Control of Renal Plasma Flow (RPF) and Glomerular Filtration Rate (GFR) - USMLE Forums
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Old 05-17-2012
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Kidney Control of Renal Plasma Flow (RPF) and Glomerular Filtration Rate (GFR)

Control of Renal Plasma Flow (RPF) and Glomerular Filtration Rate (GFR)-gpf-vs-rpf.jpg
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The graph shows the effects of MAP on GFP and RPF. Which of the following best describes the MOA that maintains nearly constant GFR and RPF as the MAP increases over the range A and B.

The answer choices are the following:

A) Afferent constric and efferent dilation
B) Afferent dilation and efferent constric
C) Efferent constric
D) Efferent constric and afferent constric
E) Efferent dilation

I dont even know where to even start this off. I thought since the FF is staying the same I need something that either decreases or increases both GFR and RPF. Answers A and D meet that criteria but the answer was A. I dont know why.
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Old 05-17-2012
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I think when you have a drop in MAP the afferent will dilate and efferent will constrict to protect GFR.

And if MAP is too high, afferent automatically constricts and the efferent will dilate or remain unchanged to protect GFR. The reason being the distal tubules will receive an increased load of Na+ and urine volume due to the increased MAP, this will inhibit the MD cells from releasing prostaglinds to stimulate the release of renin from the JG cells. So without an increase in Renin-Angiotensin-Aldosterone, the efferent remains dilated.

Just what I seem to remember, it's been a while since I reviewd renal system.
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Old 05-17-2012
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I guess constriction of afferent arteriole is the primary mechanism. Here answer will be constriction of afferent and dilation of efferent.
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Default D

I'll go with answer D
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Quote:
Originally Posted by rmpatel3 View Post
Attachment 2563
click image to enlarge

The graph shows the effects of MAP on GFP and RPF. Which of the following best describes the MOA that maintains nearly constant GFR and RPF as the MAP increases over the range A and B.

The answer choices are the following:

A) Afferent constric and efferent dilation
B) Afferent dilation and efferent constric
C) Efferent constric
D) Efferent constric and afferent constric
E) Efferent dilation

I dont even know where to even start this off. I thought since the FF is staying the same I need something that either decreases or increases both GFR and RPF. Answers A and D meet that criteria but the answer was A. I dont know why.
Glomerular pressure is determined by three factors: mean arterial pressure (MAP) or perfusion pressure, and the relative resistance of both the afferent and efferent arterioles. The initial response to an increase in MAP is an increase in afferent arteriolar resistance (RA), preventing transmission of the elevated systemic pressure to the glomerular capillaries. Efferent arteriolar resistance (RE) also may decline. This decrease decompresses the glomerulus, helping to limit the increase in glomerular capillary hydraulic pressure (PGC), and maintains constant renal plasma flow.

http://www.kidneyatlas.org/book3/adk3-06.QXD.pdf
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Default correct answer

This is kaplan Q bank question..

correct answer is (A)

as MAP increases afferent arteriole constrict and efferent arteriole dilates to keep RPF and GFR in normal range

Afferent arteriole constrict by direct response to increase in pressure on macula densa and efferent arteriole dilates due to decreased secretion of renin and angiotensin II when macula densa pressure increases...
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