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cerato
8th April 2011 - 02:03 PM
I have been given this question and with very little knowledge of physics and a book and flex studies that I am not finding very explanitory. I am not sure I am on the correct path as the vlaues I have seem very large. I would appreciate any help/assistance anyone could give.
Question
You measure the blood pressure at the bicep and find that it is 140 mmHg systolic and 80mmHg diastolic. What would you expect the blood pressure to be at the top of the head given that this point is 45cm above the measurement point? Remember thisis a gauge pressure and give your answers in both mmHg and kPa. (The density of blood is about 1060 kgm^-3)

This is what I have done but I am so unsure on what I am doing.

1. Converted the systolic and diastolic blood pressure into Pa.
Systolic Blood pressure= 140mmHg x 133.322Pa/1mmHg= 18665 Pa
Diastolic Blood pressure= 80mmHg x 133.322Pa/1mmHg= 10665.76Pa

2. Due to being gauge pressure I then converted to actual pressure. (this is something I am very unsure if it is needed)
P1 actual systolic pressure= 18665Pa + 101325Pa = 119990Pa
P1 actual diastolic pressure= 10665.76Pa + 101325Pa = 111990.76Pa

3. Due to the change in hieght I named the top of the head P2.
P2 - P1=pg h2-h1
P2=P1+pgh
P2 systolic = 119990Pa + (1060kgm^-3 x 9.81 m s^-2 x 45m) = 587927Pa
P2 diastolic = 111990.76 + ( 1060kgm^-3 x 9.81 m s^-2 x 45m) = 579928Pa

4. I then converted the P2 into kPa.
P2 systolic kPa = 587927Pa x .1kPa/100Pa = 587.9kPa
P2 diastolic KPa = 579928Pa x .1kPa/100Pa = 579.9kPa

5. I then converted the P2 into mmHg. (this seems very large)
P2 systolic mmHg = 587927Pa x 1mmHg/133.322Pa = 4410mmHg
P2 diastolic mmHg = 579928Pa x 1mmHg/133.322Pa=4350mmHg

I have spent a lot of time trying to work this out. If you can please assist me I would be very greatful.


bukh
8th April 2011 - 05:25 PM
QUOTE (cerato+Apr 8 2011, 02:03 PM)
I am not sure I am on the correct path -

You are asking for a plain physical answer to a physiological problem.

Blood pressure is as you rightly say typically being measured on the biceps - by an acoustic method where systolic pressure is assessed when the upper systolic intravasal pressure wave can open the artery for blood-flow, and the diastolic pressure is when the lower systolic pressure wave cannot any longer open the artery. During the entire cycle the pressure is dynamically changing - so one cannot meaningfully talk about any well defined intravasal pressures - but only talk about a dynamic variation over the time of the period between two heart beats.

Intravasal pressure is something specific to the very site of making the measurement - and pressure is a combination of diameter of artery, flow-rate and also elasticity of the artery - and all these parameters are dynamically changing all the time - as is the pressure.

By asking the pressure in the "top of the head" is equally meaningless - because the intravasal pressure is being determined by which vessel is being selected for measurement and at which time. And a mean pressure says nothing about pressure except that it is an integration over time - and to compare one pressure-profile with another is equally meaningless.

But I feel confident that there are several highly competent people in this Forum that can calculate a physical answer to your question.
cerato
8th April 2011 - 10:20 PM
QUOTE (bukh+Apr 8 2011, 05:25 PM)
You are asking for a plain physical answer to a physiological problem.

Blood pressure is as you rightly say typically being measured on the biceps - by an acoustic method where systolic pressure is assessed when the upper systolic intravasal pressure wave can open the artery for blood-flow, and the diastolic pressure is when the lower systolic pressure wave cannot any longer open the artery. During the entire cycle the pressure is dynamically changing - so one cannot meaningfully talk about any well defined intravasal pressures - but only talk about a dynamic variation over the time of the period between two heart beats.

Intravasal pressure is something specific to the very site of making the measurement - and pressure is a combination of diameter of artery, flow-rate and also elasticity of the artery - and all these parameters are dynamically changing all the time - as is the pressure.

By asking the pressure in the "top of the head" is equally meaningless - because the intravasal pressure is being determined by which vessel is being selected for measurement and at which time. And a mean pressure says nothing about pressure except that it is an integration over time - and to compare one pressure-profile with another is equally meaningless.

But I feel confident that there are several highly competent people in this Forum that can calculate a physical answer to your question.

Thank you for that explanation. If you take the physiological sense out of the problem can you work it out in a physical sense using the formulas?

This question, which is in the text book has caused me a lot of concern. So all assistance is welcomed.
cerato
9th April 2011 - 12:17 PM
QUOTE (cerato+Apr 8 2011, 10:20 PM)
Thank you for that explanation. If you take the physiological sense out of the problem can you work it out in a physical sense using the formulas?

This question, which is in the text book has caused me a lot of concern. So all assistance is welcomed.

I just realised I had made a very big error in writing down 45m instead of 45cm. I however am Unsure if I am any closer to the correct formulas..

You measure the blood pressure at the bicep and find that it is 140 mmHg systolic and 80mmHg diastolic. What would you expect the blood pressure to be at the top of the head given that this point is 45cm above the measurement point? Remember thisis a gauge pressure and give your answers in both mmHg and kPa. (The density of blood is about 1060 kgm^-3)

This is what I have done but I am so unsure on what I am doing.

1. Converted the systolic and diastolic blood pressure into Pa.
Systolic Blood pressure= 140mmHg x 133.322Pa/1mmHg= 18665 Pa
Diastolic Blood pressure= 80mmHg x 133.322Pa/1mmHg= 10666Pa

2. Due to the change in height I named the top of the head P2.
P2 - P1=pg h2-h1
P2=P1+pgh
P2 systolic = 18665Pa + (1060kgm^-3 x 9.81 m s^-2 x. 45m) = 23344Pa
P2 diastolic = 10666Pa + ( 1060kgm^-3 x 9.81 m s^-2 x. 45m) = 15345Pa

3. I then converted the P2 into kPa.
P2 systolic kPa =23344 Pa x .1kPa/100Pa = 23.34kPa
P2 diastolic KPa =15345 Pa x .1kPa/100Pa = 15.35kPa

4. I then converted the P2 into mmHg.
P2 systolic mmHg =23344 Pa x 1mmHg/133.322Pa =175.1 mmHg
P2 diastolic mmHg = 15345Pa x 1mmHg/133.322Pa= 115.1mmHg
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