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  #1  
Old 11-13-2011
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Kidney Difference Between Filtered Load and Freely Filtered

I am a bit confused by freely filtered in the glomerulus and filtered load.

If a substance is freely filtered in the glomerulus than does it mean that the concentration present in the plasma is the same present in the filtrate in BS? And if that's the case than why do you need to calculate the filtered load? I feel like I am missing something in my understanding.

Any help would be greatly appreciated.
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Old 11-13-2011
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Hey! The filtred load gives you information about the rate at which a substance is filtered. So if a substance is freely filtered than u know, that there is an equal concentration in both compartments. If you multiply it with the GFR (= filtered load) u get the rate(mg/min) at which the substance is filt. Is it helpful or isn't this the answer u were looking for?
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I am pretty sure I understand what you are trying to say but I just want to make sure I have it right.

So if substance X is present in the plasma at 3mg/ml the same concentrations will be present in the filtrate. By using the filtered load you can figure out how fast or slowly the substance is entering the glomerulus. Is that correct?

Thanks so much for your help.
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Yes exactly. This is how I understand it at least So lets continue your scenario: We multiply it with the GFR so: Freely filt. subst= 3mg/ml GFR=120 ml/min.
Fil.load= 360 mg/min. So concentraion in both compartments stays the same, but now u know per minute there is a total "transfer" of 360 mg. I hope it is clear to you know. Good luck with the studying.

by the way: I think this is an excellent question
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Thanks so much for your help. I tried looking for an explanation for so long yesterday!

Good luck studying to you as well.
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Old 11-13-2011
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Hello

thank you all for the explanation

I have a question

when we are dealing with these values of filtration fraction and GFR and filtration coefficient and net filtration pressure and all these stuff , what it the constant one of them that we all depend on in our calculation ????

I see that filtration fraction depends on GFR and renal plasma flow , and you cant know them directly , in guyton for example , he considers them as GFR = 120 , filtration fraction = 20% , RPF = 600 , and so on , but It is always talking about their values being changed normally and also in any abnormality or condition , like diabetes affects the permeability , hypertension affects the renal plasma flow , so what should I consider ???

another thing , could you explain the idea of physiology and GFR in two kidneys , and then when one kidney is removed, what happens and why ??? for all parameters, please ???

thank you very much
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(1) Filtration Fraction is defined as GFR/RPF.
(2) GFR is the volume of fluid per minute that is filtered out into Bowman's space.
(3) Net Filtration Pressure has to do with the four factors that are present in any capillary. They are capillary hydrostatic pressure, capillary oncotic pressure, hydrostatic pressure in Bowman's space, and oncotic pressure in Bowman's space. Now for the most part hydrostatic pressure and oncotic pressure in Bowman's space are negligible. That would change however in a condition such as obstructive uropathy where hydrostatic pressure in Bowman's space would increase which would therefore oppose filtration, thus decreasing GFR. Nephrotic syndrome as another example increases oncotic pressure in Bowman's space which favors filtration.

The way that I think of it is this: Look at what the Net Filtration Pressure, if it favors filtration that fluid will come out into Bowman's space. GFR tells you how much of the plasma is filtered out per minute. Filtration fraction therefore would change if GFR changes. However keep in mind it would also change depending on what the status of the arterioles is meaning are they vasoconstricted or vasodilated. Vasoconstriction decreases plasma flow while vasodilation increases plasma flow. So FF is dynamic and dependent on both GFR and RBF. So for example, if the afferent arteriole is constricted then that means that plasma flow will decrease. Furthermore, GFR would also be decrease because net filtration pressure decreases since not as much as coming into the capillary.

For kidney removal think of it this way:
If we remove one kidney GFR at first will be reduced by 50% because we have lost half of the total functioning nephrons we have. However over time the remaining kidney will compensate by increasing the individual GFR of each nephron. In this way, we only see a 25% reduction in GFR.

Hopefully this helps answer your question somewhat.
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Quote:
Originally Posted by woodyjr. View Post
(1) Filtration Fraction is defined as GFR/RPF.
(2) GFR is the volume of fluid per minute that is filtered out into Bowman's space.
(3) Net Filtration Pressure has to do with the four factors that are present in any capillary. They are capillary hydrostatic pressure, capillary oncotic pressure, hydrostatic pressure in Bowman's space, and oncotic pressure in Bowman's space. Now for the most part hydrostatic pressure and oncotic pressure in Bowman's space are negligible. That would change however in a condition such as obstructive uropathy where hydrostatic pressure in Bowman's space would increase which would therefore oppose filtration, thus decreasing GFR. Nephrotic syndrome as another example increases oncotic pressure in Bowman's space which favors filtration.

The way that I think of it is this: Look at what the Net Filtration Pressure, if it favors filtration that fluid will come out into Bowman's space. GFR tells you how much of the plasma is filtered out per minute. Filtration fraction therefore would change if GFR changes. However keep in mind it would also change depending on what the status of the arterioles is meaning are they vasoconstricted or vasodilated. Vasoconstriction decreases plasma flow while vasodilation increases plasma flow. So FF is dynamic and dependent on both GFR and RBF. So for example, if the afferent arteriole is constricted then that means that plasma flow will decrease. Furthermore, GFR would also be decrease because net filtration pressure decreases since not as much as coming into the capillary.

For kidney removal think of it this way:
If we remove one kidney GFR at first will be reduced by 50% because we have lost half of the total functioning nephrons we have. However over time the remaining kidney will compensate by increasing the individual GFR of each nephron. In this way, we only see a 25% reduction in GFR.

Hopefully this helps answer your question somewhat.
If the left kidney compensates for the lost one, the individual GFR increases, how can this result in the 25% reduction in GFR?

Thank you in advance for your further explanation!
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You see about a 25% reduction in GFR because the compensation never comes back to what it would have been if both kidneys were present. There is compensation but the compensation isn't complete. Hope that helps!
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Quote:
Originally Posted by woodyjr. View Post
You see about a 25% reduction in GFR because the compensation never comes back to what it would have been if both kidneys were present. There is compensation but the compensation isn't complete. Hope that helps!
Yes, the function cannot be fully compensated. But the GFR can still be 30% less than normal or about 10% less. Why is 25%?
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I don't think you should necessarily focus on the number per se. Because not every individual has the same GFR. Just know that the compensation isn't complete.

As far as where I got the percentage from: It was from a physiology lecture given by a prof. of ours. Its just something that has stuck with me.
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Quote:
Originally Posted by woodyjr. View Post
I don't think you should necessarily focus on the number per se. Because not every individual has the same GFR. Just know that the compensation isn't complete.

As far as where I got the percentage from: It was from a physiology lecture given by a prof. of ours. Its just something that has stuck with me.
Thank you woodyjr! I just want to verify I've got the correct understanding.
Thank you for your explanation!!
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Old 11-14-2011
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thanks for your reply

what you told me is what I really understood , but what I will consider now , is that they are all dependent on each other and I should not consider them as constant or normal, even the GFR isn't always 125ml/min .. always look at the condition of the forces and the diseases if present and judge based on them .. am i right ?

for the second question I did also knew the fact that it decreases to 25% , it is written in guyton , But I just wanted to know the physiological reason behind it .. and now as it is less than normal, will the urine output change ?
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Old 11-14-2011
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They might not give you the condition of the forces because that might make it too easy. Boohoo. But regardless this was a question I went through a week or so ago in UWorld. They had a presented a guy with chronic NSAIDs use. Then they asked for what would his kidney functions look like. There was other filler information but this was the gist.

So to solve the question you would have to think something like this to get the right answer: With chronic NSAID use you have a decrease in prostaglandin synthesis. That decrease in prostaglandin synthesis leaves angiotensin 2 as your primary molecuke responsible for your vascular status. AT2 cinstricts the efferent more than the afferent. So with that said you know that plasma flow will decrease. However renal function would be preserved because glomerular capillary hydrostatic pressure would increase. GFR would increase. This would then decrease serum BUN and increase creatinine clearance. However you need to keep in mind that this increased GFR will eventually lead to hyperfiltration injury which would then reverse these parameters.

So my point is if they won't necessary give you the forces or the GFR; you will have to infer it from the history given and go from there. You should know the general concept of how things will work and then apply them by doing questions.

For kidney removal the only physiological reason I know that GFR decreases is because you lose half of all your functioning neurons but the remaining neurons compensate by increasing there individual GFRs. I would assume that urine output would decrease slightly.

Hope this helps.
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Old 07-30-2015
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Could someone pls explain the difference betweem filtered load amd plasma clearance?Thankyou in advance
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