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===Stimulus===
Secretin is released into circulation and/or intestinal lumen in response to low duodenal pH that ranges between 4 and 4.5 depending on species.<ref name="isbn0-07-022001-8">{{cite book | author = Frohman, Lawrence A.; Felig, Philip | authorlink = | editor = P. K. Ghosh and T. M. O’Dorisio | others = | title = Endocrinology & metabolism | edition = | language = | publisher = McGraw-Hill, Medical Pub. Div | location = New York | year = 2001 | origyear = | chapter =  Gastrointestinal Hormones and Carcinoid Syndrome | pages = 1326 | quote = | isbn = 0-07-022001-8 | oclc = | doi = | url = | accessdate = }}</ref>

It is the active form of [[prosecretin]].This acidity is due to [[chyme]], which contains [[hydrochloric acid]], entering from the stomach via the [[pyloric sphincter]].Secretin targets the [[pancreas]], which cause the organ to secrete a bicarbonate-rich fluid that flows into the [[intestine]]. [[Bicarbonate]] ion is a base which neutralizes the acid, thus establishing a pH favorable to the action of other digestive enzymes to the small intestine and preventing acid burns<ref>http://www.vivo.colostate.edu/hbooks/pathphys/endocrine/gi/secretin.html</ref> Other factors are also involved in the release of secretin such as bile salts and fatty acids  which result in additional bicarbonate being added to the small intestine.<ref name="pmid631638">{{cite journal | author = Osnes M, Hanssen LE, Flaten O, Myren J | title = Exocrine pancreatic secretion and immunoreactive secretin (IRS) release after intraduodenal instillation of bile in man | journal = Gut | volume = 19 | issue = 3 | pages = 180–4 | year = 1978 | month = March | pmid = 631638 | pmc = 1411891 | doi = | url = http://gut.bmj.com/cgi/pmidlookup?view=long&pmid=631638 | issn = }}</ref> Secretin release is inhibited by H2 receptor [[antagonists]] which reduce gastric acid secretion. As a result, the pH in the duodenum increases above 4.5, and secretin cannot be released.<ref name="pmid7249893">{{cite journal | author = Rominger JM, Chey WY, Chang TM | title = Plasma secretin concentrations and gastric pH in healthy subjects and patients with digestive diseases | journal = Digestive diseases and sciences | volume = 26 | issue = 7 | pages = 591–7 | year = 1981 | month = July | pmid = 7249893 | doi = | url = | issn = }}</ref>