# TBR: artery pressure vs atmospheric pressure

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#### ThiaminePyroPhosphate

##### Full Member
7+ Year Member
Hey,

I came across a set of questions TBR physics page 96 passage 3 that dealt with measuring the blood pressure by inserting cannula into the artery. The passage stated that average gauge pressure of an artery was 10^4 Pa. One question required the understanding of whether the atmosphere or the artery pressure was greater. I thought that the atmospheric pressure was a lot greater than the gauge pressure since atm pressure = 101.3 kPa. However the answer stated that "Since the average gauge pressure in the artery is greater than zero, the average pressure in the artery is larger than the atmospheric pressure". I know that P total = P gauge + P atm. In this case was I supposed to use the Ptotal (instead of just P gaguge) and compare it with atmospheric pressure? When I think about it conceptually it makes sense, when you get a cut, blood leaks out due to the difference in pressure.

Generally speaking when do I add P atm to P gague?

In that drawing that I attached, would I include P atm to calculate the total pressures of fluid in test tube A and test tube B, even though they're not open to the atmosphere directly?

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Well the gauge pressure is just what you read off of a pressure gauge. It sets the atmospheric pressure as the 0 point and only measures up from there. I think using the equation as P_gauge=P_actual-P_atm is a little more intuitive. I like to think of pressure as the force of particles hitting a specific area (P=F/A), I think it makes it easier to think about it from a microscopic point of view.