L30: Intestinal Transport of Electrolytes & water Flashcards
where is water lost
- skin (sweat)
- lungs
- GI tract (urine, feces)
- kidneys
- milk
what part of the GI tract does a majority of water reabsorption
MOSTLY the small and large intestine, followed by the colon which has a high absorptive capacity; provides “reserve capacity”
what is the site for control of enterosystemic fluid balance
epithelium, where abs and secretion occur simultaneously
secretion occurs at the crypts of the small & large intestine
absorption occurs in epithelial cells of villi in the SI
the secretory mechanisms for fluid in the epithelium are activated in response to…?
a meal
they change in response to changes of diet
the submucosal plexus is involved in mediating the secretory response
why is water so important in the kidneys
needed to be able to eliminate urea which is water soluble
what are the sources of fluids
- oral intake
- saliva
- secretions from stomach, pancreas, liver, small and large intestine
where is fluid absorption isomolar
SI
what plexus is involved in mediating the secretory response of the luminal surface in response to the passage of food
Submucosal plexus
what is the effect of mechanical stimulation of the luminal surface
- release of prostaglandins
- Cl- secretion by acting on secretomotor neurons that release VIP
- release of serotonin (5HT) from enterochromaffin cells
where are chloride secretion mechanisms located along the GI tract
the entire way
cystic fibrosis transmembrane conductance regulator or CTFR
Calcium and cAMP both activate what ion channel
CFTR
how do increasing cAMP levels increase Cl- secretion
cAMP activates PKA which phosphorylates CFTR causing Cl- secretion down electrochemical gradient
what agonists activate the basolateral K+ and CFTR channels?
ACh, histamine, bile acids
what molecules can stimulate PKA
VIP or PGE2
what transporter supplies the cell with Cl-
Na-K-Cl- channel on basolateral membrane
how does cholera toxin activate CFTR chloride channel
toxin activates intracellular Gprotein, active Gprotein activates adenylyl cyclase which increases cAMP levels; high cAMP levels open CFTR channel: draws Na+H2O into lumen –> Diarrhea
can do this even in the absence of food!
water diffuses across epithelium via ?
tight junctions; water follows absorbed substances into blood
most important mechanism for fluid reuptake in the GI tract
Na/glucose (SGLT-1) & Na/amino acid co-transporters
(secondary active)
how do rehydration salts work
make use of secondary active Na/Glucose transport (SGLT-1) in small intestine
what channel is responsible for Na uptake from luminal fluid
ENaC; modulated by Aldosterone
aldosterone antagonist that blocks ENaC & inhibits Na absorption in the distal colon & kidney
Amiloride; promotes loss of Na and water
where is K+ absorption passive, where is it active?
passive: in SI by solvent drag
active: in distal colon by K/H+ ATPase
where is the Cl-/HCO3 exchanger present (and coupled with a Na/H exchanger)
epithelial cells of villi & surface cells of ileum and proximal colon
how is Cl- mostly absorbed
paracellularly
1. voltage dependent Cl- absorption driven by electrochemical gradient
2. Cl-/HCO3 exchange (esp in LI)
what triggers the Na/H exchanger
high luminal pH caused by secretin
