Upper Gastrointestinal Tract
When food is smelled and/or tasted, saliva containing the enzyme amylase is released. As food is broken down into smaller bits and mixed with saliva by chewing, the amylase begins the process of breaking down starches into simpler sugars. When swallowed, the food proceeds through the pharynx and esophagus to the stomach.
In the stomach, gastrin causes the pyloric sphincter to constrict and slow the flow out of the stomach. Gastrin also causes the chief cells to release pepsinogen and the parietal cells to release hydrochloric acid. The hydrochloric acid activates the pepsinogen into the enzyme pepsin, which breaks down the proteins into simpler peptides. Once the food has digested some and the gastrin has diminished, the pyloric sphincter relaxes and lets the partially digested food exit the stomach.