RBF and RPF

[aspiringmd1015]

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if the clearance of PAH gives you plasma flow, adn to find out renal blood flow, you take the RPF/1-HCT. But when you get the renal plasma flow, shouldnt the absence of erythrocytes already be accounted for? as its already plasma?

 

[dorster]

Im a little confused as to what you're asking, but the absence of erythrocytes from renal plasma is accounted for thats why its the equation is 1-HCT. HCT is a percentage which is usually around 50% so that means lets say normal RPF is 600ml/min then that means that RBF = 600/(1-0.5) which means RBF = 600/0.5 = 1200. so RBF is much larger than RPF. Im not sure if that answers your question but basically RBF includes both plasma and RBCs so in order to find RPF you have to subtract out the hematocrit.

 

[aspiringmd1015]

it makes sense, but if we already have the RPF on the numerator, like 600, dividing it by 1-HCT would only again be subtracting the RBC conc out. and then youre basically dviding the RPF/plasma?

 

[aspiringmd1015]

side note: 2 questions, dont want to make another thread, 1 the contraction alkalosis in CF, i understand you lose Na and h20 into the cells bc of the dysfunction CFTR, so the K/H wasting would be due to activatoin of the RAAS axis right?

 

[zhopv10]

it makes sense, but if we already have the RPF on the numerator, like 600, dividing it by 1-HCT would only again be subtracting the RBC conc out. and then youre basically dviding the RPF/plasma?

Sounds like there is a little confusion on the math, the 1-hematocrit is generating a multiplier if you will. So what you are doing is saying "I know plasma is x% of the actual blood volume (let's say 50% for our thought experiment). So plasma volume is blood volume * X. So then if I have plasma volume and want blood volume I do the opposite: plasma volume /X.

So if I know my flow of plasma but I want to calculate the actual blood flow I need to "undo" the equation. I.e if my blood is 50% RBC and 50% plasma. And I know that the plasma flow volume was say 600 (like the above examples) then I would I would say well I know my value is .5 or 50% of the total blood volume so how do I work backwards to get blood volume. I would multiple by 2. Mathematically, to get plasma volume I would multiply total blood volume by .5 (1/2) so how would I reverse that to go back to blood volume? I would DIVIDE the plasma volume I found by (1/2 [the plasma percentage]) which is the same as multiplying by two.

So what is all this saying? You divide RPF by 1-hematocrit because 1-hematocrit is the percentage of the blood volume that the plasma is (what you would multiply RBF by to get RPF, and you do the opposite of multiplication, you divide by that number.

Sorry it's so round about but hopefully makes sense, it would be easier to draw out I think and work through the equation. Going both ways through the equation helps I.e you can take RBF X (1-hematocrit) to approx. RPF and the. You can do the opposite and take RPF/(1-hematocrit) and get the RBF again.


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[worldbeater]

Look at the World question associated with it, this was literally the hardest question in the bank for me. I don't have the ID#, but I remember writing it out on a piece of paper and staring at it because it wasn't making any sense to me. The explanation is really good though after you comb through it. It's not going to go deeper that that, so don't stress out about it.

 

[dorster]

side note: 2 questions, dont want to make another thread, 1 the contraction alkalosis in CF, i understand you lose Na and h20 into the cells bc of the dysfunction CFTR, so the K/H wasting would be due to activatoin of the RAAS axis right?

Yes I think in any type of volume contraction causing alkalosis you will also have activation of the RAAS system to try and bring the blood pressure up and make up for the volume lost. So since aldosterone is working on the Na/K+ exchange channel of the principal cells in the collecting duct you will have Na+ retained in the blood and in doing that K+ is lost. Also aldosterone causes H+ excretion from the alpha-intercalated cells through a separate channel. There might be another mechanism going on in the cells as well but thats whats happening in the kidney.

 

[aspiringmd1015]

i see what youre saying, i guess my issue is the 1-HCT part, where i keep trying ot make it equal to plasma. i guess the issue is Plasma and plasma flow are different entities.

 

[worldbeater]

If anybody just wanted the basics for this type of question and is lost, it is as follows:
Renal blood flow (afferent artery)-> Creatinine, Inulin
Renal plasma flow (efferent artery) -> BUN, PAH

In between the afferent artery and efferent artery is your GFR. Filtration fraction is your GFR/RPF. Depending on if your numerator or denominator is higher, that is the direction your filtration fraction will go in.

 

[worldbeater]

Yes I think in any type of volume contraction causing alkalosis

Yeah, any type of low volume state is going to cause metabolic alkalosis (World Q)

 

[zhopv10]

I think it is more one is plasma flow the other one is PERCENT plasma. It's just a fraction. Remember that you have your RPF and then this percentage that you can use to change RPF to RBF and back again


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