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#1
12-27-2012
 USMLE Forums Guru Steps History: Not yet Posts: 315 Threads: 115 Thanked 155 Times in 84 Posts Reputation: 165
GFR and RPF Relationship

I understand what each is on their own...

RPF = the amount of plasma entering and then leaving the kidney per unit time
GFR = the glomerular filtration rate (sum of the starling forces)

But can the RPF somehow influence the GFR? if so, then how?

Or is this a concept which is not important?

#2
12-27-2012
 USMLE Forums Addict Steps History: 1+CK+CS Posts: 141 Threads: 16 Thanked 51 Times in 26 Posts Reputation: 61

its a very important concept. lots of questions based on it.
#3
12-27-2012
 USMLE Forums Guru Steps History: Not yet Posts: 375 Threads: 17 Thanked 131 Times in 81 Posts Reputation: 147

I am not very good at it, and will appreciate if someone corrects me if I am wrong...I am going to explain it in simple words.

All you need to know is the normal values and a bit of calculation.

Normal GFR= Approx 125
Normal RPF = Approx 600

Now FF = GFR/RPF
So FF= 125/600 = Approx 0.20

Most of the questions they ask about this concept is up and down arrows. They will tell you that an experiment is being done and the either afferent or efferent arteriols are constricted or expanded.and ask you the effect on GFR, RPF and FF.

So Now if efferent arteriols are constricted, it will increase GFR. and no change in RPF.
suppose GFR is increased to 140 ,then FF will be 140/600= Approx 0.23 ...means FF will increase.

And then You can calculate the others according to the data they provide.

Hope this helps u a bit.

Good Luck.
#4
12-27-2012
 USMLE Forums Guru Steps History: Not yet Posts: 315 Threads: 115 Thanked 155 Times in 84 Posts Reputation: 165

Quote:
 Originally Posted by Doc4Step1 I am not very good at it, and will appreciate if someone corrects me if I am wrong...I am going to explain it in simple words. All you need to know is the normal values and a bit of calculation. Normal GFR= Approx 125 Normal RPF = Approx 600 Now FF = GFR/RPF So FF= 125/600 = Approx 0.20 Most of the questions they ask about this concept is up and down arrows.They will tell you that an experiment is being done and the either afferent or efferent arteriols are constricted or expanded.and ask you the effect on GFR, RPF and FF. So Now if efferent arteriols are constricted, it will increase GFR. and no change in RPF. suppose GFR is increased to 140 ,then FF will be 140/600= Approx 0.23 ...means FF will increase. And then You can calculate the others according to the data they provide. Hope this helps u a bit. Good Luck.

Hmm. this is how i understood the concept. (btw, my question was a little differrent, but i think its good to review this also)...i made a reference to this in my previous thread too.

whever you get vasoconstriction (whether its the afferent arteriole, or the efferent arteriole), you will ALWAYS get a decrease in RPF.

as for GFR, that depends whether its afferent or efferent constriction...efferent vasoconstriction will INCREASE GFR, because you are stopping the blood from leaving the glomerules, which in effct, will increase its pressure there (like putting your thumb at the end of a hose pipe, you can feel the pressure under your thumb build up...goes back to starling forces in glomerular capillary...which will increase filtration rate.

(and the opposite is true for afferent vasoconstriction...it will decrease GFR)

i understand that filtration fraction = GFR/RPF

but my question is this: are GFR and RPF both independent variables when it comes to FF?

can a change in RPF cause a change in GFR? and vice versa? is there any concept within the scope of physiology that answers this? or am i going too deep here?
#5
12-27-2012
 USMLE Forums Master Steps History: 1+CK+CS+3 Posts: 824 Threads: 66 Thanked 463 Times in 298 Posts Reputation: 473

Quote:
 Originally Posted by ReggieMiller Hmm. this is how i understood the concept. (btw, my question was a little differrent, but i think its good to review this also)...i made a reference to this in my previous thread too. whever you get vasoconstriction (whether its the afferent arteriole, or the efferent arteriole), you will ALWAYS get a decrease in RPF. as for GFR, that depends whether its afferent or efferent constriction...efferent vasoconstriction will INCREASE GFR, because you are stopping the blood from leaving the glomerules, which in effct, will increase its pressure there (like putting your thumb at the end of a hose pipe, you can feel the pressure under your thumb build up...goes back to starling forces in glomerular capillary...which will increase filtration rate. (and the opposite is true for afferent vasoconstriction...it will decrease GFR) i understand that filtration fraction = GFR/RPF but my question is this: are GFR and RPF both independent variables when it comes to FF? can a change in RPF cause a change in GFR? and vice versa? is there any concept within the scope of physiology that answers this? or am i going too deep here?
decrease in renal plasma flow (as for example in hypotension) decreases GFR but FF proportionately remains same due to decrease in both gfr/rpf
in case of ang 2 wben efferent is constricted u get increased gfr due to increased hydrostatic pressure in capillaries and decreased rpf which increases FF

Thats how i understand this concept
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#6
12-27-2012
 USMLE Forums Guru Steps History: Not yet Posts: 315 Threads: 115 Thanked 155 Times in 84 Posts Reputation: 165

Quote:
 Originally Posted by MedicalExaminer decrease in renal plasma flow (as for example in hypotension) decreases GFR but FF proportionately remains same due to decrease in both gfr/rpf in case of ang 2 wben efferent is constricted u get increased gfr due to increased hydrostatic pressure in capillaries and decreased rpf which increases FF Thats how i understand this concept
yup ,thanks for confirming. i believe in a situation like hypotension, or hemorrhage, you'll get increased sympathetic activity, which prefrentially constricts afferrent more than effrent, which causes BOTH GFR and RPF to decrease, resulting in FF to be the same.

and as for angiotensin II, that will have the opposite effect - efferent constricted, causing DECREASED RFP, and INCREASED GFR, which results in increased FF.

which is exactly what you posted.

my question on whether RPF influences GFR was a dumb question, and i now realize why. i was confused on what RPF actually was, but after looking at how the formula using PAH was derived, i know have a complete understanding of what RPF is, and because of that, i know understand (more completely) what is meant by Filtration Fraction.

Thanks though!

 Tags Physiology-, Renal-

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