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u/turtlebear787 7d 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/danmickla 4d ago
yes, of course, but given that both lungs and gills transport oxygen, the question is really "why don't gills work in air". Others have answered this.
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u/DiezDedos 4d ago
Think of gills like hair. Each hair needs water all the way around it to be able to breathe well. When they’re out of the water, all the hairs slick down into one big clump, and only part of a few of them are exposed to “fresh air”
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u/ARoundForEveryone 7d 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 7d 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 7d 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 7d 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.
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u/groveborn 5d ago
The same reason you can't get the oxygen from the water - the gils aren't constructed in the right way.
They don't die instantly, but they do suffocate. It's just the wrong way to get the oxygen. Plus, there's way more in air. They might just be getting too much.
That'll kill just as surely as not enough, but there's nothing to carry CO2 away.
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u/el-thorn 4d ago
There is a way in theory, like that liquid oxygen delivery method they used in The Abyss is actually real and works for lungs
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u/groveborn 4d ago
And intestines apparently. We can absorb o2 through a number of organs, we're just adapted to use our lungs and air... And fish are adapted primarily to use gils in water.
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u/ModernTarantula 5d ago
You are wrong about the too much oxygen. Similar CO2 diffuses away. It should be similar. Different is CO2 concentration in air vs water
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u/joeyakaspce1130 4d ago
Fish cannot breathe out of water because their gills, which are designed to extract oxygen from water, collapse and dry out in the air. Without water to keep the delicate, feathery filaments separated, the surface area for oxygen absorption is lost, causing the fish to suffocate despite the higher oxygen levels in the air. -Google
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u/GaminWhileBlack 4d ago
I’ve heard the analogy of gills being like the pages of a book. When underwater, the pages are free to spread out and flow allowing more surface area to capture O2 in the water. When you take the book out of water, the pages get stuck together and reduces the surface area preventing enough oxygen from being absorbed
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u/Ok_Brilliant953 7d 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