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Blood Pressure taken at different heights (No calculations involved)

  • #1

Homework Statement



When blood pressure is measured, why must the jacket be held at the level of the heart?
I don't quite understand the answer given.


The Attempt at a Solution



It's obvious that if the the height changes the pressure reading will change due to P=ρgΔh, thus giving a 'false' reading when it's blood pressure from the heart that we are wanting to know.
I thought that if the jacket if lower than the heart, then the pressure reading will be lower.
And if the jacket if higher than the heart, then the pressure reading will be higher

Just taking some random numbers here for an example,
P= ρgh
P= (1.05x10^3) x 9.8 x 2
P= 20580 Pascals compared to...

P= ρgh
P= (1.05x10^3) x 9.8 x 3
P= 30870 Pascals

However the answer in book says that if the jacket is lower, then blood pressure will be higher and vice versa without giving an explanation and I can't work out why this is. It doesn't seem logical to me. Can someone please explain?
 

Answers and Replies

  • #2
Now think about what would happen if you 1) had your arm hanging down by your side 2) stuck your arm straight up in the air.

In the first scenario, if you took your blood pressure it would be like having the "jacket" be below your heart level, and for the second scenario it is like having the "jacket" above your heart.
Also, the blood pressure I am assuming we are talking about is the pressure on the veins/arteries caused by the heart pumping blood.

Ok, now in scenario 1. If you can imagine the blood in your arm circulating. If you measure the blood pressure of your arm, you are not measuring only the pressure due to your heart pumping, but also the pressure of the blood above where you are measuring from (given by rho g h). This will make the pressure reading greater than what it truly is.

For scenario two, there is the pressure from your heart, but gravity is pulling blood from your arms to lower in your body, which causes lower pressure.

I hope that makes sense. Also, I have pretty much no biological knowledge, so don't take what I say for fact, just my guess.

Also, I think the equation normally reads P = - rho*g*delta h
 

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