Digestive System Part 2 Flashcards
(21 cards)
Cephalic phase of digestion
Stimulated by
- Sight
- Smell
- Taste
- Chewing
- Emotional states
Parasympathetic Nervous System (PNS) fibers activate GI nerve plexus
* Contractions
* Secretory activity
Gastric phase of digestion
Occurs when food reaches the stomach. Regulated by four
stimuli that initiate reflexes
- Distension
- Acidity
- presence of peptides/amino acids
These stimuli initiate reflexes:
* Short reflexes: Via local enteric nervous system
* Long reflexes: Via CNS
Gastrin: Amplifies acid secretion and gastric motility during this phase.
Intestinal phase of digestion
When chyme enters the small intestine. Regulated by four
stimuli in small intestine:
- Distension
- Acidity
- Osmolarity
- Digestive products
These stimuli initiate reflexes:
* Short and long reflexes
- Hormones involved: Secretin, CCK, GIP
Secretin
Stimulates bicarbonate secretion to neutralize acidity in small intestine.
Cholecystokinin (CCK)
Promotes bile and pancreatic enzyme release in the small intestine.
Glucose-dependent insulinotropic peptide (GIP)
Enhances insulin release in the small intestine.
Gastrin
During the gastric phase
Produced in antrum (lower part) of stomach. Stimulates acid secretion and stomach motility
Stimuli for release:
* Amino acids & peptides in stomach
* ↑ firings PNS neurons
Mucous cells
Mucous
* HCO3-
Parietal cells
Acid (HCl?)
* Intrinsic factor
* Vitamin B12
Enterochromaffin-like cells
Histamine
Chief cells
Pepsinogen
D cells
Somatostatin: acting as a
“brake” for the gastrointestinal system
Secretion of HCl by Parietal Cells
Stomach secretes 2L of HCl per day
- Gastric fluid – pH 1.5-3.5
- Production stimulated by
- Gastrin
- Histamine
- Acetylcholine (ACh)
- Inhibited by
- Somatostatin
Stimulate insertion of H⁺/K⁺ ATPase pumps into the apical membrane
of parietal cells (facing the stomach lumen)
- Greater secretion of H+
- H+ made from carbonic anhydrase reaction in epithelial cell
- HCO3- secreted outside in exchange for Cl⁻
- Cl⁻ enters the lumen and joins H⁺ to form HCl
- Maintains electroneutrality:
One H⁺ out, one K⁺ in, one HCO₃⁻ out, one Cl⁻ in
Cephalic phase and HCl
↑ PNS activity= ↑ACh release from gastric plexus neurons
- Multiple regulatory systems:
- Histamine potentiates ACh & Gastrin
- Gastrin & Ach potentiate histamine
Increased HCl:
* Inhibits gastrin secretion
* Stimulates somatostatin
* Directly inhibits more acid secretion
Pepsin Secretion
- Chief cells in stomach secrete inactive pepsinogen
- Low pH (high H+) of stomach acid stimulates reaction
- Autocatalytic
- Pepsin cleaves pepsinogen to pepsin
- Protects from autodigestion
- Pepsin accelerates protein digestion
- Inactivated in small intestine where pH is higher
Pancreatic functions
Endocrine: Insulin and glucagon
Exocrine: “Pancreatic juice,” bicarbonate, digesting enzymes
Bicarbonate (HCO₃⁻) Secretion into the Small Intestine
- HCO3- secretion into small intestine via Cl-/HCO3- ion exchanger
- H+ secretion into blood via Na +/K + exchanger
- Concentration gradients maintained by Na +/K + ATPase & K + leak channels
- Cl- removed by CFTR (cystic fibrosis
transmembrane conductance
regulator
Secretin
- Produced in small intestine
- Stimulated by acid in small intestine
- Stimulates
- HCO3- secretion from pancreas & liver
- Inhibits
- Stomach acid secretion
- Stomach motility
- Potentiates cholecystokinin CCK’s actions
Trypsin Secretion
- Secretion of inactive enzymes prevents autodigestion
- Enterokinase
- Membrane-bound enzyme in small intestine
- Catalyzes trypsinogen to active trypsin
- Trypsin activates other enzymes
Cholecystokinin (CCK)
- Produced in small intestine
- Stimulated by amino acids & fatty acids in small intestine
- Stimulates
- Enzyme secretion from pancreas & liver
- Gallbladder contraction
- Inhibits
- Stomach motility & acid secretion
- Potentiates secretin’s actions
Control of Bile Secretion
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