The predominant effect of secretin on the pancreas is to stimulate duct cells to secrete water and bicarbonate. As soon as this occurs, the enyzmes secreted by the acinar cells are flushed out of the pancreas, through the pancreatic duct into the duodenum. This hormone, which is very similar to cholecystokinin, is secreted in large amounts by the stomach in response to gastric distention and irritation. In addition to stimulating acid secretion by the parietal cell, gastrin stimulates pancreatic acinar cells to secrete digestive enzymes. Stop and think about this for a minute - control of pancreatic secretion makes perfect sense.
Pancreatic secretions contain enzymes which are needed to digest proteins, starch and triglyceride.
When these substances enter stomach, and especially the small intestine, they stimulate release of gastrin and cholecystokinin, which in turn stimulate secretion of the enzymes of destruction. Pancreatic secretions are also the major mechanism for neutralizing gastric acid in the small intestine. When acid enters the small gut, it stimulates secretin to be released, and the effect of this hormone is to stimulate secretion of lots of bicarbonate. As proteins and fats are digested and absorbed, and acid is neutralized, the stimuli for cholecystokinin and secretin secretion disappear and pancreatic secretion falls off.