Prep for USMLEPrep for USMLE
         Forum      |     Resources New Posts   |   Register   |   Login





 GFR question?Please help!  



Post Reply  
  • 0/5
  • 1
  • 2
  • 3
  • 4
  • 5


Author21 Posts
  #1

Hi there,
Ok this is renal physiology question,it souns so simple but I dont know where I am getting all confused-
Ok here is the question.If the afferent artery resistance decreases more than that of the efferent,then what changes do you expect in Filtration fraction,RBF and GFR? :? :?:
Look in this ok we are expecting RBF to increase but at the same time GFR also increases but then why would the filtration fraction decrease?
I am going nuts :x ,I would really appreciate if any one can help me.
Thanks smiling face




  #2

Please help!


  #3

think what would happen to the time available for filteration. does this help?


  #4

Hey thanks mani,
but you know what in the explanation it says that the rise in GFR is less as compared to the fall in RPF thats why FF decreases.
2.they also mention that if the efferent arteriole were to have lesser resisitance than the afferent then FF would increse.Can you please explain.
Thanks once again!
May God bless you!


  #5

FF=GFR/RPF
if both increased but the rate of inc RPF is more than GFR then the FF will be decreased.

if afferent a. dilatation and eff const--> GFR max and RPF min so the FF will be highest

the opposit will give u lowest FF,

between the above two option we have either both aff and eff dilated or both of them constricted, FF fraction depend on the grade of dilatation or constriction between the two arteries whether infavour of incr GFR(or FF) or infavour of reduced GFR(orFF).

plasma osmolality is another factor influences the equation

hope this help


  #6

"Ria" wrote:
Hey thanks mani,
but you know what in the explanation it says that the rise in GFR is less as compared to the fall in RPF thats why FF decreases.
2.they also mention that if the efferent arteriole were to have lesser resisitance than the afferent then FF would increse.Can you please explain.
Thanks once again!
May God bless you!


Not sure about ur second point. but possibly by decreasing RPF ONLY(GFR constant or minimally decreased) which lead to increase FF,

another interpretation, as mani mentioned , decreased RPF lead to increased filtration time and lead to inc FF


  #7

hiwa, i didnt mean to say that FF will increase. Rather i meant that when RBF will increase time available for filteration will decrease (due to rapid flow) and hence FF will decrease. i remember i.too, saw this question but couldnt make out why increase in GFR will be less than increase in RBF


  #8

if resistance of afferent is decreasing more than dat of efferent===>afferent is dilated more than the eff(bcoz Resitance is inversely related to the radius^4)
RBF will increase bcoz aff is dilated AND efferent arteriole is constricted(as compared to afferent ) -->stimulate renin AT sys--->further inc in RBF

GFR is increased bcoz time available fr filtration will increase and aff is dilated
bcoz inc in RBF> Inc in GFR
FF=GFR/RBF===>FF will decrease


  #9

Thanks all of you guys,I think mash explains it pretty well.Now I get it,but wont you guys say that this is a preety tough question,cause I was really discouraged as I wasnt even thinking on the Rennin activation system.Anyways thanks for the help.


  #10

thanx mash, i get it


  #11

I think in addition to what Mash is saying I think its a good idea to at least remember this chart and the opposite.

GFR RBF FF

constriction decrease decrease no change
of afferent

constriction increase decrease increase
of efferent

increased
plasma decrease no change decrease
protein


Sorry couldn't get the chart format.


  #12

i was wondering why y'all say that increased time for filtration leads to increased GFR? GFR is a rate, the unit is volume/time (e.g. ml/min). this means if you incr time, u incr volume too and rate won't change. actually the main factor determing GFR is capillary pressure, 2nd factor is blood flow. also, is renin secreted by efferent arteriole? my long-time impression is JG cells in afferent a. instead.

any advice?

for the Q itself, I think FF decr becoz the resistance for renal blood flow is reduced, and blood flow rate is faster. just think of it as overperfusion, too much blood to filter and no time to do it, although the absolute amount of filterred blood incr, the proportion decr.


  #13

u r right dat JG cells r in AFF arteriole and they monitor pressure there

P=QR
if resistance in affferent arteiole is decreasing then press will also decrease--->stimulate renin AT sys--->inc renal bld flow

2) aff is dilated more than the efferent --->hydrostatic press is more in the glomerular cap--->promote filtration--->so, GFR will be more
(main factor dat determines GFR is glo cap press)


  #14

yes tongu, i agree with u. this is what i said in my post above, agree iwth mash,s explanation as well


  #15

Thanks guys even I was wondering that JG appratus determines pressure in afferent arteriole.Anyways so let me get this thing straight what you all mean is there is decrease in resisntance in afferent arteriole-----> dilatation of the afferent art---->Increase bld flow(RPF)
2.Since there is increase in RPF----->increase in capp pressure therefore GFR also increases
3. FF=GFR/ RPF
but here I am back to my original quest : WHY DOES RPF INCREASE MORE THAN GFR?
I am sorry I thought I got it but I just dont understand.PLEASE EXPLAIN!
Maybe there is some point that Iam missing,but you guys can you clear this thing up please.


  #16

Ria,

I think mani and I had answered ur Q already. the key factor in determining FF is blood flow. but to help u figure it out easier, try this.

think about the extreme condition, blood flow incr a big deal and filtration already reached its upper limit. in this situation, when u incr RPF, there is no change in GFR (saturated), but whats increased is the 'Unfiltered plasma flow', which equals RPF-GFR. so its clear under this siutation: no change in GFR, great incr in RPF and unfiltered plasma, therefore decr in FF.


  #17

Thanks tonguo,
I get it now,thanks for making it all that simple for me.anything I can help you with please do ask,thanks once again


  #18

thank u, Ria and everyone who posted, for bringing up this challenging topic. could u take a look at my post about countercurrent multiplication? that concept drove me nuts.


  #19

if afferent artery resistance decrease :arrow: afferent artery be come dilates :arrow: GFR inc, RPF inc, FF no change
efferent artery resistance decrease :arrow: GFR dec, RPF inc, FF dec
in your Q; afferent artery resistance decrease more than that of the efferent then net change of FF decrease :?:
please correct me if i'm wrong


  #20

No, tdr.. ur first equation is wrong..
If afferent arteriole dilates, RPF increases, GFR increases but FF decreases.. I think tonguo explained it pretty well, Read his post once again..
Just remember, whenever RPF increases, the time that the plasma spends in the glomerular capillary network decreases leading to a decrease in FF





Bookmark and Share



This thread is closed, so you cannot post a reply.



Login or Register to post messages








show Similar forum topics

GFR
GFR vs Reabsorption rate
renal blood flow and GFR
show Related resources

Conrad Fischer's Internal Medicine Question Book








Advertise | Support | Premium | Contact