Afferent Upstream versus Efferent Downstream Dilation! - USMLE Forums
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  #1  
Old 07-07-2012
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Vascular Afferent Upstream versus Efferent Downstream Dilation!

Can someone please help me with understanding this mechanism.
When the afferent is dilated there is an increase in RBF, when constricted there is a decrease in RBF

The same with the efferent arteriole, but why does the pressure decrease when the efferent is dilated and increase when the afferent is dilated?

I don't understand the upstream pressure decrease in a series if dilated and increase pressure downstream, what is downstream and up stream?

Thanks in advance
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Old 07-07-2012
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I'll try to explain this as simple as possible. But to be honest this might be something you want to do some extra reading for because of how important it is.

So basically Filtration Fraction= Glomerulo Filtration Rate divided by Renal plasma flow.

FF=GFR/RPF

and 1 more important formula:


GFR=(hydrostatic Pressure(within the glomerulus)-Hydrostatic pressure(bowmens space)) - ((Oncotic pressure (glomerulus) -oncotic pressure (bowmens space)

(If you don't know what those terms mean I suggest looking them up)



To keeping it very simple:
If you look at the formula just plug in some numbers. GFR=120 and RPF=600. Making FF = 0.2

So lets do an example of constricting the efferent artery.
So if you constrict the efferent artery the renal plasma flow would slow down. Along with that, the hydrostatic pressure within the glomulerlus increases! (its pushing against a constricted artery)
So throw in some numbers to help you visualize it.
GFR 150 AND RBF 400.
Thus 150/400 = .375

So now constricting the efferent artery we just increased filtration fraction.


Now just 1 more example for kicks.

Increasing the plasma proteins

Is there a change in renal plasma flow? No
Is there a change in GFR... YES a decrease
why?


With the increase in proteins within the glomeulus we just increased the oncotic pressure opposing the filtration fraction filtration (the second bold formula).

If understanding just those 2 formulas you should be able to knock out all the glomerulo dynamic questions outta the park.

I hope i was some help.
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  #3  
Old 07-07-2012
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Quote:
Originally Posted by ashoka1202 View Post
I'll try to explain this as simple as possible. But to be honest this might be something you want to do some extra reading for because of how important it is.

So basically Filtration Fraction= Glomerulo Filtration Rate divided by Renal plasma flow.

FF=GFR/RPF

and 1 more important formula:


GFR=(hydrostatic Pressure(within the glomerulus)-Hydrostatic pressure(bowmens space)) - ((Oncotic pressure (glomerulus) -oncotic pressure (bowmens space)

(If you don't know what those terms mean I suggest looking them up)



To keeping it very simple:
If you look at the formula just plug in some numbers. GFR=120 and RPF=600. Making FF = 0.2

So lets do an example of constricting the efferent artery.
So if you constrict the efferent artery the renal plasma flow would slow down. Along with that, the hydrostatic pressure within the glomulerlus increases! (its pushing against a constricted artery)
So throw in some numbers to help you visualize it.
GFR 150 AND RBF 400.
Thus 150/400 = .375

So now constricting the efferent artery we just increased filtration fraction.


Now just 1 more example for kicks.

Increasing the plasma proteins

Is there a change in renal plasma flow? No
Is there a change in GFR... YES a decrease
why?


With the increase in proteins within the glomeulus we just increased the oncotic pressure opposing the filtration fraction filtration (the second bold formula).

If understanding just those 2 formulas you should be able to knock out all the glomerulo dynamic questions outta the park.

I hope i was some help.
Thanks so much it makes a lot more sense now, I just have another question if you dont mind, how come the pressure decreases when we constrict the afferent arteriole? But rather increases when we dilate it?
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  #4  
Old 07-08-2012
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If u constrict the aff. arteriole, u limit the flow of blood going into the glomerular capillaries (i.e., downstream) and the pressure decreases. On dilating aff. arteriole, more blood flows through it and greater hydrostatic pressure is produced in glomerular capillaries.

Just be sure that the system is like a balloon (just endothelium lined glomerular capillaries) connected to two pipes, one on each side (aff. and eff. arterioles with muscule in their walls) and you are measuring pressure inside the balloon. Now imagine for yourself what happens to balloon by manipulating the two pipes (remember: dilating a vessel increases flow and pressure of blood downstream while decreasing the pressure upstream & vice versa; upstream is the part of the vessel from which blood is coming and downstream is that one to which it would go).
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