Eli5: Where does the air on the lungs of deep divers go to?


Let’s say someone is trying to get the world record for deepest dive without equipment. They inhale all the air they can and start going down, but as they go, their lungs decrease in size even though they’re still holding the same amount of air. Surely all the air being compressed has to go somewhere at some point, right? Am I missing some really basic concept?

In: 5

It doesn’t go anywhere, it just gets compressed. You can have one cubic meter of air at atmospheric pressure (I believe 14.7psi?) and compress it down into the size of say a shoe box. Same amount of air, just higher pressure.

Assuming they can continue to hold their breath and their lungs don’t explode, the gas compresses.

The space between the gas atoms decreases. But the overall about of gas atoms stays the same, with the exception of the exchange for breathing

We have 2 things to talk about: lungs and air.

As you dive, the air compresses and takes up less volume. So it stays in place, but takes up less room. So how does this affect the lungs? To the lungs, this is the equivalent of exhaling all of the air in the lungs. However, when on land, when you exhale all of the air in your lungs, you always still have a lot of air left–approximately 1 liter of air is in your lungs at complete exhale. This is called residual volume. you *cannot* exhale it. And the lungs NEED all of that air in them in order to keep them inflated. When you dive, every 33 feet is one atmosphere of pressure. This means at 33 feet depth, the air in your lungs has decreased to half of its capacity. Full lungs contain around 4 to 6 liters of air. So if you go deep enough, like beyond 120 feet, the air in your lungs will compress to less than the residual volume. What happens then? Your lungs collapse, but that collapse can partly be staved off by fluids being pulled into your lungs by the relative vacuum that now exists in the lungs. Deep divers also stave off collapse by storing air in their cheeks, then inhaling it as the air in their lungs compresses and there is more room. When they ascend, the air expands again to fill up the lungs. [Occasionally, deep free divers end up dying](https://www.nytimes.com/2013/11/19/sports/testing-limits-of-a-niche-sport-diver-met-fate-72-meters-down.html) because they damaged their lungs with their deep dives–lungs aren’t supposed to ever be empty.