Tubular Function 1 Flashcards
Where are the basolateral and apical/luminal membranes found?
. Basolateral on ‘exterior’ surface of PCT epithelial cells, touching peritubular fluid
. Apical/luminal membrane found on ‘interior’ surface of PCT epithelial cells, lining tubular lumen
Which transporter is found exclusively in the basolateral membrane?
Na+/K+ ATPase pump
Which transporters are found in the apical/luminal membrane?
Na+ channel proteins, Na+ symporters (Na+ with glucose or AAs), Na+/H+ exchangers
By what mechanisms does interstitial fluid move into the capillaries?
. Moves down hydrostatic gradient (low hydrostatic pressure in capillaries)
. Moves down colloid osmotic gradient (More proteins in capillaries, so moves via osmosis)
What substances are absorbed in the PCT?
Sodium, potassium, chloride, glucose, AAs, water, bicarbonate
How is tubular fluid in the PCT kept isosmotic with plasma?
. Solutes taken out from tubular fluid (reabsorbed) in PCT
. Water follows Na+ via AQP1 (aquaporin-1) and leaky tight junctions
. Therefore no build up of osmotic gradient (if water didn’t follow, would have lots of water in tubule without sodium, as sodium reabsorbed)
Give 4 mechanisms by which sodium can be transported across epithelial membranes.
. Diffusion into lateral space (can be facilitated with channel)
. Diffusion into epithelial cell (can be facilitated with channel)
. Na+ symporter (Na+ transported with glucose/AAs)
. Na+/H+ exchanger
How is sodium most commonly transported?
Na+/H+ exchanger
In the Na+/K+ ATPase pump, how many sodium and potassium ions are pumped in/out?
3 Na+ out for every 2 K+ in
Describe how filtered bicarbonate ions are transported (reabsorbed) in the PCT
. H+ (from Na+/H+ exchanger) and HCO3- combine to form H2CO3 (carbonic acid)
. Carbonic acid –> CO2 + H2O
. CO2 and H2O diffuse across apical membrane (impermeable to carbonic acid/bicarbonate ions, permeable to CO2 and H2O) into cell
. Carbonic anhydrase catalyses CO2 + H2O –> H2CO3, which dissociates to give H+ and HCO3-
. Bicarbonate leaves cell across basolateral membrane via
. H+ goes back out of cell across apical membrane into tubular lumen via Na+/H+ exchanger (NHE3) –> H+ used to combine with bicarbonate to form carbonic acid and the cycle can start again etc.
Why does reabsorption of filtered bicarbonate not correct acidosis?
Amount of bicarbonate that returns to PCT is the same as that which left it and the same goes for H+
How is water absorbed in the PCT transcellularly and paracellularly?
. Absorbed transcellularly by AQP-1 (aquaporin-1) and paracellularly by leaky tight junctions in PCT cells
. Water reabsorbed due to osmotic gradient (Na+ ions reabsorbed, water follows), higher oncotic pressure in peritubular capillaries (unfiltered proteins), and hydrostatic gradient
Compare the osmolarity of the reabsorbed fluid and filtrate
Isosmotic because water moves down osmotic gradient –> Equilibrium
What does it mean when we say that the osmotic flow of water results in ‘solvent drag’?
Solutes such as Na+, K+, Mg2+, Ca2+ are carried with the osmotic flow of water
Which substances does the Na+ symporter transport?
Glucose, AAs, lactate, phosphate
