this post was submitted on 31 Aug 2023
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I get the meme, but I'm going to be pedantic: submarines are full of air at atmospheric pressure so therefore they can be crushed. Living things are full of water which is effectively incompressible so they can't - they'll always have the same pressure on the inside as on the outside without changing size.
This is the reason human scuba divers can go quite deep without feeling any discomfort. Humans do contain big air bubbles: their lungs. The lungs aren't crushed because the scuba equipment automatically provides air at the same pressure as the outside water, but divers have to remember not to hold their breath as they swim up - as the water pressure decreases, having all that high pressure air in the lungs can rupture them. (Lungs withstand being squeezed much better than being stretched so that's why free-divers can hold their breath while swimming down.)
The reason you can't just scuba dive to the bottom of the ocean is actually because the behavior of dissolved gasses in the blood changes as the pressure increases. That's the consequence of pressure that these animals are adapted to.
I appreciate your pedantic nature.
Oxygenated liguids have entered ~~the chat~~ your lungs.
Won't let you go all the way down, but the Navy has had some successful tests with the stuff. It still feels like you're drowning at first though.
The Abyss
What about free divers? Why don't their lungs get crushed?
Last sentence of the second paragraph mentions free diver lungs.
Ah, sorry. Didn't see that.
Your lungs can compress to equalize the pressure as you go deeper and expand as you come back up. As long as you start with ambient pressure air in your lungs you won't have issues.
The problem is breathing against the external pressure, you need gas pressure to help expand your lungs again after you exhale. The regulator keeps the air pressure equal to the external water pressure so breathing feels the same no matter how deep you go. With an open loop system you use air faster with depth because each breath is higher pressure and gets wasted when you exhale.
I should have phrased that differently. The lungs don't shrink because they're filled with high-pressure air, but shrinking wouldn't damage them. "Crushed" implies damage and it was the wrong word to use. Lungs are soft and effectively can't be crushed, the way that a balloon can't be popped by deflating it.
The guy with the world record for free-diving had the air in his lungs squeezed to a twentieth of its original volume, but lungs are built for that sort of thing. Simply going from inhaling as much as you can to exhaling as much as you can reduces the volume of air in your lungs to a fifth of its original volume, a much bigger absolute change.
(He still had to practice and prepare for years, and he was probably born with an exceptional natural aptitude. Don't try this at home!)