eli5 Gills can absorb oxygen from an aqueous environment. Why can’t gills absorb oxygen from a non-aqueous environment?

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Additionally, in the instances like salamanders that absorb oxygen through their skin is this the same function?

In: Biology
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They still can as long as they are wet. Gills don’t work that much different from lungs really, the big difference being that gill’s dry out much more quickly.

When you first pull a fish out of water, at first it is not suffocating. It actually has the opposite problem, and is dealing with too much oxygen, poisonous levels of oxygen. If a fish could keep its gills wet long enough, this would end up killing it. But instead of eventually the gills dry out and then it dies of suffocation.

They can, but they collapse and stick together when out of the water and as a result there’s far far far less surface area for oxygen exchange exposed to the air than exposed to the water. If you could somehow keep gills fully open and exposed in air, they would likely work. Gills work in water because there is a huge amount of surface area presented to the water. Lungs don’t need to present nearly as much surface area for the same amount of oxygen absorption because there’s nearly 20 times the amount of oxygen in air as their is in water. Lungs avoid collapse by having lots and lots of tiny pockets that stay open in air, so there’s plenty of surface area for gas exchange.

Gills are like tiny little hairs made out of special meat that can pick up oxygen in the water. These meat hairs are better at picking up oxygen when they are in water. When they are in the air, they can still pick up oxygen, but they dry out really fast because they are so used to the water. Remember Spongebob when he went to visit sandy for the first time? He got really dry and started to crack. The same thing happens to the gills and they can’t pick up oxygen as well anymore.

The thing is though, if you can keep your gills wet somehow, you can go in air just fine. Crayfish are a good example; the gills are located in the thorax – inside the body – so they have some control of humidity (plus the enclosed space helps), and also temperature to some extent by their location in sun/shade. This is how they colonize new areas: they go for a walk over land and look for new water bodies to live in. How far they can walk depends on species (some are better at it than others), but still, they have more distance before drying out than you may think.

The main issue is interface, which water provides in a much better way than anything else. Gills have the blood vessels all disposed in an array that allows the water to pass through, which in turn allows the oxygen to be captured by the oxygen-carrying molecules of the organism. Land crustaceans, like some crabs and woodlouses, also breathe through gills, secreting a liquid over the gills to capture the oxygen in the air
Salamanders start their life with gills, some never lose them, and some develop lungs but still keep the skin as a breathing interface… but it needs to be always wet.

More concisely, just about anything living needs water in some way to work (specially if involves proteins) , and breathing is such a case.