<P> Secretin is synthesized in cytoplasmic secretory granules of S - cells, which are found mainly in the mucosa of the duodenum, and in smaller numbers in the jejunum of the small intestine . </P> <P> Secretin is released into circulation and / or intestinal lumen in response to low duodenal pH that ranges between 2 and 4.5 depending on species; the acidity is due to hydrochloric acid in the chyme that enters the duodenum from the stomach via the pyloric sphincter . Also, the secretion of secretin is increased by the products of protein digestion bathing the mucosa of the upper small intestine . </P> <P> Secretin release is inhibited by H antagonists, which reduce gastric acid secretion . As a result, if the pH in the duodenum increases above 4.5, secretin cannot be released . </P> <P> Secretin primarily functions to neutralize the pH in the duodenum, allowing digestive enzymes from the pancreas (e.g., pancreatic amylase and pancreatic lipase) to function optimally . </P>

Chyme stimulates enteroendocrine cells to secrete secretin and