Human stomach acid, also known as gastric acid, is primarily produced by parietal cells located in the stomach lining. This is a complex and energy-intensive process.
Parietal Cells: The Acid Producers
- Location: Parietal cells reside within the gastric glands of the stomach lining.
- Function: Their main function is to secrete hydrochloric acid (HCl), a key component of gastric acid, into the stomach lumen. They also produce intrinsic factor, which is important for vitamin B12 absorption.
- Structure: These cells have a unique structure, featuring an extensive network of intracellular channels called canaliculi, which greatly increases their surface area for acid secretion.
The Acid Production Process: A Step-by-Step Overview
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Carbon Dioxide and Water Involvement: Inside the parietal cells, carbon dioxide (CO₂) and water (H₂O) combine to form carbonic acid (H₂CO₃) with the help of the enzyme carbonic anhydrase.
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Dissociation of Carbonic Acid: Carbonic acid then dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻).
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Pumping Hydrogen Ions: The parietal cells use a proton pump, specifically the H⁺/K⁺ ATPase, to actively transport hydrogen ions (H⁺) into the stomach lumen (the interior space of the stomach). This pump exchanges H⁺ ions from the cell for potassium ions (K⁺) from the lumen. This process requires a significant amount of energy.
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Chloride Ion Transport: Chloride ions (Cl⁻) are transported into the stomach lumen through chloride channels, following the electrochemical gradient created by the hydrogen ion movement.
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Formation of Hydrochloric Acid: In the stomach lumen, the hydrogen ions (H⁺) and chloride ions (Cl⁻) combine to form hydrochloric acid (HCl).
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Bicarbonate Transport: As hydrogen ions are pumped into the stomach, bicarbonate ions (HCO₃⁻) are transported out of the parietal cell into the bloodstream in exchange for chloride ions (Cl⁻). This process is known as the "alkaline tide" and can temporarily increase the pH of the blood after a meal.
Regulation of Acid Production
Several factors regulate the secretion of gastric acid:
- Histamine: Released by enterochromaffin-like (ECL) cells, histamine stimulates parietal cells via H2 receptors to produce acid.
- Acetylcholine: Released by vagal nerve endings, acetylcholine stimulates parietal cells directly and also stimulates ECL cells to release histamine.
- Gastrin: Released by G cells in the stomach antrum, gastrin stimulates parietal cells directly and also stimulates ECL cells to release histamine.
- Somatostatin: Released by D cells, somatostatin inhibits the release of gastrin, histamine, and acid.
Why is stomach acid important?
Stomach acid serves several crucial functions:
- Digestion: It helps to denature proteins, making them more susceptible to enzymatic breakdown.
- Sterilization: It kills most bacteria and other microorganisms that enter the stomach with food.
- Enzyme Activation: It activates pepsinogen into pepsin, a key enzyme for protein digestion.
