medchick
Forum Newbie
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Hi, i'm a newbie on this site; so i'm not as informed as the rest of u are....but here's my Q: if filtration fraction (FF) increases when renal plasma flow (RPF) decreases, then how is it possible that GFR is increased when flow is increased (edited; said decrease last time when i meant to say increase) ???? help...i'm confused by this.... isn't FF directly proportional to GFR ?
Edited by medchick on 04/11/06 - 12:36 PM
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| doc179
Forum Guru
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the main factor determining GFR is the glomerular capillary hydrostatic pressure . when this pressure is increased as in efferent arteriolar constriction GFR would incease. and the filtration fraction also increases because of decreased RPF. but when afferent arteriole is constricted the capillary hydrostatic pressure in the glomerulus if decreased which decreases GFR and the dereased RPF increases FF.
hope this helps.
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| medchick
Forum Newbie
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thanks doc179 i understood that part...my mistake though i'm gonna edit the question so others will understand ....but i'm just confused on the flow and GFR aspect, since kaplan states: "Flow does have a small effect on GFR; an increase in flow will independently increase GFR" this is the part i don't understand.
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| doc179
Forum Guru
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Posts: 1,217
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I think I understand your Q . what you are saying is when RPF decreases FF increases and FF is directly proportional to GFR so GFR should also increase when RPF decreases.
But the formula is FF = GFR/RPF or GFR = FF * RPF
so you are right according to the both versions of the above formula that FF is directly proportional to the GFR. but the 2nd version of the same formula also says that GFR is directly proportional to RPF.
Conclusion
GFR increases when either of the 2 ( FF or RPF ) increases and it dcreases when either of the 2 decreases. and FF increases when GFR increases and decreases when RPF increases.
I dont know whether I have answered to your Q or confused you a lot more. I am sorry if I did confuse you. will think of a better way to tell you abt all this, if not lets wait for someone else to explain this in a better way.
Thanx for you patience.
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| doc179
Forum Guru
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its like in CVS ; MAP = CO * TPR when TPR decreases MAP decreases but CO increases to compensate. the same way when RPF decreases FF is increased and GFR is maintained.
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| medchick
Forum Newbie
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Ahhh...i get it....Thanks for the help doc179 
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| krsna
Forum Senior
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i thot i'ld add something here...
as i see it gfr is dependent on pressure in the capillaries and plasma flow....filtration fraction is dependent on gfr not the other way round...thats wat i think
correct me if wrong
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