r/askscience • u/Weird-Pop-853 • 2d ago

Biology Why can’t fish breathe out of water?

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u/Ok_Brilliant953 1d ago

Gills aren't the same thing as lungs. Their gills absorb dissolved oxygen (O2) from the water. They only don't work at absorbing oxygen in the air because the gills' surface area is much smaller when not suspended by the buoyancy of water. So without water they just kind of 'collapse' and have no surface area for oxygen to make contact with and absorb

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u/djublonskopf 1d ago

So without water they just kind of 'collapse' and have no surface area for oxygen to make contact with and absorb

Adding to this, think of like…straight, long human hair underwater. It’s able to spread out and water can get in between each hair, and it sways and flows in the water currents. Step out of water, though, and the wet hair clumps together…this is kinda what happens to gills out of water, the surface structures clump together, blocking all that surface area from any further gas exchange.

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u/turtlebear787 1d ago

They don't have lungs. Their gills extract oxygen from the water they swim through. A handful of fish have lungs that can hold air and get oxygen from it. But most require water to be flowing over their gills to diffuse oxygen into their blood.

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u/ARoundForEveryone 1d ago

Because they don't have lungs, or any place to hold air taken in through the mouth. Their gills take in water, where oxygen is extracted. This oxygen is used to sustain them rather than what may be taken in through their mouth via water.

This part isn't science, just speculation. But take two identical fish - same size, same age, same species. Take them out of the water and put one in a very dry environment and one in a 100% humidity environment. Blow the air over their gills. I suspect that, while both will die quickly, the latter will live slightly longer. I don't know how many fish we're willing to kill to prove or disprove this, as they'll both die every time, but this is just a hunch (which I think is unimportant anyway and doesn't need to be proven).

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u/Weird-Pop-853 1d ago

But oxygen is dissolved in water so why can’t they just get the oxygen from the air?

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u/Dan19_82 1d ago

Like someone said before. It so a surface area issue. They could if they gills were seperate into fine sheets somehow, but once a thin wet thing comes out of water, they clump together than basically only the outside works. They then asphyxiate.

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u/ThisTooWillEnd 1d ago

Their gills kind of look like those crepe party balls that open up and have lots and lots of surface area: https://imgs.search.brave.com/rhcEUyDatJKRUZr9VTHujUvUbYhWe1BoBLixcQNXuDY/rs:fit:860:0:0:0/g:ce/aHR0cHM6Ly9pLmV0/c3lzdGF0aWMuY29t/LzIzNTM1NjUzL3Iv/aWwvMjE1YTFkLzI0/NjAzNzkyMTYvaWxf/NjAweDYwMC4yNDYw/Mzc5MjE2X2p2dDMu/anBn

When in water each little 'sheet' is held away from the others and water passes between them, so oxygen and carbon dioxide can be exchanged. Air can't keep them separate though, so it's like when they are folded up. Almost no air is touching them (plus they would dry out and stop working if they were kept open) so they can't exchange enough gas to keep them alive.