Eli5: why can’t fish just breathe air through their gills if they’re out of water?

Question

If fish are out of water, why can’t they just breathe air in and out through their gills. They use their gills to breathe under water……. And are there any animals with gills that live both on and off land? #justgillythings

In: 0

Answer 1

For the same reason we can’t breathe underwater: lungs aren’t designed to separate oxygen from water, and gills aren’t designed to separate oxygen from air.

Gills have lots of capillaries behind them that absorb oxygen from the water and into the bloodstream, and release carbon dioxide into the water. If you replace that process with air, it just doesn’t work.