Digestion completes with the combined action of enzymes and hormones.
Enzymes in the stomach and small intestine break down proteins into amino acids. HCl in the stomach aids in proteolysis, and hormones secreted by intestinal cells direct the digestive processes. Some of the hormones control the regulation of the entire digestive process –some even regulate the appetite.
The hormones produced in the mucosa cells of the stomach and small intestines work by stimulating these organs and their digestive juices. The three hormones responsible for the digestion of our food are gastrin, secretin and cholecystokinin (CCK).
Gastrin gives the stomach the signal to produce acid. It also plays an important role in the growth of the stomach, small intestine and colon lining – which, as we now know, is needed for absorbing nutrients and excreting digestive juices. Secretin communicates with all the major digestive accessory organs. In the pancreas, a call from Secretin causes the excretion of those helpful digestive juices. Then Secretin calls the stomach, causing it to produce pepsin, an enzyme used to digest protein. Secretin's final call is the liver, which then produces more of that much–needed bile. CCK communicates with the organs: the pancreas and the gallbladder. With the help of this hormone the pancreas grows and produces more enzymes. Once the gallbladder hears from CCK, it knows to release all the bile it has been storing for the liver. As we mentioned, a few hormones also work to encourage us to start and stop eating that food. The first of these is ghrelin, which both the stomach and small intestine produce when there is no food in them. So, when ghrelin levels run high, your appetite is stimulated. Peptide YY, a hormone made in the gastrointestinal tract, works in an opposite manner. Once we finish taking our food, it's excreted to quell our hunger. Pepsin, an enzyme used to digest protein.