbonic anhydrase. The result of this reaction is the formation of an unstable molecule called carbonic acid (H₂CO₃).

Step 2: Dissociation of Carbonic Acid

The carbonic acid (H₂CO₃) quickly breaks down into two ions one hydrogen ion (H⁺) and one bicarbonate ion (HCO₃⁻). This breakdown is a spontaneous process and happens immediately after carbonic acid is formed inside the parietal cell.

Step 3: Active Secretion of Hydrogen Ions into Stomach Lumen

The hydrogen ions (H⁺) formed inside the cell are actively pumped out into the stomach lumen. This is done by a special protein present on the apical membrane of the parietal cells known as the H⁺/K⁺ ATPase pump. This pump uses energy from ATP to exchange hydrogen ions from inside the cell with potassium ions (K⁺) from the stomach lumen. This is an active transport process and is very important because hydrogen ions are needed in a large amount for acid formation.

Step 4: Movement of Bicarbonate Ions into Blood

While hydrogen ions move into the stomach, the bicarbonate ions (HCO₃⁻) are transported out of the parietal cell into the bloodstream. This is done through a process called the chloride-bicarbonate exchanger located on the basolateral side of the cell. This movement causes a temporary rise in the pH of blood after meals, a phenomenon known as the alkaline tide.

Step 5: Entry and Secretion of Chloride Ions into the Lumen

At the same time, chloride ions (Cl⁻) from the blood enter the parietal cells through chloride channels. These chloride ions then move towards the apical side and are secreted into the stomach lumen. This passive movement of chloride ions is essential because they are needed to form hydrochloric acid.

Step 6: Formation of Hydrochloric Acid (HCl)

In the final step, the hydrogen ions (H⁺) secreted into the stomach lumen combine with the chloride ions (Cl⁻). The combination of H⁺ and Cl⁻ leads to the formation of hydrochloric acid (HCl). This HCl is highly acidic and brings down the pH of stomach contents to about 1 to 2, making the environment very acidic, which is ideal for digestion.






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