Lecture 25; Intestinal fluid and electrolyte balance Flashcards
What increases the SA of the SI?
- Folds of kerchring a.k.a plicae circularis
- Microvilli and crypts of lieberkuhn
- Submicroscopic microvilli
What increases the SA of the LI?
- Semilunar folds
- Crypts, but NO VILLI
- Microvilli
What are the primary secretions of the small intestine?
- Pancreatic secretions
- Bile secretions
- HCO3
NOT POTASSIUM
What does the SI absorb?
- Water
- Na (Glucose, Protein)
- K
- Cl
6.5L
What does the LI secrete?
- Potassium
- HCO3
What does the LI absorb?
- Na
- Cl
- Water
NOT NUTRIENTS
(~2.0L)
What happens with increased secretion in the SI and LI?
Secretory diarrhoea
SI = far worse LI = not as bad (because less fluid at this point)
Why do they need to give HCO3 in solutions to people who have secretory diarrhoea?
Both SI+LI secrete lots of HCO3 (Not pulled with electrolytes)
What is notable about epithelium?
They are polarised (Concentration gradient b/w lumen and apical surface)
What are the transporters present in the intestinal epithelium?
- Na/K ATPase
This creates a Na and K gradients, (electro chemical) which facilitates diffusion from the lumen into the cell.
What are the ways solutes can move across the epithelium?
- Either absorptive or secretory
Transcellular: (Crossing one membrane therefore requires active transport)
OR
Paracellular: (Passive movement between tight junctions)
How does water move across the epithelium?
- Osmosis
Transcellular and paracellular (predominantly)
Where does majority of water absorption occur?
Jejunum
What is the huge water movement responsible for?
Solvent drag, i.e osmosis is coupled with movement of Na and urea in the jejunum
Where does Na absorption occur in relation to mucosal structures in the SI and LI?
SI = Villus epithelial cells
LI = Surface epithelial cells
What are the sodium transporters of the epithelium?
- Na/glucose or Na/AA co-transporters
- Na/H exchanger
- Parallel Na/H and Cl/HCO3 exchangers
- Epithelial Na channels
Na/K ATPase for Na transport across the apical membrane
What follows Na movement?
Cl-
How does Cl move?
- Passive Cl absoprtion
- Cl-HCO3 exchanger
- Parallel Na-H and Cl-HCO3 exchangers
What is the channel for Cl secretion?
CFTR (cystic fibrosis)
What can increase CFTR insertion?
Inflammation / microbes etc cause an increase of intracellular Ca or cAMP
= insertion of CFTR into lumen membrane (from subapical vesicles)
Whats the other ion to consider? not too important…
Potassium
- Passive K absorption and secretion
- Active K absorption and secretion
What changes the absorption and secretion of the gut?
- ENS = ACh
- Endocrine = Aldosterone
- Paracrine system = 5HT
Summarise the net movement of ions in the SI and LI;
SI
- Net absorber of Water, Na, Cl and K
- Net secretor of HCO3
LI
- Net absorber of Na and Cl
- Net secretor of K and HCO3
Describe the potential origins of diarrhoea and implications?
SI = Voluminous LI = Small volume
