27.19
Intestinal Phase of Digestion
The intestinal phase of digestion is the third and final stage of the digestive process, occurring after the cephalic and gastric phases. It begins wh…
After the gastric phase, the chyme from the stomach moves into the small intestine, gradually decreasing the stomach distention.
In contrast, the arrival of chyme expands the duodenal end of the small intestine, stimulating the local stretch receptors and chemoreceptors to trigger the enterogastric reflex.
This reflex inhibits stimulation of the stomach via the submucosal and myenteric plexuses, as well as parasympathetic stimulation via the medulla.
As a result, both gastrin production and gastric contractions are inhibited.
Additionally, the pyloric sphincter contracts, preventing further discharge of chyme from the stomach to the small intestine.
The incoming chyme stimulates the enteroendocrine cells scattered in the duodenal mucosa to release enterogastrone, such as cholecystokinin or CCK, secretin, and gastric inhibitory peptide or GIP.
Lipids and carbohydrates in the chyme stimulate the release of CCK and GIP. These hormones inhibit gastric secretions and trigger the pancreas to release digestive enzymes and the gallbladder to release bile.
Acidic chyme stimulates the release of secretin. This in turn, stimulates the release of pancreatic juice to increase the pH of chyme, which is favorable for digestion.
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Q1: What triggers the enterogastric reflex when chyme enters the small intestine?
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.
Q2: How does the enterogastric reflex inhibit stomach function?
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.
Q3: Which hormones are released by enteroendocrine cells during the intestinal phase?
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.
Q4: What is the role of cholecystokinin and GIP in intestinal digestion?
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.
Q5: How does secretin regulate pH during the intestinal phase?
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.
Q6: Why does the pyloric sphincter contract during the intestinal phase?
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.
Q7: How does chyme distension of the duodenum coordinate digestive processes?
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.