# Blood pressure and atmospheric pressure

1. May 4, 2010

### ananthu

The normal atmospheric pressure is 760 mm of Hg. But the normal human blood pressure is around 120/80 mm only. In that case how equilibrium of our body is achieved? For any vessel to retain its original shape its inside pressure and outside pressure should remain equal. If the outside pressure exceeds the vessel will be crushed inward and if the inside one exceeds it will blow up. In the case of our body, assuming it as vessel, how the pressure of atmosphere which is nearly six times higher than the blood pressure inside, could not crush the body inward?
Are there any other inward forces make up for the rest in order to counteract the atmospheric pressure? Also why do we not feel the atmospheric pressure acting on our body surface at all?

2. May 4, 2010

### uart

The blood pressure measurments are "relative pressure", meaning the figures that you state are above atmospheric pressure.

3. May 4, 2010

4. May 4, 2010

### Staff: Mentor

760+120=880

Think about it: the measurement is taken with a pressure measuring device open to atmosphere.

Last edited: May 4, 2010
5. May 4, 2010

### ananthu

Does it mean that the actual blood pressure is 880 mm of Hg? In that case it means the actual blood pressure is 120 mm higher than that of the outside pressure.If so how balance is achieved? If there is no balancing mechanism, I should feel as if my blood is pushing my body from inside!

6. May 4, 2010

### xxChrisxx

Mechanical strength of blood vessels.

Not everything gets crushed because of a pressure difference. Think of, a can of fizzy drink, a high pressure hose pipe, gas/water pipes, an aeroplane. All are pressure containers.

So just like a waterpipe with high pressure water rushing through it doesn't explode (usually) neither do blood vessels. Note that if blood pressure get's too high, your blood vessels can break.

7. May 4, 2010

### Staff: Mentor

120 mm is not a lot of pressure for a hose like your blood vessels to hold.

8. May 4, 2010

### Andy Resnick

That may seem true, but recall that hypertension starts around 140 mmHg: that's an increase of only 2% absolute pressure. (16% increase in gauge pressure, tho).

9. May 5, 2010

### ananthu

Thank you for all the replies. I am happy that my doubts and the chain of answers have led to a new realm of interesting details and facts which are seldom found in text books.

10. Nov 7, 2011

### juzlam

ok, i guess its really2 fair explaination. but, if thts the case, when 1 person die, we must observe tht the body would shrink with 880 mmHg pressure. 880 is a huge pressure difference. if the difference is maintain to 120-80, meaning, our heart is just the elevation (pumpung power of only 120)...dont u guys think so?

I still damn impressed with the design of the heart to sustain pumping on average of 100000 times a day even with that 120 pressure ramp.

11. Nov 7, 2011

### Staff: Mentor

Welcome to PF!

Pressurized and compressed are two different things. Our blood is pressurized, but is not noticeably compressed.

12. Nov 7, 2011

### juzlam

ok, get the point. (correct me if its not allign with what u try to explain)..basically, after someone die, there is no more pressure generated from inside, that would definately damage the body as pressure inside is not in equilibrium with pressure outside. however, due to the physical properties of human body which built from very minimal compressible substances, so tht, our body could still maintain its original state even without internal pressure generated from the blood circulation.is tht the point?

13. Oct 2, 2013

### PhiloVita

From my understanding a pressure gradient is required for gas to exchange from one system to the next, such as from the atmosphere into our blood. The atmospheric pressure is not that high if you compare it to our circulatory pressure - atmospheric pressure is 14.69psi and our systolic blood pressure is 2.32psi. Our blood vessels are composed of soft and hard tissues that enable our circulatory system to with stand a certain amount of pressure from outside as well as from within. 14.69psi is a small amount of pressure - enough for gaceous exchange but not enough to crush a human body or a blood vessel. Consider that Oxygen has partial pressure of 0.21atm (160mmHg), which is necessary for Oxygen to enter the blood as our systolic blood pressure should be about 120mmHg. The atmospheric pressure of CO2 is less than 0.0397atm (30mmHg), which is also necessary for the CO2 to diffuse out of our blood and into the atmosphere. A fine balance indeed.