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cathy4208
#1 Posted : Wednesday, July 01, 2015 12:22:48 AM
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Why does the body induce fainting when circulation to the brain is insufficient?

A) The CNS ischemic response increases the pressure of blood travelling to the brain
B) Brain response to this insufficiency cause brain BP to exceed ventricular BP
C) An active brain results in reduced metabolic need, allowing circulation to catch up
D) Fainting reduces the effect of gravity which reduces the blood pressure as blood flows from the heart to the brain

I was debating between A and D, and wasn't sure why A is wrong or why D is correct. The answer mentions that pressure is proportional to height and inversely proportional to depth, and fainting prevents blood pressure from dropping. Where should I assume the zero point in this scenario and why?
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heatherjudd
#2 Posted : Tuesday, July 07, 2015 5:58:02 PM
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Blood is a non-ideal fluid, and if we are talking about lack of blood in the brain we are really talking about the heart having trouble getting it there. If we could just increase the pressure (as in A) we wouldn't need to fall down, increasing pressure just helps push blood up. If you lower the head relative to the heart (and to everything) the heart doesn't need to work quite so hard to 'defy' gravity and pull the blood from your feet to your head. This means even as the heart does less work, more blood with naturally flow to the brain. And since no part of your head it very high above your heart when lying down, the heart need to excerpt much less pressure to get it there.

This question forces you to step a little outside physics and consider how the body works to move blood - a true MCAT style question.
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robinluo
#3 Posted : Sunday, June 12, 2016 4:29:44 PM
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The question stem is asking for a reason to explain why fainting is the "natural/right/beneficial" response to low brain circulation.

While A) may be a valid reason, it does not address fainting.

D) is the answer because it explains how fainting helps the brain recover from low brain circulation.

-Robin
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