What are the three phases of gastric secretion and what triggers each phase?

The three phases of gastric secretion are the cephalic, gastric, and intestinal phases. The cephalic phase is triggered by the sight, smell, taste, or thought of food, acting through the vagus nerve. The gastric phase is triggered by food in the stomach through distension and chemical stimuli such as partially digested proteins and rising pH. The intestinal phase is briefly stimulated by partially digested food entering the duodenum, and then inhibited by distension, acidity, fat, or hypertonic chyme in the duodenum.

In the cephalic phase, sensory cues like aroma, taste, sight, or thought of food excite the cerebral cortex and hypothalamus, which send impulses through the vagus nerve to increase gastric gland secretion before food arrives. In the gastric phase, stomach distension activates stretch receptors, while chemical stimulants such as partially digested proteins, caffeine, and rising pH directly activate G cells in the gastric antrum to release gastrin, which strongly stimulates acid secretion. The intestinal phase begins as partially digested food enters the duodenum, causing intestinal mucosal cells to release intestinal (enteric) gastrin, which briefly encourages further gastric secretion; however, duodenal distension and the presence of acidic, fatty, or hypertonic chyme soon trigger inhibitory signals, including the enterogastric reflex and release of enterogastrones such as secretin and cholecystokinin, to slow gastric secretion.