Lec 43 Intro To Transport and Proximal Tubule Function Flashcards
What parts of nephron associated with vasa recta?
- loop of henle
- collecting duct
What parts of nephron associated with peritubular capillary?
- proximal and distal tubules
Equation for filtered load?
F.L. = Px * GFR
Equation of urinary excretion rate?
UEx = Ux * V.
Equation for tubular reabsorption rate?
TRx = FLx - UEx
= filtered load - urinary conc
Equation for tubular secretion rate?
TSx = UEx - FLx
= urinary conc - filtered load
Equation for fractional secretion?
FEx = UEx/FLx
= urinary excretion/filtered load
What is mech of transceullular transport?
- through membrane through cell
- through channels, simple diffusion, or active transport
What is mech of paracellular transport?
- between cells through tight junctions
- no protein carriers
- requires driving force
- simple diffusion down electrochemical gradient
What are 3 forms of passive transport?
- simple diffusion
- facilitated diffusion
- solvent drag
What are 2 forms of active transport?
- primary active
- secondary active [cotransporter or countertransporter]
Is tight junction leaker in proximal tubule of ascending loop of henle?
- very leaky in prox tubule for reabsorption
- very impermeable in ascending loop
How much of tubule content reabsorption occurs in proximal tubule?
2/3
How is Na transported in first half of proximal tubule?
on apical: Na couple transport into cell [Na-H, Na-gluc, or Na-AA, etc]
on basolateral: Na leaves via Na-K ATPase [2 K in, 3 Na out]
Describe Cl transport in first half of proximal tubule?
Cl trapped in lumen –> no Cl transport in early PT
What is predominant extracellular cation anion transported in first half of proximal tubule?
HCO3- goes to balance out charge of Na
What happens to Cl conc along early PT?
- Cl conc increases as fluid moves down early PT since Cl trapped in lumen
What two types of Na transport in second half of proximal tubule?
- paracellular: Cl driven Na reabsorption
- transcellular: Na-H exchanger
What is mech of Cl driven Na reabsorption in second half of proximal tubule?
- Cl diffuses between cells into interstitium
- driving force for Cl because of the increased conc from first half PT
- causes +4 mV lumen positive potential
- electrical gradient favors movement cations paracellular –> Na is most abundant cat so moves a lot
What is mech of transcellular Na reabsorption in second half of proximal tubule?
apical:
- Na-H exchanger brings Na into cell
- Cl-anion exchanger brings Cl into cell [anion often formate]
- H and formate into lumen then re-transported back into cell to be used again
basolateral
- Na leaves via Na-K ATPase [3 Na out, 2 K in]
- Cl leaves via Cl-K cotransporter [1 K out, 1 Cl out]
What is mech of HCO3 reabsorption in PT?
- filtered HCO3 trapped in lumen unless converted
- H comes out of cell into lumen via Na-H exchanger or H ATPase
- In lumen: HCO3 + H –> H2CO3 –> H20 + CO2 via luminal membrane bound carbonic anhydrase
- H2O and CO2 diffuse into cell
- CO2 and H2O converted back to HCO3 via intracellular carbonic anhydrase
- on basolateral HCO3 leaves by
- – HCO3-Cl exchanger [1 HCO3 out, 1 Cl in]
- – HCO3-Na cotransporter [3 HCO3 out, 1 Na out]
What are the 2 ways HCO3 leaves epithelial cell basolateral membrane for reabsorption to capillary in PT?
- – HCO3-Cl exchanger [1 HCO3 out, 1 Cl in]
- – HCO3-Na cotransporter [3 HCO3 out, 1 Na out]
Is there net H secretion in HCO3 reabsorption in PT?
No – H is recycles back into cell via H2O and CO2
How is glucose reabsorbed in PT?
- in proximal tubule
- if you saturate reabsorptive capacity in PT there is no downstream glucose transporter
- apical: comes in by Na-glucose cotransporter
- basolateral: leaves by Glut1/2
