<P> There are at least five hormones that aid and regulate the digestive system in mammals . There are variations across the vertebrates, as for instance in birds . Arrangements are complex and additional details are regularly discovered . For instance, more connections to metabolic control (largely the glucose - insulin system) have been uncovered in recent years . </P> <Ul> <Li> Gastrin--is in the stomach and stimulates the gastric glands to secrete pepsinogen (an inactive form of the enzyme pepsin) and hydrochloric acid . Secretion of gastrin is stimulated by food arriving in stomach . The secretion is inhibited by low pH . </Li> <Li> Secretin--is in the duodenum and signals the secretion of sodium bicarbonate in the pancreas and it stimulates the bile secretion in the liver . This hormone responds to the acidity of the chyme . </Li> <Li> Cholecystokinin (CCK)--is in the duodenum and stimulates the release of digestive enzymes in the pancreas and stimulates the emptying of bile in the gall bladder . This hormone is secreted in response to fat in chyme . </Li> <Li> Gastric inhibitory peptide (GIP)--is in the duodenum and decreases the stomach churning in turn slowing the emptying in the stomach . Another function is to induce insulin secretion . </Li> <Li> Motilin--is in the duodenum and increases the migrating myoelectric complex component of gastrointestinal motility and stimulates the production of pepsin . </Li> </Ul> <Li> Gastrin--is in the stomach and stimulates the gastric glands to secrete pepsinogen (an inactive form of the enzyme pepsin) and hydrochloric acid . Secretion of gastrin is stimulated by food arriving in stomach . The secretion is inhibited by low pH . </Li> <Li> Secretin--is in the duodenum and signals the secretion of sodium bicarbonate in the pancreas and it stimulates the bile secretion in the liver . This hormone responds to the acidity of the chyme . </Li>

How does digestion take place in the stomach