Lungs vs Gills

I tried to find the difference between lungs and gills. I know some of you already gave me the answer, but I'm not satisfied with "lungs work in air and gills work in water".

I couldn't find much detailed information in the vast web of internet stupidity, but their chemical workings seem similar. The surface area and the way to create this surface area appear to be the only difference between lungs and gills. It's a structural difference and not a chemical one. Water contains about 20 times less oxygen than air, which explains why we can't breathe water. However, this makes you think fish should be able to breathe air.

The problem is that the structure that creates the enormous surface area of their gills has to be supported by water. "Out of water, the gills collapse like wet tissue paper, and very little surface area is left exposed for gas exchange. Most fish, therefore, can only survive a short time out of water before oxygen deficiency catches up with them and they asphyxiate." What about zero-g? "even in a humid air-filled chamber at zero gravity, the gill filaments will simply adhere to one another.". Too bad...

Can we breathe a liquid if it contains enough oxygen? "If water could hold about 20 times more oxygen than it does, things would be different - there are apparently a few liquids (though not water) that can hold that much dissolved oxygen, and one can breathe a liquid of this sort". Despite their similarities, it is currently believed that lungs evolved from gas bladder that fish use to float, and not from their gills. All vertebrates use Hemoglobin for oxygen transport.