Acidic Secretion
A corrosive biological fluid that digests proteins and protects against pathogens.
via Wikipedia: Gastric acid · see source
Gastric acid, also known as stomach acid, is the acidic component – hydrochloric acid – of gastric juice. It is produced by parietal cells in the gastric glands of the stomach lining. In humans, the pH is between one and three, much lower than most other animals, but is very similar to that of carrion-eating carnivores that need protection from ingesting pathogens. With this higher acidity, gastric acid plays a key protective role against pathogens. It is also key in the digestion of proteins by activating digestive enzymes, which together break down the long chains of amino acids.
Lore & Background
Gastric acid secretion is a complex and relatively energetically expensive process. Parietal cells contain an extensive secretory network called canaliculi from which the hydrochloric acid is secreted into the lumen of the stomach. The pH level is maintained by the proton pump H+/K+ ATPase. The parietal cell releases bicarbonate into the bloodstream in the process, which causes a temporary rise of pH in the blood, known as an alkaline tide.
The highest concentration that gastric acid reaches in the stomach is 160 mM in the canaliculi. This is about 3 million times that of arterial blood, but almost exactly isotonic with other bodily fluids. The lowest pH of the secreted acid is 0.8, but the acid is diluted in the stomach lumen to a pH of between 1 and 3. There is a small continuous basal secretion of gastric acid between meals of usually less than 10 mEq/hour. A typical adult human stomach will secrete about 1.5 liters of gastric juice daily.
Reader's Guide
Gastric acid production is regulated by both the autonomic nervous system and several hormones. The parasympathetic nervous system, via the vagus nerve, and the hormone gastrin stimulate the parietal cell to produce gastric acid, both directly acting on parietal cells and indirectly, through the stimulation of the secretion of the hormone histamine from enterochromaffin-like cells (ECLs). Vasoactive intestinal peptide, cholecystokinin, and secretin all inhibit production.
Four types of cells are involved in this process: parietal cells, G cells, D cells and enterochromaffin-like cells. The release of histamine is the most important positive regulation mechanism of the secretion of gastric acid in the stomach. Its release is stimulated by gastrin and acetylcholine and inhibited by somatostatin. In the duodenum, gastric acid is neutralized by bicarbonate.
The secretion of bicarbonate from the pancreas is stimulated by secretin. This polypeptide hormone gets activated and secreted from so-called S cells in the mucosa of the duodenum and jejunum when the pH in the duodenum falls below 4.5 to 5.0.
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