any of a group of digestive hormones secreted by the wall of the pyloric end of the stomach (the area where the stomach joins the small intestine) of mammals. Gastrin is a polypeptide. Released into the bloodstream when food enters the stomach, gastrin is carried by the circulatory system to the gastric cells in the stomach wall, where it triggers the secretion of gastric juice. This juice consists primarily of hydrochloric acid, which helps break apart fibrous matter in food and kills bacteria that may have been ingested, and pepsinogen, which is a precursor of the protein-splitting enzyme pepsin. Gastrin also increases the motility of the stomach, thereby helping to churn the food and eventually to empty the stomach; to a lesser degree, gastrin also increases the motility of the upper small intestine and the gallbladder.