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Gastric acid is the main secretion of the stomach, characterised by H2O, hydrochloric acid and several enzymes (mainly pepsinogen).
Physiology
 Diagram summarising control of stomach acid secretion, emphasising interaction between the body and antrum. Gastic acid is produced by the parietal cell (wall cell) of the gastric mucosa. The pH of gastric acid is 2-3, the acidity being maintained by the proton pump, a H+/K+ ATPase. The parietal cell releases bicarbonate into the blood stream in the process.
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. Vasoactive intestinal peptide, cholecystokinin, and secretin all inhibit production.
Role in disease
In atrophic gastritis and anchlorhydria, there is decreased gastric acid production and thus decreased disinfectant properties of the gastric milieu, leading to an increased risk of infections of the digestive tract (such as the foodborne infection with Vibrio vulnificus).
In Zollinger-Ellison syndrome and hypercalcemia, there are increased gastrin levels, leading to excess gastric acid production, which can cause ulcers.
In diseases featuring excess vomiting, patients develop hypochloremic metabolic alkalosis (decreased blood acidity by H+ and chlorine depletion).
Pharmacology
The proton pump enzyme is the target of proton pump inhibitors, used to increase gastric pH in diseases which feature excess acid.
See also
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