HCl Secretion by Parietal Cell of Gastric Mucosa

· Acid is secreted when the parietal cells of the stomach are stimulated

· The pH of the acid is about 0.8

· The Hydrochloric Acid (HCl) is formed at the vilus-like projection inside the canaliculi of the parietal cells

· Proton pumps powered by H⁺/K⁺ ATPases actively transport H+ into the lumen while bringing potassium ion (K⁺) into the cell

· At the same time, Chloride ion (Cl^-) and K+ diffuse out into the lumen through the Cl^- and K⁺ channel.

· Carbonic Anhydrase catalyze the formation of carbonic acid from water and carbon dioxide. Carbonic acid dissociates into H⁺ and bicarbonate ion.

· As bicarbonate ion builds up in the cytosol, it diffuse out from the parietal cell into the ECF in exchange to the movement of Cl^- from ECF to the parietal cell

· Bicarbonate ion diffuses into nearby capillary

· Water passes into the canaliculus by osmosis. Thus, the final secretion contain water, hydrochloric acid, potassium chloride and sodium chloride