A hormone produced by S cells in the lining of the anterior duodenum in response to the presence of hydrochloric acid and partially digested food (chyme) entering the small intestine from the stomach. Secretin circulates in the bloodstream and causes the pancreas to secrete alkaline pancreatic juice and stimulates bile production in the liver. Secretin, whose function was first demonstrated in 1902, was the first substance to be described as a hormone.