Phases of gastric secretion | Three phases (cephalic, gastric, and intestinal) of gastric secretion

Gastric secretion unfolds in three stages—cephalic, gastric, and intestinal—with each phase triggered by specific regulatory signals. Specialized gastric cells play distinct roles: chief cells secrete pepsinogen for protein digestion, neck cells produce mucus, and parietal cells release a highly acidic hydrochloric acid along with intrinsic factor. The extensive canaliculi in parietal cells boost the surface area, enabling the creation of a potent acidic environment essential for digestion.

Food intake triggers a finely tuned increase in gastric secretion, while an empty stomach reduces it. Neural, hormonal, and chemical signals coordinate to stimulate secretion with factors like gastrin, histamine, and acetylcholine, and to inhibit it with agents such as gastric inhibitory peptide, VIP, cicrotin, and duodenal glucagon. The cephalic phase initiates the process through vagus nerve stimulation, promoting pepsin and acid production, while local nervous reflexes and chemical mediators further refine gastric activity.

The cephalic phase of digestion is triggered by sensory signals such as seeing, smelling, or thinking about food, which activates the vagus nerve to initiate gastric acid release even before food enters the stomach. When food is ingested, the gastric phase engages both neural and chemical mechanisms to control secretion. Stomach distension is detected by mechanosensory receptors that signal the medulla oblongata, reinforcing acid production. Local hormones like gastrin and stimulants such as histamine further enhance the secretion of hydrochloric acid, ensuring efficient digestion.

Gastrin is essential for digestion, stimulating parietal cells to produce hydrochloric acid, maturing the fundal region, and prompting chief cells to release pepsinogen. It enhances stomach muscle mobility and contractions, facilitating efficient gastric emptying while also modulating pancreatic secretion and gallbladder emptying. When food enters the duodenum, the dropped pH and lipid by-products trigger inhibitory feedback through the brain. Peptides such as GIP, secretin, and CCK then block further acid production, fine-tuning the digestive process.

Gastric emptying is primarily regulated by the volume of ingested material, the duodenal osmolarity and acidity, as well as hormonal and nervous factors. Distinct mechanisms in each of the three phases of gastric secretion work together to modulate this process. These factors act as negative regulators, reducing secretion when necessary and ensuring precise control over gastric emptying.