Intestinal Phase of Digestion

The arrival of chyme in the small intestine distends the duodenum, stimulating local stretch receptors and chemoreceptors. This sensory input triggers the enterogastric reflex, a neural response that slows gastric activity. The reflex inhibits stomach contractions and gastrin production, ensuring the small intestine has adequate time to process nutrients effectively.

The enterogastric reflex reduces activity in the submucosal and myenteric plexuses, neural networks regulating gastric secretions and contractions. It also diminishes parasympathetic stimulation through the medulla, decreasing nervous signals promoting stomach activity. Additionally, the pyloric sphincter contracts, limiting further chyme release into the small intestine.

Enteroendocrine cells in the duodenal mucosa release enterogastrones including cholecystokinin (CCK), secretin, and gastric inhibitory peptide (GIP). Lipids and carbohydrates stimulate CCK and GIP release, while acidic chyme triggers secretin release. These hormones inhibit gastric secretions and stimulate pancreatic enzyme and bile release.

Cholecystokinin (CCK) and gastric inhibitory peptide (GIP) are released in response to lipids and carbohydrates in chyme. These hormones inhibit gastric secretions and contractions, allowing the small intestine to focus on digestion. They also trigger the pancreas to release digestive enzymes and the gallbladder to release bile for nutrient breakdown.

Acidic chyme entering the small intestine stimulates enteroendocrine cells to release secretin. Secretin stimulates the pancreas to release pancreatic juice rich in bicarbonate, which neutralizes stomach acid and raises chyme pH. This optimal pH environment is favorable for enzyme activity and nutrient absorption in the small intestine.

The pyloric sphincter contracts as part of the enterogastric reflex response to chyme entering the small intestine. This contraction prevents further discharge of chyme from the stomach, protecting the small intestine from becoming overwhelmed. The controlled release allows adequate time for proper digestion and absorption of nutrients.

Chyme distension of the duodenum activates both neural and hormonal responses that coordinate digestion. The stretch triggers the enterogastric reflex to inhibit stomach activity, while stimulating enteroendocrine cells to release hormones that enhance pancreatic and gallbladder secretion. This coordinated response ensures efficient absorption of nutrients throughout the intestinal phase.