27.18
Gastric Phase of Digestion
The gastric phase of digestion begins as soon as food enters the stomach. The incoming food bolus triggers neural and hormonal mechanisms, which last…
Local, neural and hormonal mechanisms trigger the gastric phase after food enters the stomach, lasting approximately 3-4 hours.
The incoming food bolus distends the stomach and triggers the stretch receptors, activating the local short reflexes mediated through the submucosal and myenteric plexuses.
As a result, the secretory activity of the gastric cells is activated, and the muscularis externa produces powerful contractions called mixing waves.
In addition, chemical stimuli like changes in pH and partially digested food also stimulate the release of hormones.
Gastrin secreted by the G cells, stimulate the enterochromaffin-like cells to release histamine, which activates the parietal cells to secrete hydrochloric acid. Additionally gastrin induces the chief cells to release pepsinogen.
This mixture of hydrochloric acid, pepsinogen, and mucus, secreted by the stomach lining, constitutes the gastric juice.
Hydrochloric acid destroys bacteria and partially denatures proteins. It also converts pepsinogen into pepsin, which breaks down proteins into smaller peptides.
Gastric juice and mixing waves further disintegrate food and form chyme.
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Q1: What triggers the gastric phase of digestion?
The gastric phase begins when food enters the stomach and stretches the stomach walls, activating stretch receptors. These receptors trigger local reflexes through the enteric nervous system, mediated by the myenteric plexus. Chemical stimuli like pH changes and partially digested food also stimulate hormone release, initiating the phase lasting approximately 3-4 hours.
Q2: How does the stomach produce gastric juice during digestion?
Stretch receptors and chemical stimuli activate gastric cells to secrete gastric juice containing hydrochloric acid, pepsinogen, and mucus. Gastrin, released by G cells, stimulates enterochromaffin-like cells to release histamine, which activates parietal cells to secrete hydrochloric acid. Chief cells release pepsinogen in response to gastrin, creating the complete gastric juice mixture.
Q3: What are the functions of hydrochloric acid in the stomach?
Hydrochloric acid destroys bacteria and partially denatures proteins, making them easier to digest. It converts pepsinogen into pepsin, an enzyme that breaks down proteins into smaller peptides. This acidic environment is essential for initiating protein digestion and creating an antimicrobial barrier within the stomach.
Q4: What role do mixing waves play in gastric digestion?
Mixing waves are powerful contractions produced by the muscularis externa of the stomach. These contractions work with gastric juice to further disintegrate food and form chyme, a semi-liquid mixture ready for the next digestive phase. Mixing waves are activated by local reflexes triggered when food distends the stomach.
Q5: How does the stomach regulate gastric juice production?
Gastric juice production is regulated by a feedback mechanism: accumulation of hydrochloric acid lowers stomach pH, triggering somatostatin release. Somatostatin inhibits acid production to maintain balance and prevent excessive acidity. When pH becomes too low, gastric juice production decreases, protecting the stomach lining from damage.
Q6: How do emotions affect the gastric phase?
Emotions such as fear and stress inhibit gastric secretion by overriding vagal digestion controls through sympathetic nervous system activation of the fight-or-flight response. This emotional regulation demonstrates how the central nervous system influences digestive processes, potentially disrupting normal gastric function during stressful situations.
Q7: What is chyme and how is it formed?
Chyme is a semi-liquid mixture formed when gastric juice and mixing waves disintegrate food in the stomach. It consists of partially digested food, hydrochloric acid, pepsin, and mucus. Chyme is the product of the gastric phase and is ready to move into the small intestine for continued digestion and absorption of nutrients.