MCAT assessment Physics 79: fluid mechanics

[LuminousTruth]

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In a healthy person standing at rest, how does the arterial BP compare in the arm vs the leg? The pressure in the leg is:

A) Lower, because blood rate is less
B) Lower, because viscous flow resistance causes pressure loss
C) Same, because viscous pressure loss compensates for hydrostatic pressure increase
D) Greater, because the column between the arms and legs have a greater hydrostatic pressure

The answer was "D"

At first, I thought it was B, since the pressure of the blood decreases as it leaves the aorta and towards the extremities of the body. That's why when blood comes back, the venous pressure is low. And in order for this to happen, there would be some resistance causing the pressure loss.

Can someone explain why my reasoning is incorrect?

 

[rickrun]

nvm I was thinking artery vs vein, way off.

 

[Eastfolding]

Remember that this question is part of the physics section of the test. Don't use any extraneous biology knowledge to answer this question.

Break the question down. It's asking which point on the body has a higher blood pressure. The only major different between the arm and the leg is that the arm rests at a greater height than the leg.

What do you know about height and fluid pressure?

 

[sciencebooks]

Remember that this question is part of the physics section of the test. Don't use any extraneous biology knowledge to answer this question.

Break the question down. It's asking which point on the body has a higher blood pressure. The only major different between the arm and the leg is that the arm rests at a greater height than the leg.

What do you know about height and fluid pressure?

👍 Also how I solved it. Good call on the whole using-section-relevant-information thing -- *so* true.

 

[Trayshawn]

This is a good question, and I'm stumped!

Can someone please explain this?

 

[Trayshawn]

Hmm I thought about it a bit more and I think I got it.

Gauge pressure increases with depth since there is more fluid on top [P(gauge) = rho*gh].
Legs are much lower than the arms, so they should have a much higher pressure.

I just don't get what "column between the arms and legs have a higher hydrostatic pressure" means.
The answer should include something about the legs and something about the arms, to differentiate them. Not something connecting them (which is apparently some random column).

 

[SSerenity]

Bernoulli's Principle states that:

StaticPressure + Kinetic Pressure + Hydrostatic Pressure = Constant (Along a Streamline)

I made the following assumptions:
Static Pressure = Constant

Kinetic Pressure = Varies with velocity which varies with Hydrostatic Pressure.

Hydrostatic Pressure = (rho)gh


So as we descend to the legs, our hydrostatic pressure term INCREASES.
BUT wont this in turn make our KINETIC PRESSURE term DECREASE? If all these terms are to sum up to a constant.

I know Bernoulli's equation applies to a 'single streamline', but what does this really mean? Isn't our Arteries in our Arms and Legs part of a single streamline?

 

[The_Sunny_Doc]

Bernoulli's Principle states that:

StaticPressure + Kinetic Pressure + Hydrostatic Pressure = Constant (Along a Streamline)

I made the following assumptions:
Static Pressure = Constant

Kinetic Pressure = Varies with velocity which varies with Hydrostatic Pressure.

Hydrostatic Pressure = (rho)gh


So as we descend to the legs, our hydrostatic pressure term INCREASES.
BUT wont this in turn make our KINETIC PRESSURE term DECREASE? If all these terms are to sum up to a constant.

I know Bernoulli's equation applies to a 'single streamline', but what does this really mean? Isn't our Arteries in our Arms and Legs part of a single streamline?

You're exactly right about how descending within the arm/leg fluid column causes a hydrostatic pressure increase. Remember to include the PE of the fluid due to gravity. 🙂 That would be decreasing as h decreases (you get closer to the ground, if you set the ground at your reference value).

Just remember that no matter the size or shape of the container, the depth from the surface of fluid is what tells you how much hydrostatic pressure there is on a layer of fluid. So if you picture your whole body being a human-shaped balloon filled with H2O, your legs are at a lower height, so there are a lot more "layers" of fluid bearing down to create pressure in your toes than in your fingertips.

Bernoulli's equation doesn't work with pumps, btw. The fluid not doing work, or not having work done on it, is one of the stipulations that comes with using the equation. Your heart would qualify as a pump. So, we can't say much about the kinetic energy/speed of blood using Bernoulli's equation. EK has an awesome diagram of how the hydrostatic pressure, PE, and KE change with each other on pg 87.

 

[LookingForASpot]

I don't have time to read all of this. But in summary, the answer is D yeah??

 

[Cawolf]

Sorry you didn't have time to read all of this 2+ year old thread.

Reread the first post where he says the answer. . .

 

[LookingForASpot]

haha okay tricky and slick. NICE