Would swimming from a submarine cause the bends?

Chris - This is a good question, one I've actually pondered on myself which is why I guess it's come my way.

The answer is, probably not. Now the reason for this actually is that, when you're on a submarine, the air that you're breathing in the submarine is not under pressure or at least not under demonstrably higher pressure than ambient pressure - ie surface pressure - because the submarine can be thought of as an incompressible tin can underwater; so although the water is pushing in very hard on the submarine, the air inside is not feeling any extra pressure when the submarine goes down to the bottom of the ocean.

Therefore, you're breathing air as though you're breathing at the surface, and that means, if you escape from the submarine - say you went up one of the torpedo tubes or something in a submarine that was stuck underwater - then although you would immediately be exposed to extremely high pressure, depending on how deep the submarine was, then pressed in on you would be the surrounding water pressure and it would be subjecting the air already in you to that higher pressure; this means that the air would dissolve more in your blood, including the nitrogen that was in your lungs at that time, but there won't be very much of it. And, as you went up to the surface, you would find that nitrogen coming back out of solution and back into your lungs, and back into your blood, but again, there would only be a very small amount of it.

If you were a scuba diver, on the other hand, you would have a problem. What happens there is that scuba divers go down to the bottom of the ocean - say, 40, 50 metres underwater - and they're breathing compressed gas (in order to inflate the diver's lungs, to compete with the surrounding ambient pressure underwater, you've got to deliver the gas at higher pressure than the surrounding water) - therefore, you're breathing gas which becomes increasingly dense and increasingly high pressure the deeper you go; therefore more of the oxygen is going to dissolve in the bloodstream the lower down you go, but also the nitrogen is going to be forced into solution. Nitrogen is not very soluble, so then when you come back up again, because you have a body which has a lot of dissolved gas in it, as the pressure comes off, that nitrogen comes back out of solution and it forms bubbles in the tissue, and you get the bends.

If you're not breathing compressed gas, this won't happen. Whales don't get the bends or at least not often unless they resurface incredibly rapidly. We've made a very nice video of this. If you look at Naked Scientists Scrapbook on YouTube, you can actually see the footage we did recently - it's 'Why Don't Whales Get The Bends But Divers Do?' and it goes through the physiology of this.

Dave - So the problem with the divers is that their breathing air compressed for a very, very long time, whereas if you escape from a submarine there are only a couple of lungfuls at most, and there's not enough time for it dissolve?

Chris - Exactly. You're breathing air which is under pressure so it's pushing a lot more nitrogen to dissolve and nitrogen doesn't like dissolving. It's very insoluble. You also can't metabolise it, so even if it's in a tissue somewhere, it won't get used up. So you're increasing your total body load of nitrogen because you're breathing compressed gas, and the longer you spend underwater, the higher the burden of nitrogen in your body. As you go back to the surface, the pressure on you comes off and that pressure that was keeping the nitrogen dissolved is now gone so the nitrogen comes back out of solution and turns into bubbles again.

Dave - But I guess if you escape from a submarine, at some point, you're going to have to take a breath of compressed air, otherwise you're going to get flattened when you get out under high pressure so you come out through sort of an airlock or something so you'll be breathing compressed air for a bit.

Chris - That's true. As the, say, torpedo tube floods, it's going to compress the air in there and that will in effect subject you to compressed air. If you don't breathe it, then what I say, stands. If you do breathe it, you would have a little bit more nitrogen in your body but assuming you did it only for a very short time, I don't think it would make very much difference really.