Renin Angiotensin Aldosterone Sysatem and Cardiac Output

[MudPhud20XX]

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Here is one of Kaplan physio q:

A drop in blood pressure would result in which of the following?
A. Arteriolar vasoconstriction
B. Decreased aldosterone release
C. Decreased extracellular fluid volume
D. Decreased renin release
E. Increased cardiac output

The answer is A and I did choose A.

But why not E? Won't Renin-angiotensin-aldostereon system also eventually increase CO since it basically increases H2O reaborption which will eventually increase venous return and increase CO?

 

[ChEMD]

It's not the best question, but I think it's referring to what is the immediate response to a drop in BP, and you have to compare it to the baseline levels before the drop occurred. So the drop in BP leads to increased sympathetic output, which leads to your increased arteriolar tone. It would also lead to increased cardiac contractility and HR, which would increase the cardiac output, but the cardiac output would overall still be lower than the baseline value (before the BP drop).

I don't think you need to think about the renin-angiotensin system at all, since (at least in my interpretation) it is referring to an acute decrease in BP.

 

[teotuf]

If you think about the question in terms of the equation I = V/R, or

CO = MAP/TPR, assuming right atrial pressure is negligible.

Then you can see that a drop in MAP is going to lead to a drop in CO. There can be compensatory changes, but there is no reason to think that a compensatory increase in CO will OVERSHOOT your baseline, or because once you go beyond the baseline, the stimulus for the compensatory change is gone. (i.e. pH of someone with metabolic acidosis even with respiratory compensation is not generally going to be basic, the reaction should not over compensate unless there is a defect somewhere)

 

[MudPhud20XX]

If you think about the question in terms of the equation I = V/R, or

CO = MAP/TPR, assuming right atrial pressure is negligible.

Then you can see that a drop in MAP is going to lead to a drop in CO. There can be compensatory changes, but there is no reason to think that a compensatory increase in CO will OVERSHOOT your baseline, or because once you go beyond the baseline, the stimulus for the compensatory change is gone. (i.e. pH of someone with metabolic acidosis even with respiratory compensation is not generally going to be basic, the reaction should not over compensate unless there is a defect somewhere)

Thanks guys. I just looked at Kaplan Physio and it says:

"This (Renin-Angiotensin-Aldosterone System) system regulates both resistance, via vasoconstriction, and cardiac output, via preload.