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I am not well versed in physiology, and I've found the literature available to me confusing on this point.

Mostly I'm going off of wikipedia, mind you. The article on Decompression Sickness' Mechanism Heading states:

Under normal conditions, most offgassing occurs by gas exchange in the lungs.[53][54] If inert gas comes out of solution too quickly to allow outgassing in the lungs then bubbles may form in the blood or within the solid tissues of the body.

What I can't figure out is if the actual ambient pressure matters, if we suppose that that is separate from the pressure in the lungs. From what I've understood, in SCUBA diving the water pressure applies pressure to the air you're breathing (somehow? through the air hose? I'm not certain my understanding is correct) and thus the air you're breathing and the air in your lungs is also at a higher pressure. If I'm wrong about that, that may be the ultimate answer of my question.

I'd like to present a hypothetical, to clarify the exact focus of my question:

Let's say we can somehow form a perfect airtight seal around the waist of an otherwise naked human being. Setting aside the practicality of that, of course. The point is that the human is otherwise unaffected, but it blocks air.

That air tight seal around the person's waist makes it so that their lower body and legs are exposed to a very low pressure environment, maybe even pure vacuum. The person's upper body (and notably their nose and lungs) are in a standard sea-level terrestrial atmosphere.

Putting aside the vacuum bruising and painful swelling and other detriments caused by the low pressure environment on their lower body, my question is this: could the person in this hypothetical get the bends, simply because of the lower pressure on part of their flesh? Or would decompression sickness not be an issue because their lungs are pressurized?

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  • $\begingroup$ Your scenario is impossible because of blood vessels. If you cut the lower half of the body away while sealing it, and reduced the pressure dramatically, yes, bubbles would form in the now non-functional blood vessels. $\endgroup$
    – anongoodnurse
    Commented Mar 4 at 3:23
  • $\begingroup$ @anongoodnurse I don't understand. I'm not implying that the seal cuts physically through the body, just around it but forms a tight seal with the skin. $\endgroup$
    – Mia
    Commented Mar 4 at 13:30
  • $\begingroup$ I understand that. But I'm saying because blood vessles keep ferrying blood to and from the lower body, your thought experiment is impossible to carry out. $\endgroup$
    – anongoodnurse
    Commented Mar 4 at 14:14
  • $\begingroup$ If you did carry it out you wouldn't get bruising in the lower limbs, you would extrude everything in the upper body through and into the lower body. It would get very traumatic and gory very quickly. $\endgroup$
    – Bryan Krause ♦
    Commented Mar 4 at 14:30
  • $\begingroup$ Ah. I was under the impression that connective tissue is rigid enough to compensate for that, and that the heart would be doing its best to maintain blood pressure for at least a short time. Obviously swelling and bruising would occur, but I wasn't sure that it would necessarily follow that nitrogen would emerge from solution as in the bends. $\endgroup$
    – Mia
    Commented Mar 4 at 15:07

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The solubility of a gas in a liquid is proportional to the partial pressure of the gas above the surface of the liquid Henry’s Law (On the ground at sea level air pressure is about 15psi, nitrogen is about 80% of air, so the partial pressure of nitrogen then is about 12psi.)

When a diver is 32ft deep the total pressure is about 30 psi and at 64ft deep pressure is about 45psi. This means there’s twice as much nitrogen in her blood and tissues at 32ft and three times as much at 64ft. If you are on the ground at sea level, the pressure in your lungs is also about 15psi. When you breathe in it is slightly less, and when you breathe out it is slightly more. This small difference in pressure is what makes the air move in and out of your lungs.

Because the human body is flexible, when subject to an external pressure, that pressure is experienced equally throughout the whole body. This comes from pressure on the skin - it doesn't come through the airline (though that does need to be at the same pressure of course). If you want the pressure inside to be lower than the outside pressure then you need a rigid encasement such as a submarine or bathysphere where that exterior can support the pressure difference. You can see how an air-filled balloon expands as it rises in this video (from about 3:25)

The pressure in our arteries is a little higher than outside pressure; on average that’s about 100 mmHg or about 2 psi, as the heart has to pump blood around the body. It’s also a little higher in our tissues, as the cell walls have to keep their contents inside against the force of gravity etc. Healthy human lungs have a surface area about equal to the size of a tennis court which allows for efficient diffusion of gases across this barrier. However it takes a much longer time for gases to diffuse out of tissues into the blood stream and be conveyed to the lungs. A bubble of air inside water has a higher pressure inside than that in the surrounding water due to surface tension of the surface of the bubble which is trying to contract, the smaller the bubble the higher the pressure. This means that, in general, dissolved gases have any easier time diffusing out of a cell than forming a bubble.

According to Navy diving tables the maximum time at 60ft is about 55min without the need for decompression. But ascent should be slow at about 0.5ft/s. Longer dives at this depth do require decompression protocol. wikipedia as illustrated by Naval sea systems command - US Navy Diving Manual, Revision 6. SS521-AG-PRO-010, Public Domain decompression

As I said, this whole process is slow, so that if there is a sudden change in outside pressure dissolved gases will nucleate and form bubbles. This can happen even when the outside pressure is originally atmospheric. For example passengers flying at great height in a plane with a pressurized cabin can experience DCS if there is a sudden decompression, as when a window blows out. So reducing the external pressure on the lower half of the body will only make the situation worse as dissolved gases will rapidly form bubbles there.
The same thing can happen to fish! recent news

Edit: According to David Hammen’s answer to this question air on ISS there is no chance of DCS when using pure oxygen at 30kPa. which astronauts do in their EMUs. It’s nitrogen which is the big problem. They do however have to reduce the nitrogen levels in their blood before donning these suits as air in the ISS is at atmospheric pressure with 79% nitrogen.
Also, high pressure oxygen is the best treatment for DCS. Hyperbaric Oxygen Therapy

In case of rapid decompression of the EMU due to accident or failure, getting back inside the ISS would seem to be the priority.

Good luck with your suit design.

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  • $\begingroup$ A diver who goes to 60 feet can stay underwater for about 45 minutes and ascend at a rate of half a foot per second (not per minute) with a three-minute stop at 15 feet - about 40 minutes down and 5 surfacing. Decompression is only required for deeper or longer dives. $\endgroup$
    – Eonema
    Commented Mar 19 at 4:55
  • $\begingroup$ @Eonema You're right. I'll correct that. $\endgroup$
    – Rich
    Commented Mar 19 at 14:39
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Your body isn't rigid. All of your blood vessels are under pressure from the atmosphere, from the pressure of the air pushing on your skin which pushes on the underlying tissue.

When nitrogen or anything else is dissolved in blood, it's dissolved under that atmospheric pressure. If that pressure suddenly drops, it bubbles out, just like the bubbles in a carbonated soda that is under pressure until you open the can. Those bubbles happening in blood vessels is bad because it blocks them. That's the bends.

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  • $\begingroup$ That makes sense. But skin is gas-tight by itself. From what I've read about research into mechanical counter-pressure suits, it's not like we strictly need to be surrounded by an atmosphere so much as we need pressure (and air to breathe). $\endgroup$
    – Mia
    Commented Mar 4 at 13:34
  • $\begingroup$ @Mia You can squeeze a balloon and increase the pressure inside without any gas moving into the balloon. Your body is a big balloon of biology. $\endgroup$
    – Bryan Krause ♦
    Commented Mar 4 at 14:25
  • $\begingroup$ Sure, yes. Swelling and bruising would occur, you'd be generally uncomfortable and in some period of time you'd die. My question is much more specific - would nitrogen come out of solution and form bubbles in your bloodstream? This is the documented mechanism of decompression sickness and it is not clear to me if it would still occur if pressure in the lungs remained unchanged. $\endgroup$
    – Mia
    Commented Mar 4 at 15:11
  • $\begingroup$ @Mia No one would ever be able to do your experiment, it would have disastrous consequences for all of circulation. But yes, if you had just a closed system of blood with dissolved gasses in a tube and reduced the pressure, bubbles will form. That's the mechanism. It has nothing to do with the lungs. No source is talking about bubbles forming because of the lungs. The quote you have is about the other place gas can go to not produce bubbles which is that it is breathed out if the change is slow enough. $\endgroup$
    – Bryan Krause ♦
    Commented Mar 4 at 15:22
  • $\begingroup$ Fair enough, thank you for your responses. My mind has been on questions of pressure suit design. Wondering if hybrid designs involving a helmet-volume kept at ~100kPa to avoid the need for pre-breathing and a body-volume kept at ~30kPa for mobility purposes would be possible. It sounds like decompression sickness would still occur. Obviously the body can handle the lower pressures in general since existing designs are ~30kPa throughout the entire suit, but I guess inconsistencies in pressure over the entire body would be too big an issue. $\endgroup$
    – Mia
    Commented Mar 4 at 15:36

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