l9: GI salt and water transport Flashcards
what does the jejunum reabsorb
Na+ cl- H20 & K+
What does the ileum reabsorb and secrete
Na H20 cl k
Secretes- bicarbonate
What in total does the colon secrete and absorb
Secretes- k+ and bicarbonate
Absorbs- Na cl and H20
Describe enterocytes
columnar epithelial cells in small intestine
Polarised
Apical side- faces lumen; has microfolds
Basal side: Communicates with bloodstream & lymphatic lacteals
Seperated by tight junctions
function of tight junctions
Allow passage of water &
Restrict passive flow of solutes after secretion or absorption
What are the 2 types of epithelial transport & describe each
Paracellular: Forms regulated and restrictive Permeation in both directions through tight junctions: uses electrochemical gradients to move water & solute
Transcellular: Employs membrane transporters to move H20 & molecules through cell-active process
Sets up gradient, via solute movement, for passive transport of H20 & solutes.
What are the 3 types of transepithelial channels
Channels- gated and ion specific, via electrochemical gradient
Carrier -
pumps
What are the 3 types of transport, describe each
Passive
active - Primary-uses ATP, against electrochem gradient: ie Na-ATPase channel
Secondary- Cotransport- Na-glucose transporter
solvents drag-water leaks from lumen, through paracellular space to reach osmotic equilibrium on basolateral side. -upper small intestine
- moves/pulls additional solutes
For all concepts look at the diagrams from slides 36 to 41
Describe concept 1
NA/K ATPase- Basolateral active transport. Creates Na electrochemical gradient, between enterocyte and lumen
Concept 2
Na coupled transport-AA or glucose-Apical
the NA gradient from C1 allows coupled Na transport from lumen.
secondary transport- couples uphill movement of AA or glucose to downhill movement of NA, helping to set up Na/K-ATPase
Process is electrogenic as lumen becomes more negative
Concept 3
Apical side: Separate NA and cl antiporters.
Na-H exchanger + HCO3-Cl exchanger, creating NACL
Concept 4
Apical and Basolateral: CL- secretion occurs in conjunction with basolateral NA K CL absorption
Na-k-ATPase drives NA gradient allowing Apical Cl- secretion through CFTR channel and coupled import of k cl and na from basolateral
Due to high chloride conc CFTR widely expressed in colon
concept 5
Water will travel through intercellular tight junctions in the setting of NACL absorption
from lumen to blood
H20 Absorption in the small intestine
Basolateral Na/k-ATPase pumps
Leads to build up of NACL in paracellular spaces between enterocytes
Provides osmotic gradient that draws water into paracelluar spaces
Oral rehydration therapy
Utilises mechanism of glucose-coupled NA absorption
oral rehydration solutions to promote fluid absorption by coupling Na with glucose in solution
Channel SGT-1 binds 2 Na molecules to 1 glucose, transporting them into cell.
How do GI cells secrete water
In small intestine cl- secretion draws Na and H20 across tight junctions
How does the jejunum absorb water
Solvent drag
Highest NA absorption coupled with nutrient absorption
How does the ileum absorb water
Highest NACL absorption
How does the colon absorb water
Apical NA and k
Aldosterone increases synthesis of Na channels and increased k secretion
What are the effects of the cholera toxin
Stimulates excess cAMP in crypt cells cAMP activates CFTR channel therefore massive influx of cl- Na follows forming NaCL Followed by H20 secretion, more than can be reabsorbed Result: H20 and electrolyte loss-fatal
Large Intestine: regulated by what hormone
Aldosterone
What is the function of secreting cl- and HCO3- into lumen
Buffers for acid produced by bacteria
what are the Na channels called
Basolateral-NHE1
Apical: NHE 2&3
For Na/H exchanger
Na+ and H+ exchangers coupled with cl-/HCO3- result in
NaCl absorption
