Question about "the bends"

BishopUser

From what I understand a SCUBA diver's breathing equipment will supply him with air at a pressure that correlates to the surrounding water (probably to make it easier for your chest to expand/contract naturally under high pressure water). The higher pressure air means more nitrogen gets dissolved in your blood. When you come up too fast you start breathing lower pressure air which makes room for the extra nitrogen to come bubbling out and cause all sorts of problems. That all makes sense except for the following which is copied from wikipedia's article on the bends:

"DCS may be the cause of the disease called taravana by South Pacific island natives who for centuries have dived by breath-holding for food and pearls."

http://en.wikipedia.org/wiki/Decompression_sickness

If you are holding your breath then you aren't breathing in any high pressure air which means no extra absorbtion of nitrogen. How is it possible to get the bends if you take a deep breath at the surface, dive deep, then come back up quickly?

stevenb

This statement does not sound perfectly worded to me. It's not so much the low pressure air that you are breathing when you come up. The issue is that the nitrogen in your blood is in higher concentration from breathing the compressed air, and that high concentration can only stay in solution at that high pressure. Once you come up, your entire body (internally) has lower pressure, and the nitrogen will come out of solution as a result. This is exactly the same as when you open a bottle of cola. Here the CO2 comes out of solution as a result of the low pressure atmosphere and the high CO2 concentration in the liquid.

This is a good question because traditional wisdom says that you can't get the bends from free diving. The idea is that the limited air and the brief periods of being at depth make it too difficult to get enough nitrogen into the blood. Apparently, when people get really good at free diving, they can stay long enough to have issues, I think rarely though. My guess is that the air that is in the lungs gets compressed at depth (this is true for sure) and even this small volume has enough nitrogen if the event is repeated over and over (this is just a guess on my part). I think this must be rare because I personally know a number of free divers who go to over 100 ft depth over and over again. Although I'm a scuba diver and can hold my breath much longer than average, I would never do this myself. The bigger risk is shallow water blackout that can occur. There have been cases of this that have resulted in drowning.

http://scuba-doc.com/latenthypoxia.html

mgb_phys

Another theory is that (at least at recreational diving depths) DCS isn't caused so much by N2 being absorbed simply due to pressure as by micro tears in the lungs leading to gas directly entering the bloodstream and causing embolisms.

stevenb

Interesting. Yes, good point and I should stress that I really don't know the answer. Previously, I was taught that you don't get the bends when snorkling or free-diving.

I don't think that the bends can't happen in principle because at depth the lungs are compressed and the nitrogen (what little there is) is at higher pressure. However, if you consider the length of time that a person can hold their breath (< 3 minutes) the time it takes to ascend and desend (which is hopefully slowly if one is not being reckless), the limited amount of nitrogen in the lungs, the fact that the lung volume is compressed down to a tiny chamber at depth, and then compare this with dive tables used by scuba divers, it seems very difficult for a human to get the bends without regulators and tanks.

Still, some people push the limits and it's possible to descend and ascend very quickly, and to do multiple dives with brief periods between dives. It's hard to rule out the possiblity of the nitrogen induced bends completely. But again, this would not be my first worry given the other more significant risk factors.

BishopUser

How can the depth affect your internal pressure? I imagined the body as a closed system and while the water is trying to squish me, it should have no affect on my internal pressure. The only open part of the system is when I am breathing the high or low pressure air into my blood stream. If I am holding my breath and assuming the body is somewhat rigid (my lungs are protected by my rib cage), then my internal pressure should not be a function of depth.

stevenb

Alas, no matter how much we pump iron and think we are rock solid, were are really as squishy as a jelly fish.

Actually, this is a serious issue for scuba divers. If we do not continually breathe out as we ascend, our lungs will burst. Also, in cases where you are correct, there is trouble. If our ears hold back the pressure, then they hurt like hell. So we make efforts to equalize.

The critical invention for scuba divers was the regulator (was it Jacques Cousteau who invented it? probably not but he adapted it successfully and made it and himself famous). The reason for needing the regulator is that the pressure we breath air at needs to be very close to that on our external body. Otherwise we do not have the physical strength to breath and our lungs can collapse or explode.

nismaratwork

@ Bolded: good stuff.

For the rest, it's not just serious for SCUBA, but military maneuvers as well. It is our very "jelly" nature that allows us to dive in the first place, which is quite a feat, but without the proper techniques, and at greater depths a different gas mixture, you get trouble in river city.

IMHO, I think the bends is USUALLY caused by the classically described mechanism, but Mgb's explanation is probably a factor in some cases. There is systemic stress within and without, and it doesn't take much to cause pain, sickness, or death, just as with high-speed flight. It is damnably hard to isolate a single cause, but fortunately the "cure" is simple: adjust slowly.

curioso

Though you are not breathing in pressurized nitrogen at the surface, the nitrogen in your lungs becomes pressurized with increasing water depth. Conceivably with repetitive deep diving of long duration, some of this nitrogen can go into solution in body water, particularly if intervals at the surface between dives are short and the number of dives is high in a relatively short period of time. Since the symptoms of taravana are similar to those of DCS, this may be the explanation but the answer is not known for certain.