Renal Transport Flashcards
Describe the polarity of epithelial cells.
(functionally distinct sides)
- apical membrane
- basolateral membrane
Where are tight junctions, and what do they do?
near apical surface
segregate membrane surfaces (and associated proteins) from one another – this determines function of epithelium (transport directionality)
Where are there transepithelial potential differences?
across peritubular/interstitial space (0 mv) and tubular lumen
- interstitial space is always used as the references (0)
- tubular lumen can be electronegative, neutral, or electropositive – has implications for ECs in terms of what types of transport is being favoured
What is the brush border?
membrane folding and villi on renal epithelial cells that amplify/enhance the apical surface area
- greater SA for transport to occur across
- especially important in proximal tubule, where bulk of transport occurs
Na+ Reabsorption
Where does the bulk of Na+ reabsorption occur?
proximal tubule
~2/3 reabsorbed by the time we reach the end of PT
Na+ Reabsorption
Where does regulation of Na+ reabsorption occur?
collecting duct
Na+ Reabsorption
What is the filtered load?
amount of Na+ in ultrafiltrate per day
GFR x [Na+]plasma
Na+ Reabsorption
What is fractional reabsorption?
fraction of Ma+ that has been successfully reabsorbed
1 - [Na+]tubular/[Na+]ultrafiltrate
where [Na+]tubular is the % of Na+ remaining along nephron
where [Na+]ultrafiltrate is the % of Na+ remaining in Bowman’s space (100%)
Na+ Reabsorption
What is fractional excretion?
how much Na+ being lost vs. how much has been filtered – amount of excretion is quite low compared to how much is filtered
excretory rate of Na+ / filtered load of Na+
= [Na+]urinary x V / GFR x [Na+]ultrafilitrate
Na+ Reabsorption
What is Na+ transport dependent on?
- available transporters
- regional permeability (regulated by tight junctions)
- composition of the delivered tubular fluid (changes based on flow rate through nephron)
- transepithelial voltage gradient (helps favour come types of transport)
Na+ Reabsorption
How much Na+ is reabsorbed (beginning from proximal tubule to collecting duct)?
almost 100% of the Na+ that gets filtered from glomerulus into the tubule
Na+ Reabsorption
Proximal Tubule – Mechanism
see notes
Na+ Reabsorption
Thin Descending Limb – Is the thin descending limb permeable to NaCl?
NO
Na+ Reabsorption
Thin Ascending Limb –Is the thin ascending limb permeable to NaCl?
YES – some/minimal passive Na+ reabsorption occurs (not much transport proteins contributing here)
Na+ Reabsorption
Thick Ascending Limb – Mechanism
see notes
Na+ Reabsorption
Distal Tubule – Mechanism
see notes
Na+ Reabsorption
Distal Tubule – Does the composition of proteins change along the tubule?
YES – composition of proteins (especially found on apical membrane) varies depending on part of distal tubule
Na+ Reabsorption
Distal Tubule – How can hypertension be reduced?
reduced amount of Na+ transport, paired with reduced H2O absorption
Na+ Reabsorption
Distal Tubule – What are thiazides?
diuretics – help get rid of excess H2O, and slightly reduces blood presure
Na+ Reabsorption
Distal Tubule – What are the principal cells?
DCT1 and DCT2
Na+ Reabsorption
Collecting Duct – Mechanism
see notes
What is the last portion of the nephron where fluid in the tubule is still modifiable?
collecting duct
Na+ Reabsorption
Collecting Duct – Why is this part of the tubule so important?
regulates Na+ reabsorption
more fine tuning and regulation of the final composition of fluid
Na+ Reabsorption
Collecting Duct – Mechanism
see notes
Where does bulk K+ reabsorption occur?
proximal tubule
Where is K+ reabsorption the same for all K+ levels? Where is it different?
PT and TAL reabsorption is same for all K+ levels
CD reabsorption is different for normal/elevated vs. low K+ levels
Where does regulation of K+ reabsorption/secretion occur?
collecting duct
What happens if there is normal/elevated K+ levels in the collecting duct?
secretion predominates – K+ leaves blood, and going to be excreted
What happens if there is low K+ levels in the collecting duct?
reabsorption predominates – excrete < 1% of K+ filtered
What does K+ concentration determine in the collecting duct?
direction of transport – secretion vs. reabsorption
K+ Reabsorption
Proximal Tubule –Mechanism
see notes
What does transport do?
alters tubular fluid composition and decreases flow rate
What is the TF/P ratio?
ratio of concentration of solute (x) in tubular fluid to solute (x) in plasma at a given distance along PT
- describes how composition of tubular fluid is changing compared to fluid in plasma
What is the filtrate in the glomerulus very similar to?
composition to plasma – EXCEPT there is no protein in filtrate
What is used as the reference point when observing TF/P ratios? Why?
inulin
- substance is freely filtered at glomerulus, and is NOT reabsorbed/secreted – therefore, amount of inulin in tubular fluid is constant
- use as reference to observe how other things in PT fluid are changing
What decreases TF/P?
reabsorption of valuable substances – such as glucose, amino acids, HCO3-
What increases TF/P?
reabsorption of Na+ (less) and Cl- (more)
From beginning to end of proximal tubule, how does fluid composition resemble?
goes from closely resembling plasma (without protein) to becoming essentially NaCl
In the proximal tubule why is transepithelial potential (PD) inegative at beginning, but then becomes positive?
initially negative because there is very little Cl- movement
later movement of Cl- makes it positive
How does proximal tubular fluid flow rate change along the tubule?
decreases progressively – result of bulk H2O reabsorption into bloodstream
K+ Reabsorption
Thick Ascending Limb – Mechanism
see notes
K+ Secretion
Distal Convoluted Tubule – Mechanism
see notes
K+ Handling
Collecting Duct – Mechanism
see notes
Where does bulk K+ reabsorption occur?
- PT
- TAL – paired with Na+ reabsorption
