Lungs underwater

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The OP was referring to lung volume during a free dive, this is one area where scuba has no relationship to free diving. The lungs are a flexible container, within a semi-flexible container (the thoracic cavity). At a certain pressure the body does the blood shift that @Angelo Farina mentioned, so the lungs are no longer air filled spaces, but are fluid filled spaces that are incompressible. The chest is noticeably compressed though. If you watch videos of deep freedivers, you can see how baggy and loose their wetsuits are around the chest, they are just flapping from the current (seen here at 3:15 on the ascent ). It all happens naturally, with no specific effort required by the diver. It is also why the deep safety divers can't just give the freediver a regulator if things go wrong. The freediver can't overcome the surface tension between the alveoli in the collapsed lungs to be able inhale.
Here is Herbert Nitsch doing a 702 foot no-limits dive (with deco stop!). You can see the compressed chest at the :47 mark during the warm up dives (I suspect he was doing exhaled dives to promote the blood shift, as that is pretty significant compression given how shallow he appears)


Other air spaces are more of a challenge. Masks are not worn, they made fluid filled goggles for a while, I'm not sure if that is still a thing or not, it seems most just go with no vision aids. Ears are cleared from air in the mouth initially, then from air pumped up from the lungs, and eventually have to be cleared by flooding the sinus cavities with water!

I remember seeing Tanya Streeter give a presentation on no limits competitions, and she mentioned should could only do a few per year because she would have horrible sinus infections afterward. Not surprising when you waterboard yourself at 200 feet.
 
I really wish this concept would apply to my belly.
 
I really wish this concept would apply to my belly.

No you don't. Well, maybe if you dive dry -- air bubble in the midsection is a b*tch.
 
No you don't. Well, maybe if you dive dry -- air bubble in the midsection is a b*tch.


I meant the compression and thereby decreasing of circumference.
 
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