Blood Pressure

[ieatshrimp24]

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This was a question from a TPR source.

Q: When the aorta diameter narrows, what happens?
A: Decrease in pressure in the lower limbs.

Can anyone provide an explanation to this answer? I thought constriction of the aorta would increase the pressure in the systemic circuit.

My only reasoning would be that a decrease in the diameter of the aorta results in increased resistance which leads to decreased cardiac output which results in decreased blood pressure.

Any explanations? Thanks.

 

[sazerac]

Yeah, your second explanation about the increased resistance is what they wanted. It is kind of a bogus question, I mean was the aorta full of blood when it suddenly shrank, or what?

If you understand the concept, I wouldn't worry about it too much. The actual MCAT is much clearer with their questions.

 

[el_duderino]

One helpful thing to do in this sort of situation is to take it to the extreme.

What would happen if the aortic diameter were decreased to 0.1 mm? You have the heart pumping against a pipe 0.1 mm wide. You would not be able to get much pressure across that small of a hole, would you?

Keep in mind that you'll be measuring BP at some distal artery, such as the brachial. In the case of aortic narrowing, you might well be generating higher pressure in the heart but you will not be able to generate as much pressure distally.

 

[ElectricNoogie]

This was a question from a TPR source.

Q: When the aorta diameter narrows, what happens?
A: Decrease in pressure in the lower limbs.

Can anyone provide an explanation to this answer? I thought constriction of the aorta would increase the pressure in the systemic circuit.

My only reasoning would be that a decrease in the diameter of the aorta results in increased resistance which leads to decreased cardiac output which results in decreased blood pressure.

Any explanations? Thanks.

Think about it this way (though again they have over idealized the CV system), if the aorta is narrowed, barring any typical physiological compensations by the heart (that's typically too complex for the MCAT, unless they bring it up in the passage) that means less blood is now passing through the aorta. This would cause a drop in pressure in any downstream vessels.

In real life, when aortic stenosis (narrowing) occurs, the left side of the heart will pump harder to compensate, raising pressure in the left ventricle (which slowly works its way upstream), cause muscular hypertrophy to occur as the heart tries to maintain constant systemic BP. Shortness of breath is a common symptom. It reflects the cardiac muscle's failure to compensate for the extreme pressure load of aortic stenosis. Shortness of breath is caused by increased pressure in the blood vessels of the lung due to the increased pressure required to fill the left ventricle. Initially, shortness of breath occurs only during activity but as the stenosis worsens, shortness of breath occurs at rest. Eventually the heart fails.