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If your blood pressure ever falls below the ambient atmospheric pressure would it be possible for you to bleed. Because if blood pressure was less than that of atmosphere air would move in if you get cut, bleed or something rather than blood moving out as it normally does.

So ultimately will it be possible bleed if blood pressure falls below atmospheric pressure

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    $\begingroup$ BP < 1 atm will crush you $\endgroup$ – nzaman Aug 17 '16 at 5:23
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    $\begingroup$ If your blood pressure falls below the atmospheric pressure, then the atmosphere will crush your body until the pressures are equalized. In other words, the air "moves in" before you get cut. So it's impossible for the situation you describe to arise. $\endgroup$ – ApproachingDarknessFish Aug 17 '16 at 5:33
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    $\begingroup$ A question that is impossible is no reason for a downvote. $\endgroup$ – Cem Kalyoncu Aug 17 '16 at 9:50
  • $\begingroup$ @CemKalyoncu Maybe not, but a question that demonstrates a lack of research is a valid reason for downvoting. (The downvote isn't mine.) $\endgroup$ – Frostfyre Aug 17 '16 at 12:13
  • $\begingroup$ Where do you even begin to research something like this? If it is not on this site chances of you finding anywhere is quite slim. $\endgroup$ – Cem Kalyoncu Aug 17 '16 at 14:50
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For the sake of answering the question I am going to making a couple of assumptions.

  • Blood vessels in this “human” don’t collapse do to external pressure being greater than internal.
  • The blood of this “human” does not move because the heart pumps it. It moves because of some other organ that shoots the blood through the non-collapsing blood vessels. (maybe it works similar to a Magnetohydrodynamic Drive: https://en.wikipedia.org/wiki/Magnetohydrodynamic_drive)

If this organism were cut, the wound would quickly fill with air. This would bring the pressure in the creature up to atmospheric pressure. If this sudden increase in pressure does not cause any harm to the organism then as blood passes the wound, some would exit the wound.

TL:DR, under the right condition yes the “human” would bleed.

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You cannot bleed — or live — with zero blood pressure.

The absolute blood pressure must be at least 1 atmosphere of pressure or the atmosphere itself would clamp down on all your limbs like a blood pressure measuring sleeve and restrict blood flow. The blood pressure must overcome the atmosphere or blood cannot flow. So a blood pressure of absolute zero is not possible.

But I suppose what you meant was "blood pressure of zero compared to the atmosphere", which is what we normally mean. I am sorry but that too is impossible. There are 100 000 km of blood vessels in the human body, and getting a fluid to flow — especially a fluid as viscous as blood — requires a force to push it through all of these tiny tubes. Blood vessels always offer resistance — a kind of friction — to the blood, and this must be overcome to make the blood flow. You cannot have blood flow without blood pressure.

If you rupture a blood vessel however, then you have a path for the blood that offers no resistance. So simple physics say that this is where the blood will go, literally taking the path of least resistance.

So you have two incompatible demands here. In order to not bleed, you need zero blood pressure, but then no blood is flowing through your body. It goes without saying that this is very bad for you. Or you have non-zero blood pressure, but then you will bleed if any of your blood vessels are ruptured.

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