There's oxygen in water, why can't you breathe underwater?

The answer to your question though is that fish bodies, in particular their gills, cannot handle being out of water and thus can't handle breathing and getting oxygen through their blood.

What about air can they not handle?

Gills function by having a very high surface area with a bunch of “plates” that are stacked on top of each other. Water flows in between all these plates fully wetting the top and bottom of each for a huge total surface area.

Out of the water the plates stick together like wet leaves and the fish suffocates with 90+% of the gill surface now blocked and the exposed area dangerously dry.

Some fish do have some creative solutions to breathing air though, as an adaptation to nasty swamp water or low tide.

the gills are very delicate structures that rely on being "fluffed out" to have enough surface area to absorb the oxygen.

this is fine while underwater, but when you take them out into the air, their gills hang down all matted.

it's like your hair that floats freely underwater but straggles down in wet clumps when you surface.

the water also forms a barrier preventing circulating air from touching the gills. so until the gills are dry and fluffed out again, there's no chance of absorbing any fresh oxygen at all.

even then the surface area just isn't enough to sustain life. we enclose our oxygen absorbing surfaces inside our chest and our lungs fill the majority of our chest. that's a whole lot of absorbing area and mechanics to move a lot of air across it.

so basically, the fish suffocates before the gills dry. but they're too small and not structured properly to sustain life in air regardless.

Yes, that's well known. the op wants to know why. Read the question again

Who says that I can’t

Air dries out and cannot support the fish's gills, causing them to collapse. Once they collapse, they aren't contacting nearly enough air to exchange enough oxygen to keep the fish alive.

Think of how seaweed or kelp in the water can use its buoyancy to stretch itself out, but once out of water, it's just collapses into a heap.

its because the very delicate gills collapse and dry out quickly in air when supported and hydrated by water, and thus the fish effectively suffocates. Its the other side of the coin where we can't breathe underwater because the structures in our lungs are not designed to be able to efficiently take oxygen out of the water that is now surrounding them.

However this depends on the fish in question though. some fish are more hardy and can withstand longer periods outside the water. and some fish have special features that let them partially breath air, such as modifying parts of their digestive tract to function as a lung, using their swim bladder as a modified lung, or developing an entirely separate lung-like organ for use. These fish usually live in areas where water quality can become poor. Surprisingly a large number of popular fish kept in home aquariums have these features, such as Bettas, Gouramis, Plecos, Cory Catfish, Oto Catfish, and Ropefish

ELY5: Fish have special lungs called gills, which are like a stack of delicate tissue paper, with many layers of thin tissue all sitting next to each other. The surface of each tissue paper needs to not touch its neighbours in order to absorb oxygen from the water. If you soak a stack of tissue paper in water, and then take it out; it will be impossible to peel the tissues apart. This is exactly what happens to fish gills once they leave their water home.

Other people are giving you the right answer - it's about surface area and the gills collapsing. But I feel like that needs a more ELI5 description:

Picture someone with long straight hair going underwater. In water, a fish's gills are like a person's hair behaves under water. It's all spread out in the water and every individual strand is floating freely. There's a lot of hair surface touching the water.

When a fish leaves the water into air, their gills act like wet hair when you get out of the water. It all "collapses" into clumps. The amount of contact between hair strands and the surroundings is a tiny % of what it is under water.

Fish rely on their gills being all spread out and free-floating to have enough surface area to pick up enough oxygen. It's not that gills are unable to extract oxygen from air and can only get it from water. The problem is their physical structure prevents them from touching enough air to keep the fish alive.

Same subject but slightly off-topic perhaps: so when fish extract oxygen from water they're freeing the hydrogen?!?

Nope, they're extracting oxygen gas that's dissolved in the water, not the O from H2O.

That's a much more difficult process.

Alright I'm really curious, what are the creative solutions?

Mudskippers have more rigid gill plates that don't collapse, and do the whole "breathe through my skin" thing that frogs do too.

Bettas have an organ called the "labyrinth" behind the gills that they suck air into like a false lung and then bubble out later.

Corydoras Catfish eat the air and absorb oxygen in their digestive system.

And of course lungfish are close relatives of our own ancient ancestors and straight up have lungs, but decided against the whole land thing.

Its because they need water to pass through their gills, for the oxygen extraction process to work, pure air will not work.

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ELI5 Then how can some animals can breathe when they are underwater and on the ground? (i.e. crabs)

Holy crap thats so cool

>"breathe through my skin" thing that frogs do

Huh. So Kojima didn't make it up entirely out of his imagination, that's interesting. I assume there is something that would stop it working with a human though, right?

Two things

First, humans are simply much larger and heavier, and have much more mass per surface area to try and oxygenate - and it must travel much farther to reach the critical organs. It’s not a very efficient system so most amphibians are very small and many supplement with lungs, gills, and/or tricks to increase surface area.

Second, if you’ve ever met a frog you may have noticed that they spend 99% of their time motionless staring into the abyss. They’re cold blooded and have poor oxygenation capacity, and so have very little metabolic energy reserve to spend. They move infrequently and tire quickly. Humans are tireless hyper-endurance athletes by comparison. We can move all day and burn oxygen at a much faster rate - but need much more food to do so.

That’s one of our evolutionary advantages, even by warm blooded mammal standards humans are tireless athletes. We’re not the fastest or the strongest, but we can pursue for hours like a horror movie slasher until any prey is absolutely gassed.

because they are the ultimate lifeform

Thanks for the knowledge friend! I suspected that it was something to do with our warm blooded nature, nice to see it as a side by side comparison.

One last question, what sub section of learning gave you this information to so readily share with me?

A fishtank full of rowdy corydoras catfish! …and a background in chemistry.

I'm pretty sure this has been asked recently. Lungs are designed specifically to exchange gas with blood. They've got a huge amount of surface area to do this. Mammals have alveoli, and other vertebrates have parts that function similarly but the structures can be different. The big thing here is that the air sacs can expand when a breath is inhaled and there are a bunch of capillaries lining the air sacs that exchange gas through the cells in the air sacs. Gills don't really expand like that. They're flexible but they don't increase in volume nearly to the degree that lungs can. Gills are also very delicate and are dependent on the surrounding water for structural support. They can't do their job if the water isn't supporting them. They still perform gas exchange, but in the case of gills, the gas comes from the oxygen dissolved into the water. This also means that there has to be gas dissolved in the water or the fish will die from oxygen deprivation.

Crabs have gills under their shell near their legs. They have an appendage that pushes water over them called a scaphognathite and can seal the water inside the shell with articulating plates to keep the gills wet. When they are out of the water and blowing bubbles out of what looks like their mouth, that's the result of them breathing with their gills. As long as they can keep finding more oxygenated water, aquatic crabs can stay out of the water for a surpassing long time, like a day or two. Land crabs have a weird lung gills hybrid organ but they're adapted to land life and just seek out water to keep that organ wet. Coconut crabs are a type of hermit crab that can't swim and will drown in about an hour if they are swept away or fall into water