Heart Force Exertion in Diastole

[SaintJude]

Pseudo-discrete question from Kaplan's Fluid & Solid Topical Test:

Why is diastolic blood pressure murch lower than systolic blood pressure ? (Note: A typical systole/diastole reading is mmHg is 120/80)

A. Because the heart exerts more force on the blood during diastole.
B. Because the heart exerts no force on the blood during diastole.
C. Because the radii of the blood vessels increase during diastole, while the force exerted by the heart on the blood remains the same.
D. Because the radii of the blood vessels decrease during diastole, while the force exerted by the heart on the blood remains the same.

The answer is hidden in white, highlight: B.

Q: So, is the pressure in diastole then entirely due to constriction of the arterioles and arteries?

About Choice C...is the part about radii true? What happens to the radii of blood vessels during diastole?

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[theseeker4]

Pseudo-discrete question from Kaplan's Fluid & Solid Topical Test:

Why is diastolic blood pressure murch lower than systolic blood pressure ? (Note: A typical systole/diastole reading is mmHg is 120/80)

A. Because the heart exerts more force on the blood during diastole.
B. Because the heart exerts no force on the blood during diastole.
C. Because the radii of the blood vessels increase during diastole, while the force exerted by the heart on the blood remains the same.
D. Because the radii of the blood vessels decrease during diastole, while the force exerted by the heart on the blood remains the same.

The answer is hidden in white, highlight: B.

Q: So, is the pressure in diastole then entirely due to constriction of the arterioles and arteries?

Residual pressure from the previous pump of the heart, since the circulatory system is (for practical purposes) a closed system, the lack of force by the heart doesn't mean the pressure goes instantly to 0. The pressure bleeds down (in your example to 80mmHg) as the blood flows throughout the system until the next "pump" of the heart.

 

[SaintJude]

Hmm, I see. About Choice C...is the part about radii true? What happens to the radii of blood vessels during diastole?

 

[theseeker4]

Hmm, I see. About Choice C...is the part about radii true? What happens to the radii of blood vessels during diastole?

That I am not positive about, and maybe the constriction of vessels has something to do with diastolic pressure, but at least a large part of it has to do with the fact that blood is pumping into a closed system of vessels rather than simply being pumped out of the aorta into empty space.

 

[MedPR]

Hmm, I see. About Choice C...is the part about radii true? What happens to the radii of blood vessels during diastole?

It's not true. Systole/diastole do not directly change the radii of blood vessels. Systole is just contraction of the ventricles and of greater interest the left ventricle. Increased blood flowing through it might stretch the vessels a bit, but I don't think that's what the answer choices are getting at.

I didn't know that B was true, but I could rule out all the other answers, so I guess it must be.

The pressure in diastole is due to there being something in the vessels. Remember, we just call it "blood pressure," though technically it is just hydrostatic pressure.

 

[chiddler]

It's not true. Systole/diastole do not directly change the radii of blood vessels. Systole is just contraction of the ventricles and of greater interest the left ventricle. Increased blood flowing through it might stretch the vessels a bit, but I don't think that's what the answer choices are getting at.

I didn't know that B was true, but I could rule out all the other answers, so I guess it must be.

The pressure in diastole is due to there being something in the vessels. Remember, we just call it "blood pressure," though technically it is just hydrostatic pressure.

what about the stretching of the aorta during systole?

 

[MedPR]

what about the stretching of the aorta during systole?

You don't measure BP at the aorta, so it shouldn't matter.

Well, I just reread my original post and I can see how it is misleading, since I kind of made a mistake.

Systole will indirectly increase blood vessel diameter by pumping more blood through it. Since the vessels have the ability to stretch, increased volume will make them stretch a bit. Diastole, however, does not really involve constriction even though the vessels are smaller relative to during systole. In other words, their diameter decreases because there is less blood stretching them, not because some stimulus causes vasoconstriction.

 

[SaintJude]

Took this question to the USMLE forum (oops)

2 med school students...

blood vessels radii should have no (or negligible) change during diastole or systole. Usually some other factors (drugs, pathology) change the radii.

 

[MedPR]

Took this question to the USMLE forum (oops)

2 med school students...

blood vessels radii should have no (or negligible) change during diastole or systole. Usually some other factors (drugs, pathology) change the radii.

Indeed. I recently covered some of this in my physio class. Imagine how much stress your vessels would be under if they were constantly changing diameter in some appreciable amount.

 

[SaintJude]

Yeah, I even incurred a warning from SDN for my investigation... no cross-posting anymore for me (but I had to know!)

 

[MedPR]

A warning for what? Asking a non USMLE question in the USMLE forum? If so, that's dumb. It is still a topic tested on the USMLE.