Actions of Gastrointestinal hormones
Secretin is released under acidic conditions (low pH); digested fat or bile initiates production of pancreatic juice low in enzymes but rich in salts important in neutralising the acid chyme. CCK is secreted when partially digested proteins or HCl (to a lesser extent) are present. It induces the flow of pancreatic juice rich in enzyme. Figure summaries the action of GI hormones.
Figure: Action of several gastrointestinal hormones. Gastrin is secreted in response to intragastric proteins, stomach distention and stimulation by vagus nerve. Gastrin from the lower stomach stimulates HCI secretion and pepsin from secretory cells. Cholecystokinin (CCK) stimulates pancreas to secrete digestive enzymes and bases to neutralise and digest chyme. It also induces contraction of gall bladder to secrete bile salts. CCK is secreted in response to arrival of amino acids and fatty acids in deodenum from stomach.
These two hormones inhibit stomach motility. Arrival of fat from the stomach initiates the release of CCK by intestinal mucosa, this causes gall bladder to release bile which aids in fat digestion.