Secretin is a peptide hormone produced by S-cells of the small intestinal mucosa and involved in the regulation of pancreatic secretory activity. S-cells that are located in the mucous membrane of the duodenum and in the proximal part of the jejunum secrete the polypeptide prosecretin, an inactive precursor of secretin, which turns into secretin by the action of hydrochloric acid of the gastric juice. Being absorbed into the blood, secretin reaches the pancreas and pancreatic centroacinar cells have secretin receptors in their plasma membrane. As secretin binds to these receptors, it enhances the secretion of bicarbonate-rich fluid that flows into the intestine. Bicarbonate is a base that neutralizes the acid, thus establishing a pH favorable to the action of other digestive enzymes in the small intestine. Secretin also increases water and bicarbonate secretion from duodenal Brunner's glands to buffer the incoming protons of the acidic chyme and also reduces acid secretion by parietal cells of the stomach. Secretin is used in a diagnostic test for pancreatic function.