Cut while skydiving will a person bleed faster?
June 7, 2009 9:11 PM   Subscribe

So my buddy and I watched that old Keanu Reeves film Point Break the other night, and we got to talking. We're thinking that more air pressure causes you to bleed at a more furious pace. So if someone's in a knife fight in a plane, is cut, say across the arm, then jumps out of the airplane to parachute to the ground - will they bleed to death before they hit the ground because the air pressure is causing them to bleed so quickly? It doesn't have to be skydiving, it could be on like, Everest. Will you bleed crazy fast higher up? I know that there are a lot of variables here, but I'm just curious if the thinking is sound.
posted by Sully to Health & Fitness (22 answers total) 1 user marked this as a favorite
Air pressure is lower at higher altitudes. Or is your theory that lower air pressure causes faster bleeding?
posted by pravit at 9:21 PM on June 7, 2009

Air pressure is lower as you go up, so the pressure isn't going to squeeze the blood out of you at high altitude, if that's what you were getting at.

Also, the atmospheric pressure at Everest is still pretty substantial, about a third of that at sea level.
posted by bluejayk at 9:21 PM on June 7, 2009

We're thinking that more air pressure causes you to bleed at a more furious pace.

Do you mean less air pressure?
posted by Conrad Cornelius o'Donald o'Dell at 9:22 PM on June 7, 2009

We're thinking that more air pressure causes you to bleed at a more furious pace.

I assume you did mean less air pressure.

I looked at the book The high altitude medicine handbook, and while it discusses a number of issues with blood (pressure, content, etc.) there is no discussion of bleeding times.

Bleeding is really a function of the heart pumping, not of blood pressure leaking. You are not a balloon. Any probable increase in blood flow is likely attributable to adrenaline and such.
posted by dhartung at 9:30 PM on June 7, 2009

You also have to take into consideration that the blood's coagulants start to coalesce relatively quickly, and, unless a major artery is struck, in which case the body's blood pressure is its own worst enemy, the bleeding will stop relatively quickly.

But none of this has anything to do with the elevation at which the injury occurs.

All bets are off in the relative vacuum of space.
posted by dfriedman at 9:41 PM on June 7, 2009

Besides, higher air pressure might also act on the wound, making it *less* likely to bleed. If we're going to indulge in movie-style physical logic at least. ;D
posted by Billegible at 9:46 PM on June 7, 2009

Billegible makes an interesting point.
posted by dfriedman at 9:49 PM on June 7, 2009

Best answer: You know what they say: Pressure and Elevation.

I can't imagine the difference in terms of ml/second between being stable and falling as very large at all, if any.

Okay: There ARE cuts that you can bleed to death from in a very short order. Let's take one of those:
Let's look at a cut's dangerousness as a factor of how much blood you lose over time: (x)ml/second. The human body holds an average of 5.6 liters of blood. 40% blood loss is considered very dangerous. Let's say that 50% (2.8 liters) is fatal. Let's look at the average freefall: At 15k feet you might fall for at most a minute (if memory serves) before you had to open your chute (I'm assuming a chute in this question, for obvious reasons). So the (x) would HAVE to be about 47ml/sec to be lethal during that freefall. By my math that's nearly 10 teaspoons per SECOND. 3/4 of a gallon/minute. That's one bad cut, whether or NOT you're falling.

While there certainly MIGHT be a very specific zone where a cut might be worse off in freefall than stable, (I'd imagine that it would also be a factor of position within the aerodynamic profile in terms of how much positive or negative pressure it was under; it might be WORSE in the bubble- who knows) I think it'd be nearly impossible to try and quantify the (incredibly) tiny zone that (x) was higher than 47 for the brief freefall time than on the ground.

You'd be freefalling toward a hospital. Or a hospital pool.

My apologies for the disjointed nature of this post. I'd hone this into something more cogent, but I'm a little tired. I love LOVE this kind of conjecture. Over a beer or two.
posted by asavage at 10:23 PM on June 7, 2009 [9 favorites]

Response by poster: Wow. We were dead wrong.
In our defense, we were dead drunk.

I don't know why we thought it was
higher air pressure, rather than lower.

Later on in our conversation, we got
to talking about how cold it must get in
higher altitudes. One of our speculations
was that if you have a water balloon,
and you tied it to like, 20(?) regular helium
balloons - could this structure get high
enough that the water would freeze?
Or would the helium balloons pop first?

Thanks to everyone for taking this
question so seriously, when it is so
clearly ridiculous.
posted by Sully at 10:45 PM on June 7, 2009

What's the term of art for using up your weekly askmefi quota while inebriated?
posted by asavage at 10:47 PM on June 7, 2009

Response by poster: Asavage -

I've done some drunk ebaying and I
have to say, drunk AskMefi-ing is
considerably less painful.
On the wallet, at least.

I think I need a breathalyzer on my PC.
Or something like Google Goggles for
every site that could cost me money
or my reputation.

Oh, and thanks for your wonderfully
analytical answer.
posted by Sully at 11:10 PM on June 7, 2009

FWIW, normal blood pressure seems to be about 2-3 psi; nominal sea-level atmospheric is about 14.7 psia (of course, the body is 2-3 psi above std atmosphere). I was expecting the body's pressure to be large enough to trivialize the drop in pressure in a vacuum; seems that in fact the vacuum would indeed probably "suck" the blood out of you.

For reference, Denver's pressure is 12.2 psia, approximately doubling the body's nominal blood pressure versus atmosphere (psig). HOWEVER, since blood pressure is mainly due to pumping, it is possible that the heart will respond with less pumping at higher altitudes, reducing the pressure relative to the atmosphere. Anyone know more?
posted by IAmBroom at 11:13 PM on June 7, 2009

Best answer: A few things:

1) Because venous blood pressure is low, a cut to a vein (even a major one) won't leave you bleeding out in minutes. A cut to a major artery will result in a pretty quick death at any altitude -- the primary factor in your survival is how quickly you receive proper medical care.

2) Air pressure at 10,000 feet (about the maximum altitude at which the average unacclimatized human can maintain consciousness without supplemental oxygen) is still 10.2 psi -- only 3.5 psi (~20%) less than the 14.7 psi at sea level.

3) Your cardiovascular system maintains about the same pressure differential relative to atmospheric pressure, regardless of what atmospheric pressure is. The point of this is to keep blood flowing to your brain and to your extremities, despite the effects of gravity and fluid-dynamic drag.

So, all in all, the altitude doesn't matter. A major cut will kill you, a minor cut won't. Even if you're jumping out of a plane. Even if you're not in a movie.
posted by randomstriker at 12:40 AM on June 8, 2009 [1 favorite]

What's the term for writing poetry as clarification posts in your drunk askme question?
posted by Grither at 6:10 AM on June 8, 2009

What's the term for writing poetry as clarification posts in your drunk askme question?

That's pretty much just Sully's posting style.
posted by owtytrof at 7:23 AM on June 8, 2009

Response by poster: Grither, owtytrof,

I just like to cut my meat differently.
I think of it as chopping up my sentences
into manageable bits.
posted by Sully at 7:37 AM on June 8, 2009

One thing that is being missed here (although it could be irrelevant) is the oxygenation of blood at altitude.

Higher altitude =
less oxygen in the air =
less oxygen in the blood (presuming you are not acclimated and have not generated more red blood cells) =
you'll bleed to death (specifically: you'll die of hypoxia) faster at altitude than at sea level because you have less blood carrying less oxygen.

But, like I said, this delta could be so small as to be functionally irrelevant.
posted by Cool Papa Bell at 7:58 AM on June 8, 2009

if i nick my finger or something underwater it seems to bleed normally, even if i'm at several atmospheres of pressure.
posted by snofoam at 8:23 AM on June 8, 2009

I have spent over two years of my life in airplanes, mostly at cabin altitudes of about eight thousand feet. I can say without equivocation that bleeding at that altitude is the same as at sea level.

The sex is better, though.
posted by dinger at 9:02 AM on June 8, 2009

If pressure was low enough. Your blood would boil at 98.6 degrees. (Boyles law) That would accelerate bleeding!!
posted by Megafly at 9:30 AM on June 8, 2009

I always read Sully's comments as haiku, and then get confused when they keep going.
posted by ActingTheGoat at 10:54 AM on June 8, 2009

randomstriker: Air pressure at 10,000 feet (about the maximum altitude at which the average unacclimatized human can maintain consciousness without supplemental oxygen) is still 10.2 psi -- only 3.5 psi (~20%) less than the 14.7 psi at sea level.

Just a minor correction. 10,000 feet is well below the altitude that may cause unconsciousness. For example the FAA allows pilots to fly at altitudes between 10,000 and 12,000 feet for up to 30 minutes without supplemental oxygen safely maintaining mental acuity. Out of shape weekend warriors routinely take ski lifts above 10,000 feet, where there may be a bar and restaurant. Thousands of tourists every day drive their cars to the top of Pikes Peak in Colorado at 14,110 feet or Mauna Kea in Hawaii at 13,800 feet. Most people would be extremely uncomfortable and mentally impaired but remain conscious at least for a while up to about 20,000 feet.
posted by JackFlash at 12:36 PM on June 8, 2009

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