Kaplan FL7 C/P Question # 54 "Spoiler"

[betterfuture]

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Can someone explain why the answer is what it is, please. Thinking about this is hurting my head; I don't understand the explanation.

 

[To be MD]

I would think of it simply. It takes a certain amount of work for the heart to deliver a specific amount of blood at a certain pressure. Work = (1) amount of blood delivered + (2) pressure at which it's pumped out.

So, if hypertension is resulting in increased pressure with the same amount of blood, then (1) is staying the same while (2) is higher than it would otherwise be.

That means that more work is being done on average by the heart.

 

[betterfuture]

Thanks @To be MD! Kaplan gave me an answer that seemed to be contradicting to what I thought.

Increasing the pressure in the lungs increases the pressure that the heart is working against, while keeping volume the same. This will increase the amount of work that the heart has to do. In other words, the work can be calculated as the area under the curve for a PV graph. Since the pressure increases and volume remains the same, then work must increase.​

I think this is what you are saying. But to me it seemed like if the volume did not change, but pressure did there would be no work like in an isovolumetric system. But this obviously makes no sense because the heart has to do some work. Wouldn't it seem to be opposite?

 

[aldol16]

I think this is what you are saying. But to me it seemed like if the volume did not change, but pressure did there would be no work like in an isovolumetric system. But this obviously makes no sense because the heart has to do some work. Wouldn't it seem to be opposite?

It seems to me that Kaplan is convoluting several concepts. Pressure-volume work refers to work done by gases. The heart is not doing work on compressing/expanding a gas but rather mechanically pushing blood out of the ventricles. So use of a P-V graph doesn't make much sense here.

 

[betterfuture]

@aldol16 Yeah. I didn't think the answer made sense because no volume change means no work. Also, as you stated, the PV diagrams applies to gases because they are compressible unlike liquids, and thus, are able to do work. So basically if this question applied to gases, would the answer be 'No work is done', like originally what I had picked as the answer?

 

[aldol16]

@aldol16 Yeah. I didn't think the answer made sense because no volume change means no work. Also, as you stated, the PV diagrams applies to gases because they are compressible unlike liquids, and thus, are able to do work. So basically if this question applied to gases, would the answer be 'No work is done', like originally what I had picked as the answer?

You mean if there's a gas and there's no volume change? If it's a gas and there's no volume change, it's impossible to do any pressure-volume work by definition.

 

[betterfuture]

@aldol16 Quick question. Sorry to bring this post back up. But PV diagrams apply to gases only right? Is there a case with PV diagrams for liquids?

 

[aldol16]

@aldol16 Quick question. Sorry to bring this post back up. But PV diagrams apply to gases only right? Is there a case with PV diagrams for liquids?

Yes for gases. Not really for liquids. That's because volume essentially does not vary with pressure for liquids. For instance, if you're standing in a vacuum with a container of gas and open the container, the gas will expand to fill the room. If you do the same with a liquid, the liquid does not expand at the macroscale level.

 

[betterfuture]

So PV diagrams are non-applicable with cardiovascular physiology i.e. stroke work (work done by the ventricle to eject a volume of blood aka stroke volume into the aorta, a diagram of ventricular pressure vs. volume)? I am just asking because there was some confusion with another post I read elsewhere that stated that PV diagrams are applicable in this case. It would seem this type of work is mechanical work, as you said before, or what?

 

[aldol16]

So PV diagrams are non-applicable with cardiovascular physiology i.e. stroke work (work done by the ventricle to eject a volume of blood aka stroke volume into the aorta, a diagram of ventricular pressure vs. volume)? I am just asking because there was some confusion with another post I read elsewhere that stated that PV diagrams are applicable in this case. It would seem this type of work is mechanical work, as you said before, or what?

I'm not sure what the other post is referring to because I haven't seen it but in general, you can plot pressure vs. volume for anything but it doesn't mean it will be meaningful. PV work applies only to gases because they are the only ones that change volume to do work. A liquid does not change volume to do work - otherwise, liquids would not have clearly defined boundaries.

 

[betterfuture]

Okay thanks! I will mention your quotes in the post so maybe they can understand when PV diagrams are used, if you don't mind!