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Digestive Secretion & Control
Regulatory Mechanisms in General
Glandular Secretion of the Stomach
Neural Control of Secretion
Endocrine Control of Secretion
Control of Gastric Secretion
Digestive Regulatory Mechanisms
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Allow communication between different parts of the
digestive tract
Ensure the presence of sufficient secretions when
food present
Help avoid overabundance of secretions in absence
of food
Two types of mechanisms: neural and endocrine
Glandular Secretions of the Stomach
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Recall that the main secretory products of the stomach
are:
- mucus: protection from acidic chyme and pepsin
- hydrochloric acid: acidic pH of stomach
- intrinsic factor: absorption of vitamin B12
- pepsinogen: cleaved to pepsin
- gastrin: from endocrine cells, stimulates parietal
and chief cells
How does a Cell Produce HCl?
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Parietal cells must maintain a normal pH to survive
They don’t really produce HCl internally and excrete
it, but instead release H+ ions and Cl- ions into the
lumen of the stomach
Here’s how:
- CO2 is taken in from the cell
- CO2 + H2O ==> carbonic acid (H2CO3)
(reaction catalized by carbonic anhydrase)
- H2CO3 dissociates into bicarbonate and H+
- H+ ions are actively pumped out of the cell, into
the lumen of the stomach
- Cl- ions diffuse with the charged H+ ions
- bicarbonate enters bloodstream
Production of H+ and Cl- Ions
Neural Control of Secretion
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Stimulation by the parasympathetic nervous system
- oral cavity: facial and glossopharyngeal nerves
- other GI organs: vagus nerve, and local nerve
reflexes (stretch receptors, chemo receptors)
The neurotransmitter of the parasympathetic nervous
system is acetylcholine (ACh)
Endocrine Control of Digestive Secretions
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Feedback interactions between organs, and within
organs
Allow different parts of the digestive tract to influence
each other
Main hormones:
- gastrin
- secretin
- cholecystokin (CCK)
- gastric inhibitory peptide
Source, Regulation, and Action of Hormones
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Gastrin:
- produced from the stomach and duodenum
- stimulated by stomach distension, peptides,
amino acids, alcohol, caffeine
- inhibited by acidic pH
- functions:
- increases stomach secretions
- increases rate of gastric emptying
Source, Regulation, and Action of Hormones
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Secretin:
- produced by the duodenum
- release stimulated by acidity of chyme
- functions:
- decreases gastric secretions
- stimulates pancreatic secretion of
bicarbonate
- increases bile secretion by liver
- increases intestinal mucus secretion
- decreases gastric motility
Source, Regulation, and Action of Hormones
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Gastric Inhibitory Peptide:
- produced by small intestine
- stimulated by lipids
- functions:
- inhibits gastric secretion
- inhibits gastric motility
Source, Regulation, and Action of Hormones
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Cholecystokinin (CCK):
- produced by small intestine
- stimulated by lipids
- functions:
- inhibits gastric secretion
- stimulates pancreatic digestive
secretions
- causes contraction of gallbladder
- decreases gastric motility
The Control of Gastric Secretion
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The stomach produces 2 to 3 liters of secretions per
day
Secretion increases immediately after a meal
The rate of gastric secretion is under neural and
hormonal control
There are three phases to gastric secretion:
- cephalic phase
- gastric phase
- intestinal phase
Cephalic Phase
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Stimulation: thought, chewing, taste of food
Neural response: release of ACh from the vagus nerve
Result: increased release of mucus, HCl, pepsinogen,
intrinsic factor, gastrin (gastric secretions) in the
stomach (about 500 ml/hour)
Prepare stomach for oncoming food
Gastric Phase
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Stimulus: distension of stomach, increase in pH of
gastric contents, presence of proteins and amino
acids
Neural Response: Stretch receptors and
chemoreceptors (detecting protein, alcohol, and
caffeine) stimulate ACh release, activating parietal
and chief cells
Endocrine Response: Protein and distension also
stimulate the release of gastrin
Gastrin production declines at pH < 2.0
Duration of gastric phase: many hours
The Intestinal Phase
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Purpose: Control the rate of gastric emptying, and
ensure that the small intestine is ready to digest and
absorb chyme
Stimulus: Acidity of chyme entering the small intestine
Neural Response: chyme in duodenum activates
mechanoreceptors and chemoreceptors, resulting in
the enterogastric reflex (slowing of gastric motility)
The Intestinal Phase (cont.)
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Endocrine Responses:
- low pH => increased secretin release
- decreased gastric secretion
- stimulation of pancreatic bicarbonate
- increased bile secretion
- increased intestinal mucus secretion
- decreased gastric motility
The Intestinal Phase (cont.)
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Endocrine Responses:
- lipids increase CCK and gastric inhibitory peptide
- both slow gastric secretion
- CCK also causes gall bladder contraction and
increased pancreatic digestive secretion
The Intestinal Phase (cont.)
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Endocrine Responses:
- partially digested proteins (or chyme of high pH)
cause increased release of gastrin
- result: increased acid and pepsin production
Thus, gastric digestion can be regulated for specific
components of food
SUMMARY
The secretion of digestive enzymes is tightly
regulated, such that organs of the digestive tract
signal each other to optimize effeciency of digestion
and absorption.
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Absorption