# Question about naked swimming at great depth after pressurisation

1. Dec 28, 2008

### LennoxLewis

Question about "naked" swimming at great depth after pressurisation

In the 1989 movie The Abyss, we see Ed Harris' character briefly swim without pressure suit (couldn't fit this into the title, hence the word 'naked') from one cabin to an other, several hundreds of meters below sea level.

Now, he spent days in a compression chamber, so that the air of the underwater station supposedly was at the same pressure corresponding to the depth. Exactly what does this mean anyway? We're talking about water pressure vs air pressure? I think i don't quite grasp this concept.

My main question: if indeed it is possible to swim at random depth if you spend enough time in a compression chamber, where if anywhere does the body break down? Could one theoretically swim at 6 km depth if you were sufficiently pressurized? (forgetting about temperature, breathing problems, etc).

2. Dec 28, 2008

### mgb_phys

Re: Question about "naked" swimming at great depth after pressurisation

While you are at depth the depth doesn't really matter - you are made of water so pressure inside = pressure outside.
The problem comes when you try and return to lower pressure. Gas is dissolved in your blood stream and as the pressure lowers it forms bubbles - like opening a bottle of soda. It is to allow these bubbles to release slowly that you spend a lot of time in decompression.

The main difficulty at extreme depths is the chemical reaction of different gases in your body. Oxygen becomes toxic at very high concentrations and so has to be diluted with other gases. So you constantly have to adjust mixtures and switch gases as you go deeper. The deepest commercial operations are typically around 750m.

Last edited: Dec 28, 2008
3. Dec 28, 2008

### LennoxLewis

Re: Question about "naked" swimming at great depth after pressurisation

Thanks for the response, mgb_phys. Would i be correct in stating that the bubbles are formed because the maximum partial pressure of oxygen in blood is lower at higher pressures?

4. Dec 28, 2008

### Staff: Mentor

Re: Question about "naked" swimming at great depth after pressurisation

I think bubbles are formed mostly by nitrogen (or other inert gases, if you use some mix for deep diving).

5. Dec 28, 2008

### mgb_phys

Re: Question about "naked" swimming at great depth after pressurisation

Bubbles are formed because the solubility of any gas increases with pressure.
Oxygen generally doesn't get chance to form bubbles because the blood (or rather Hemoglobin) is so good at hanging onto oxygen. The bubbles are normally of whatever dilution gas you are using, nitrogen for scuba, helium or hydrogen for deep diving.

(or at least that's how bubbles can exist - how they form is up for debate.)

6. Dec 28, 2008

### LennoxLewis

Re: Question about "naked" swimming at great depth after pressurisation

So how do fish and a few mammals living down in the oceans deal with that problem? They don't seem to be very worried about descent rate.

7. Dec 28, 2008

### mgb_phys

Re: Question about "naked" swimming at great depth after pressurisation

They don't breathe compressed air.
You breath in air at the surface as you dive it is compressed, as you rise it expands to exactly the same state as you started with.
That's why human free divers can do 200+m dives while scuba would need long decompression stops.

ps. Even on gas diving the descent rate isn't a problem - it is the ascent (coming up) rate when the bubbles form.

8. Dec 28, 2008

### Renge Ishyo

Re: Question about "naked" swimming at great depth after pressurisation

Actually the ascent rate has the exact same effect on fish providing the fish have a "gas based" organ such as a swim bladder as their buoyancy organ. This is noticed by fisherman all the time as when they pull up fish suddenly from depths it is often the case that their swim bladders explode and can be seen pushed out of their bodies/mouth etc. on the surface.

Fish with swim bladders can only change their depth in the sea gradually by adjusting the amount of gas in their swim bladders in order to avoid rupture. Hence, you typically see such fish even in aquariums swimming in straight horizontal lines more often than up and down.

Some predators like sharks lack gas filled swim bladders and can swim horizontal as well as vertical quickly, but the catch is that they must constantly swim to stay afloat (requiring lots of energy/food to maintain the existence of a single shark, and hence not too many of these can proliferate in a given area).

9. Dec 28, 2008

### LennoxLewis

Re: Question about "naked" swimming at great depth after pressurisation

I now get that part. When carbon dioxide is released from a bottle of coke, it happens because the pressure of the gas above the liquid is decreased (to 1 atm in this case). I guess a similar effect happens when you ascent.

Furthermore, do i understand your words correct if i state that nitrogen bubbles don't form if you quickly ascend, after free diving, because the air was uncompressed?

That would explain the whale-ascending problem, but then i don't understand why i was taught as a kid to always exhale all oxygen when quickly ascending from the bottom of a swimming pool to the surface.

10. Dec 28, 2008

### Staff: Mentor

Re: Question about "naked" swimming at great depth after pressurisation

Some species of fish release the gas from the bladder when they go up. IIRC thats what sardines do and from what I was told in Fishermen Museum (Vrboska, Hvar, Croatia) experienced fishermen were able to hear them and locate the school.

11. Dec 28, 2008

### mgb_phys

Re: Question about "naked" swimming at great depth after pressurisation

Exactly.

Yes, suppose that you started with flat soda and only had the 1atm of gas above the liquid. If only that amount dissolved in the liquid under pressure than when you returned to the surface you would only have the same volume of gas back. That isn't enough to cause bubbles - you only get bubbles if there is more gas than you can dissolve.

That's because a much bigger danger than bubbles is embolism.
The air in your lungs expands quickly as you ascend - if you rise too quickly without exhaling the compressed air could expand enough to burst your lungs.
Again this wouldn't matter for free diving - because it couldn't expand to more than the one lungfull you took down.

12. Dec 28, 2008

### LennoxLewis

Re: Question about "naked" swimming at great depth after pressurisation

Well, i was talking about free diving. Could it be that some of the air be distributed at other places than the lunges, meaning that when it expands, while it would not go beyond the lung volume, but perhaps beyond the local volume, of say, a blood vessel? And that for that reason, blowing out air on fast ascent is recommended even for free diving?

13. Dec 28, 2008

### mgb_phys

Re: Question about "naked" swimming at great depth after pressurisation

Yes air trapped in a small corner of a lung expanding and embolysing is a big danger.
One theory is that most bubbles in the blood stream while diving are actually caused by micro tears in small bits of lung.
I wouldn't have thought it was a risk in a swimming pool.
Deep free divers are mainly concerned about their lungs collapsing.

14. Dec 28, 2008

### Renge Ishyo

Re: Question about "naked" swimming at great depth after pressurisation

Yes, that's the way they do it. This strategy is not unlike blowing out the oxygen in your lungs when surfacing (the act of blowing out the gas compresses the lung or bladder prior to the expansion that occurs when the remaining gas in the organ expands upon ascent...the net result is the organ stays roughly the same size and doesn't rupture). The problem with the fish using swim bladders is that they can't do this quickly (or at least I have yet to see an example of a fish with a swim bladder doing this quickly), it can sometimes take hours or even days for them to change their elevation appreciably.

15. Dec 28, 2008

### LennoxLewis

Re: Question about "naked" swimming at great depth after pressurisation

It is all clear to me now. Thanks everyone for the responses.

16. Dec 29, 2008

### IMP

Re: Question about "naked" swimming at great depth after pressurisation

Also, at 6KM depth there is no gas of any kind, only liquids. You could never adjust in a pressure chamber unless you could breath some kind of oxygen filled liquid, some kind of air substitute...

17. Dec 29, 2008

### mgb_phys

Re: Question about "naked" swimming at great depth after pressurisation

The solubility of a gas in a liquid is proportional to pressure (Henry's law)

That isn't the problem of doing a breathing mix at 6km of pressure - you just lower the proportion of oxygen to keep the partial pressure below 1.2bar ( admittedly a fairly low concentration!)
The difficulty is finding a dilution gas that doesn't have any biochemical effect at these pressures. That's the reason for the exotic gas mixes needed in commercial diving.

Nitrogen causes narcosis effects at a partial pressure of 4bar (40-50msw) thats what limits recreational scuba.
If you replace the nitrogen with helium (heliox) you can go to around 500msw, below 200msw helium can cause High Pressure Neurological Syndrome (HPNS) twiching similair to oxygen toxicity, you solve this by putting some nitrogen back in (trimix).
Commercial diving tends to use trimix rather than heliox because it uses less expensive helium, military mostly use heliox.

Even deeper (750msw) you can replace the helium with hydrogen, hydrogen is lighter and easier to breathe. It has less HPNS effect than helium although it is chemically active so might have other biochemical effect at very high pressures.
If it does then that would presumably set the limit - since there is nothing smaller than hydrogen or more inert than helium to use as a dilutant.

Last edited: Dec 29, 2008
18. Dec 30, 2008

### epenguin

Re: Question about "naked" swimming at great depth after pressurisation

Hmm, I have known about this for many years,the fish can control the gas in the swim bladder by controlling the acidity there which has an effect on the oxygen affinity of a special hemoglobin. But now it occurs to me - the buoyancy... well the density difference between top and bottom of an aquarium must be very tiny. So their primary struggle must be to stay at the same height. Wouldn't the equilibrium of a compressible bladder in near-incompressible water be unstable? So that there must be constant adjustment going on to stay at the same height? Then when they need to change height there is a mechanism to override that?
Not contradicting what you say but is this a valid point?

19. Dec 30, 2008

### Renge Ishyo

Re: Question about "naked" swimming at great depth after pressurisation

Yes, it is a valid point. I think I read that the vertical limit for a "quick" change in height in the free ocean is about 10 meters or so for most fish...probably much larger than the typical aquarium display. So in an aquarium it wouldn't be much of an issue. Nevertheless, fish still behave in the aquarium in much the same way as they behave in the free ocean perhaps not out of necessity, but out of habit (it's probably similar to how insects that normally use the sun to navigate as they fly, fly towards our light fixtures when they enter our homes).

Is there constant adjustment in the swim bladder going on to stay at the same height? Yes, but keep in mind that this is what they had to do in the free ocean due to temperature fluctuations, local pressure differences due to currents, etc. and so they have mechanisms that readily accomplish these sort of adjustments. So long as the change in height is not too large at any one point they can cope. Also, I think the swim bladder takes care of the buoyancy issue passively (i.e. the gas in the bladder naturally is released from the bladder and the membrane surrounding the bladder can alter the permeability to either retain more of less gas depending on signals taken in from the environment). So they might not notice the "work" in this case (sort of like how must of us don't have to think about gravity fluctuations as we walk).

20. Dec 30, 2008

### mgb_phys

Re: Question about "naked" swimming at great depth after pressurisation

Pressure differences near the surface are proportionally greatest - going from surface to 1msw is a 10% change in volume, that's why you tell divers to equalize their ears 'early and often'.

I'm guessing that most aquarium fish are near surface dwellers (they are brightly colored and there isn't much point in being brightly colored if you live on the ocean bottom) so presumably they have either very small or very controllable swim bladders?