Secretin is a hormone produced in the duodenum that plays an important role in the digestive process and also appears to be involved in osmoregulation, the process the body uses to keep the internal balance of salt and fluid stable. This hormone can also be introduced into the body, usually in a test that is designed to check the function of the pancreas. The secretion for injection or introduction into the body through a gastric tube is available from various pharmaceutical companies, only by prescription.
A series of cascading reactions occurs as people begin to digest food. Secretin is one of many players involved. As food moves through the stomach, it passes into a partially digested state known as chyme. The chyme is very acidic as a result of the gastric juices it has been marinating in, and as it begins to make its way into the intestines, the secretin kicks in.
One of the things secretin does is trigger the production of bicarbonates which are used to neutralize the acid in the chyme so it doesn't damage the intestines. Unlike the stomach, the intestines are not equipped to deal with the strong gastric juices used in the early stages of digestion. The release of secretin also tells the stomach to stop producing gastric juices, signaling that the next phase of digestion is beginning and the stomach can take a break.
Secretin also triggers the production of pancreatic juices and hepatic bile, which are used to lubricate chyme as it moves through the digestive tract. Also, if the glucose content of food is high, secretin will also stimulate the production of insulin to handle glucose.
In the secretion stimulation test, a patient will be injected with secretin to see how the pancreas responds. If stimulation with this hormone does not produce the production of pancreatic juices, it indicates that the patient has a medical problem that needs to be identified and treated. This test can be uncomfortable, since secretin is often put in through a stomach tube, and many people gag during the tube's placement and removal.
This hormone has another distinction: it was the first hormone identified. Researchers working with the digestive tract in the early 1900s noted that this compound seemed to play an important role, sending chemical signals to moderate the digestive process. This contradicted the expectation that the digestive process was mediated by the nervous system, changing the way scientists thought about the body and paving the way for the discovery of other "chemical messengers," which were called hormones.