A molecular model of secretin, a peptide hormone secreted by S cells in the duodenum of the small intestine. As chyme enters from the stomach, secretin responds to the lower pH levels of the hydrochloric acid by signaling the pancreas to secrete a bicarbonate solution to neutralize the acidic contents. Additionally, secretin will stimulate the release of pepsin for protein digestion and glucagon to increase blood glucose levels. Atoms are colored dark gray (carbon), light grey (hydrogen), blue (nitrogen) and red (oxygen).