Humans absorb oxygen from air in the lungs, fine.
If you put water in the lungs the dissolved oxygen in the water somehow(?) doesn’t get absorbed, fine.
But, somehow(?) fish can absorb oxygen from water.
OK? How come human lungs can’t absorb oxygen from water but fish gills can absorb oxygen from water?
It’s literally just diffusion?
**EDIT: This thread is cancer. I will try to find the answer somewhere else. Turning off replies.**
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lung are not made to absorb oxygen from water, just like gills are not made to absorb oxygen from air
Fish gills are like sheets, and when they are not in water then they stick together, causing them to suffocate.
Human lungs don’t have the efficiency to move water past, absorb air, and then cycle more fresh water.
Basically lungs are less efficient so we need straight air, but the more efficient gils fail when dry.
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Earth surface water has enough partial pressure of oxygen to keep you alive, and lungs have a much greater surface area than gills.
The diaphragm, however, is not strong enough to push water in and out of the lungs at the rate needed for gas exchange, so you still drown.
To make it really simple? Yes, simple gas exchange.
There are fish who have gills and can breathe oxygen from the air, and there are liquids with enough oxygen that you can breathe through them (as shown in the 1989 movie *The Abyss*). Why can’t we all just breathe in both ways? Well, because of evolution.
When vertebrates came to land in search of a new place to feed, they became adapted to breathe in a way that was easier on land, using either the skin directly, or a with a gas exchanging internal organ. The problem with that, one that still exists today, is that the gas exchange needs a liquid interface. Amphibians need to keep themselves in high humidity, we have to keep our lungs permanently covered in mucus… while the fish just protect their gills with a simple bony flap. Even crustaceans that live mostly on land, developed a way to keep their gills in water.
These adaptations are usually one-way, depending on how far you are from a humid environment.
There is a lot less oxygen in water than in air and our lungs are terribly inefficient and also not optimized for underwater.
Fish gills are very efficient, they use [countercurrent exchange](https://en.m.wikipedia.org/wiki/Countercurrent_exchange) to pull much more oxygen out of the water. Theres other factors too like we would struggle to pump water in and out and the water would wash away surfactants, etc. that help our lungs function.
Fish’ gills are like sheets that are designed to allow water to flow over them at high rates, allowing the water that has had oxygen removed to move on quickly so that the next, oxygenated, bit of water can take its place.
Lungs are made of millions of little sacs designed to have a high surface area, sacs that would be hard to empty out without squeezing them out completely like a sponge – taking a lot of effort and squeezing room we don’t have.
Water also has a lower concentration of oxygen than air does, since you’re relying on the amount of air dissolved and how quickly you can get it out of the water, as opposed to breathing air where the oxygen is freely available. The mucus lining of our lungs just hasn’t evolved to efficiently pass dissolved air through either.
In short, you’d need to be able to breathe really really fast, and since our lungs are inside our body and would need to be emptied each time, you’d need to breathe really really hard.
There have been some experiments that have shown an ability to breathe liquids that have an extremely high free oxygen content, even through our intestines instead of our lungs.
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