Final answer:
The three phases of gastric secretion are the cephalic phase, which starts with brain stimulation; the gastric phase, which begins when food reaches the stomach and continues digestion; and the intestinal phase, which begins when chyme enters the small intestine and regulates digestion pace.
Step-by-step explanation:
The three phases of gastric secretion are the cephalic phase, the gastric phase, and the intestinal phase. The secretion of gastric juice is regulated through stimuli in the brain, stomach, and small intestine, which can activate or inhibit production depending on the phase.
Cephalic Phase
This phase begins when seeing, smelling, tasting, or even thinking about food and involves the direct stimulation of the stomach's gastric glands by the brain, especially the vagus nerve, to begin secreting gastric juice even before food enters the stomach.
Gastric Phase
The gastric phase commences when food reaches the stomach. It continues the process started in the cephalic phase and is stimulated by the distension of the stomach, a decrease in pH, and the presence of undigested material. This leads to hormonal, neural, and local responses that further stimulate gastric secretion and muscle contractions for digestion.
Intestinal Phase
This phase begins when the chyme (the mixture of partially digested food and gastric juice) enters the small intestine. It is mainly a regulatory phase that slows down gastric secretion and motility, ensuring the stomach does not empty its contents too quickly into the small intestine.