When food enters the stomach, the gastric glands secret gastric juice. The stomach stretches initiating stretch receptors that sends signal to the medulla and back to the stomach through the vague nerve. This cause the gastric glands to secrete more gastric juice. Partially digested protein activate the G cells that are found in the pyloric region of the stomach to secret gastrin. The activation of G cells stimulates the gastric glands to secrete gastric juice.
Small peptide and amino acids from the breakdown of proteins buffer the stomach acid to prevent the pH from dropping o much. As digestion continues, the peptides empty the stomach, causing the pH to drop low. When the pH reaches below 2, G cells and pariental cells are inhibited and the gastric phase is slowed down and the hydrochloric acids and pepsin decreases. Close to two third of gastric secretion occurs during this phase. Food that has been ingested stimulate gastric secretion in two ways: By stretching the stomach which activates two reflexes: a short reflex and a long reflex.
And the second gastric activity is by raising the pH.