<P> The production of gastric acid in the stomach is tightly regulated by positive regulators and negative feedback mechanisms . Four types of cells are involved in this process: parietal cells, G cells, D cells and enterochromaffine - like cells . Besides this, the endings of the vagus nerve (CN X) and the intramural nervous plexus in the digestive tract influence the secretion significantly . </P> <P> Nerve endings in the stomach secrete two stimulatory neurotransmitters: acetylcholine and gastrin - releasing peptide . Their action is both direct on parietal cells and mediated through the secretion of gastrin from G cells and histamine from enterochromaffine - like cells . Gastrin acts on parietal cells directly and indirectly too, by stimulating the release of histamine . </P> <P> 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 . </P> <P> In the duodenum, gastric acid is neutralized by sodium bicarbonate . This also blocks gastric enzymes that have their optima in the acid range of pH . The secretion of sodium 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 . The neutralization is described by the equation: </P>

Where is acid produced in the gi tract