Gastric Phase of Digestion

The gastric phase begins when food enters the stomach and stretches the stomach walls, activating stretch receptors. These receptors trigger local reflexes through the enteric nervous system, mediated by the myenteric plexus. Chemical stimuli like pH changes and partially digested food also stimulate hormone release, initiating the phase lasting approximately 3-4 hours.

Stretch receptors and chemical stimuli activate gastric cells to secrete gastric juice containing hydrochloric acid, pepsinogen, and mucus. Gastrin, released by G cells, stimulates enterochromaffin-like cells to release histamine, which activates parietal cells to secrete hydrochloric acid. Chief cells release pepsinogen in response to gastrin, creating the complete gastric juice mixture.

Hydrochloric acid destroys bacteria and partially denatures proteins, making them easier to digest. It converts pepsinogen into pepsin, an enzyme that breaks down proteins into smaller peptides. This acidic environment is essential for initiating protein digestion and creating an antimicrobial barrier within the stomach.

Mixing waves are powerful contractions produced by the muscularis externa of the stomach. These contractions work with gastric juice to further disintegrate food and form chyme, a semi-liquid mixture ready for the next digestive phase. Mixing waves are activated by local reflexes triggered when food distends the stomach.

Gastric juice production is regulated by a feedback mechanism: accumulation of hydrochloric acid lowers stomach pH, triggering somatostatin release. Somatostatin inhibits acid production to maintain balance and prevent excessive acidity. When pH becomes too low, gastric juice production decreases, protecting the stomach lining from damage.

Emotions such as fear and stress inhibit gastric secretion by overriding vagal digestion controls through sympathetic nervous system activation of the fight-or-flight response. This emotional regulation demonstrates how the central nervous system influences digestive processes, potentially disrupting normal gastric function during stressful situations.

Chyme is a semi-liquid mixture formed when gastric juice and mixing waves disintegrate food in the stomach. It consists of partially digested food, hydrochloric acid, pepsin, and mucus. Chyme is the product of the gastric phase and is ready to move into the small intestine for continued digestion and absorption of nutrients.