Gastrin is released in response to distension (via stretch receptors) and the presence of foods (via chemoreceptors). Gastrin enters the bloodstream to stimulates secretion of gastric acid. In addition, The parasympathetic vagus nerve (CN X) secretes acetylcholine (ACh) which stimulates acid secretion directly from parietal cells and indirectly by stimulating the ECL cells to release histamine. Histamine released by the ECL cells stimulates gastric acid secretion from parietal cells.
A proton pump (H-K ATPase) pumps acid into the stomach lumen in exchange for K ions. The stomach lining is protected from damage by gastric acid by a bicarbonate-rich mucous layer (mucosal barrier) that lines the gastric mucosa and is secreted by gastric mucous cells. Pumping of acid into the stomach is balanced by secretion of bicarbonate from the stomach into the blood. This alkaline tide causes an increase in the pH of the blood leaving the stomach.
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