The gastric phase of digestion begins as soon as food enters the stomach. The incoming food bolus triggers neural and hormonal mechanisms, which last approximately 3 to 4 hours. During this phase, the stomach undergoes significant changes to prepare the food for further digestion and absorption.

When food enters the stomach, it stretches the stomach walls and activates stretch receptors. This triggers local reflexes of the enteric nervous system, mediated through the myenteric plexus. These reflexes stimulate the secretory activity of gastric cells, which produce gastric juice containing hydrochloric acid, pepsinogen, and mucus.

Chemical stimuli, such as changes in pH and the presence of partially digested food, also play a significant role in the gastric phase. These stimuli promote the release of hormones, including gastrin, secreted by G cells, and histamine, released by enterochromaffin-like cells. The hormones activate parietal and chief cells, which secrete gastric juice and enzymes to facilitate further breakdown of the food.

Hydrochloric acid is a critical component of gastric juice. It destroys bacteria and partially denatures proteins, making them easier to digest. Pepsinogen, another key component of gastric juice, is converted into pepsin by the acidic environment provided by hydrochloric acid. Pepsin is an enzyme that breaks down proteins into smaller peptides.

The muscularis externa of the stomach also contributes to the gastric phase. It produces powerful contractions called mixing waves, which further disintegrate the food and form chyme.

Several factors, including satiety and pH levels, regulate the gastric phase. Stretch receptors activated by stomach distension signal satiety to the brain. Gastric juice production is regulated by a feedback mechanism: the accumulation of hydrochloric acid lowers stomach pH, triggering the release of somatostatin. This hormone inhibits acid production to maintain balance and prevent excessive acidity. Similarly, when the stomach's pH becomes too low, the production of gastric juice decreases. Emotions such as fear and stress can inhibit gastric secretion by overriding vagal digestion controls through the sympathetic division's activation of the fight-or-flight response.