Final answer:
The gastric phase of digestion starts when food arrives in the stomach, where it triggers local, hormonal, and neural responses that promote secretion and processing of the ingested material. It succeeds the cephalic phase and is followed by the intestinal phase, which begins when chyme reaches the small intestine.
Step-by-step explanation:
The gastric phase of digestion begins when food reaches the stomach. This phase is part of a larger process that starts even before food is ingested, known as the cephalic phase, which is triggered by the sight, smell, or even thought of food and results in salivation and gastric juice secretion. Once food enters the stomach, the gastric phase builds upon the cephalic phase's stimulation. The presence of food in the stomach causes distension, which leads to a decrease in the pH of the gastric contents and the presence of undigested material, all of which stimulate the gastric phase. This involves a mix of local, hormonal, and neural responses, and these responses work together to stimulate gastric secretions and contractions to process the ingested materials effectively.
In contrast, the intestinal phase is initiated later when chyme enters the small intestine and involves different hormonal and neural events to ensure proper digestion and coordination with other organs such as the pancreas and liver.