Final answer:
Secretin is a hormone secreted by the S cells of the duodenum that targets the pancreas to release bicarbonate, neutralizing stomach acidity and aiding digestion. It also helps regulate other digestive activities and is triggered by the presence of acidic chyme in the small intestine.
Step-by-step explanation:
The hormone secretin plays a crucial role in the regulation of food digestion in the gastrointestinal tract. This hormone is part of a group of hormones produced by the mucosal epithelium of the stomach and small intestine, specifically by specialized cells called endocrinocytes. The substrates and sources of secretin are the S cells in the duodenum, which release it into the bloodstream when the acidic chyme from the stomach enters the small intestine. The stimulus for the secretion of secretin is the presence of acidic chyme in the duodenum.
Once secretin is in the bloodstream, it targets the pancreas, stimulating the production of a watery bicarbonate-rich solution. The main action of secretin is to neutralize the acidity of chyme, thus creating a more favorable environment for digestive enzymes to operate in the small intestine. In this way, secretin regulates the organs involved in food digestion such as the pancreas and indirectly influences the liver and gallbladder by promoting the effects of bile secretion.
Additionally, secretin influences other aspects of gastrointestinal function, including the inhibition of gastric secretion and motility, to coordinate the digestive process effectively.