Lecture 44: GI Tract Special Secretions Flashcards
(46 cards)
What are 5 exocrine secretions and where do they come from?
- Salivary secretions - Salivary glands
- Gastric secretions - stomach epithelia, urface epithelia, gastric glands with specialized epithelial cells
- Bile - Liver
- Pancreatic secretions - Pancreas
- Intestinal secretions
What are the functions of exocrine secretions?
- Maintain the composition of the lumen of the GIT appropriate to its function
- Maintain osmolarity, pH, water, and enzyme content at the correct levels
What components make up exocrine secretions?
Electrolytes (salt), water, mucous, enzymes, acid or bicarbonate to alter pH
How are exocrine secretions made?
Components are produced or transported by epithelial cells and secreted across the mucosal surface of: GI tract lining and the lining of accessory organs
Are exocrine secretions reabsorbed?
Yes
What are the functions of salivary secretions?
- Lubrication of ingested food
- Dissolves water soluble components
- Neutralises Acid - food acid, refluxed acid (protects teeth)
What are the components of salivary secretions?
- Electrolytes (mostly Na⁺, Cl⁻ and HCO₃⁻)
- Water
- Mucous
- α amylase
- Antimicrobial factors
Where is saliva made and secreted?
Salivary glands are lined with epithelial cells that make and then modify saliva before secretion
Describe step 1 of saliva secretion:
- Occurs in the acini located at the end of the glands
- Primary saliva: isotonic NaCl, contains amylase
- Leaky epithelium secreting Cl⁻
- Na⁺ follows Cl⁻ via paracellular pathway (leaky)
- Osmotic water movement can occur
- Isotonic solution produced
Describe step 2 of saliva secretion:
- Occurs in the ducts leading out of the gland
- Modify saliva content - hypotonic
- Tight epithelium - no paracellular movement
- Reabsorption of Na⁺ and Cl⁻ ions through the cells
- Osmotic water movement can NOT occur
- Decreased salt = increased H₂O (osmolarity decreases)
- HCO₃⁻ secreted
- Final saliva is hypotonic with basic pH
Describe the salivary secretion of NaCl:
Same as intestinal NaCl secretion
1. cAMP stimulates CFTR activity:
* Sustained secretory response
* Ligands include VIP (ENS) and prostaglandins
2. Ca²⁺ stimulates K⁺ channel activity:
* Increased driving force for NKCC and Cl⁻ exit via CFTR
* Transient secretory response
* Ligands include ACh (ENS) and histamine
Describe saliva regulation from the autonomic NS:
- Parasympathetic (Primary pathway) - Acetylcholine
- Increases intracellular Ca²⁺,↑ Cl⁻ secretion - large amounts of isotonic NaCl - Sympathetic (Secondary pathway, potentiates parasympathetic pathway) - adrenaline
- Increases viscous fluid, more mucus.
Describe the indirect regulation of the acini on the ducts:
- Ion transport in the ducts doesn’t change but flow rate does - more secretion in acini = higher flow rate through the ducts
- Low flow rates = greater NaCl reabsorption = more hypotonic
- High flow rates = less NaCl reabsorption = more isotonic
- Ionic composition of saliva is dependent on flow rate through the ducts
What is the function of surface epithelia and what does it secrete?
Protective
- Mucus
- HCO₃⁻
What is the function of gastric glands and what do they secrete?
Make acid, enzymes, and hormones
- Parietal cells: HCl and Intrinsic factor
- Enterochromaffin-like cells (ECL): histamine
- Chief cells: pepsinogen
- Enteroendocrine cells, several types e.g. G cell: gastrin, D cell: somatostatin
Describe the secretion of mucus and bicarbonate:
- Mucus and HCO₃⁻ Mucins are released by exocytosis
- HCO₃⁻ and H₂O secretion occur and mucous is hydrated
What is the function of alkaline mucous?
Protection of gastric mucosa from acid
What are the functions of gastric acid?
- Denatures protein
- Activates pepsin
- Hypo-osmotic - dilutes food
- Protection (kills bacteria)
Describe how gastric acid is secreted?
Epithelial Cl⁻ and H⁺ secretion into lumen
1. Carbonic anhydrase makes H⁺ and HCO₃⁻
2. H⁺/K⁺ ATPase transports H⁺ against conc. gradient
3. HCO₃⁻ is recycled into body in exchang with Cl⁻ at basolateral membrane
4. Cl⁻ moves into lumen via a Cl⁻ channel
5. Osmotic water secretion (H₂O follows ions)
How is HCl secretion up-regulated by distention?
- Distention induces a neural response in which neurons of the ENS releases ACh
- ACh binds to receptors on parietal cells that increase intracellular Ca2+
- Activates calcium activated Cl- channels (CICC) and H+/K+ATPase
How is HCl secretion up-regulated by protein content/gastrin?
- Protein/peptides are detected by enteroendocrine cells that secrete Gastrin
- Gastrin binds to receptors on parietal cells that increase intracellular Ca2+
- Activates calcium activated Cl- channels and H+/K+ATPase
How is HCl secretion up-regulated by histamine?
- Gastrin and ACh induce the release of Histamine from Enterochromaffin-like cells (ECL cells)
- Histamine binds to receptors on parietal cells that increase intracellular cAMP and PKA
- Phosphorolates and activates CFTR channels and H+/K+ATPase
Describe how HCl secretion is down-regulated?
- Acid in the duodenum is detected by enteroendocrine cells which release secretin
- Secretin travels via blood to the stomach and causes the release of
somatostatin from D cells - Acid chyme in the distal stomach also directly causes the release of
somatostatin from D cells 4. Somatostatin inhibits the activity of chloride channels and H+/K+ ATPase
Where are pancreatic gland fluid made and secreted?
Glands are lined with epithelial cells that make and modify secretion before secretion
