aamc 6 bio question re: GFR, blood pressure

[crazymedgirl]

Ok, paraphrasing, the question is basically: if you have one guy with significantly higher blood pressure (Steve) than the other (Joe), what differences in urinary system function would be expected between the two?

The answers:
a) Joe's GFR would increase more than Steve's
b) Steve's reabsorption rate of glomerular filtrate by the peritubular capillaries would be LOWER than Joe's
c) Steve's reabsorption rate of glomerular filtrate by the peritubular capillaries would be HIGHER than Joe's
d) Steve's GFR would increase more than Joe's

Ok the correct answer is D. The explanation provided includes the statements that "arterial blood pressure would not be a factor in reabsorption, so B and C do not address the question...the process of glomerular filtration depends on the hydrostatic pressure in the capillaries of the glomerulus to force fluid across the capillary walls of the glomerulus..this would be higher in Steve than in Joe."

However...I really do not understand why B is not the right answer? I thought that GFR actually doesn't change much except in extreme physiological conditions. I thought the first thing that would change in response to arterial BP changes WOULD be reabsorption! Wouldn't the whole angiotensin II thing come into play and to reduce BP, it would reduce blood volume by reabsorbing less and excreting more Na, water, etc.???

The statement "arterial BP would not be a factor in reabsorption" is really confusing me.

Any thoughts? (Sorry for the long post.)

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[loveoforganic]

GFR and blood pressure are directly related. You increase blood pressure, you increase the pressure the blood exerts on the glomerulus. The more pressure you have, the more filtrate can squeeze through. More filtrate per unit time, higher GFR.

 

[crazymedgirl]

Ok, thank you. I guess I didn't realize that relationship well enough.

I would still appreciate if someone could comment on my confusion regarding blood pressure affecting reabsorption: it DOES affect it, right?

 

[Isoprop]

It's been a while and I don't have my renal notes in front of me, but generally, reabsoption is not affected by GFR or BP.

 

[zmatrix]

Reabsorption rates of glomerlular filtrate (everything except H2O) is dependent upon active transport and facilitated diffusion.

Since we're not messing with transport proteins, the active transport side is unaffected.

Facilitated diffusion is determined by the concentration gradient between renal circulation and the lumen. Since arterial pressure does not affect the concentration gradient between the two, it is also unaffected.

 

[crazymedgirl]

Ok, maybe I'm not being clear...
This is what I thought was true:

If BP rises, body needs to bring it back down, so activates the renin/angiotensin pathway which results in release of aldosterone. Aldosterone acts on the distal tubule to increase reabsorption and fluid retention, thereby increasing blood volume and thus blood pressure.

Am I completely off in this line of thinking??

thanks for all the reponses.

 

[zmatrix]

If BP rises, body needs to bring it back down, so activates the renin/angiotensin pathway which results in release of aldosterone. Aldosterone acts on the distal tubule to increase reabsorption and fluid retention, thereby increasing blood volume and thus blood pressure.

-First, to clarify for others, the Renin-Angiotensin pathway is a response to a DECREASE in blood pressure which acts to INCREASE blood pressure.

-Also, although aldosterone promotes the reabsorption of Na+, it also promotes the excretion of K+.

These are key points which are commonly found in MCAT so I would recommend you review and gain a thorough understanding of the Renin-Angiotensin pathway.


That aside, you're simply over-thinking the problem. As with any abnormal condition, there will be a plethora of responses from the human body. Unless the problem specifies a particular response, then you should assume the problem is asking: "Which of the answers is a result of the abnormal condition?"

 

[crazymedgirl]

Ok, thanks very much, that helps😉

I did know that aldosterone acts to raise BP, not lower it, I wasn't thinking while writing my last post.

Thanks again.