How do fish gills work?

Diana - Well I think they work a little bit like really efficient lungs. So, when a fish opens its mouth, in goes the water, it goes into the gill, and they've got a really thin membrane over which the water flows, and on the other side of membrane, blood is flowing and it's flowing in the opposite direction which means that any oxygen which is dissolved in the water can then go through this membrane into the blood. And they're also really, really, finely made so they've got these things call lamellae and what happens when a fish is out of water, the reason it can't breathe is that these little structures collapse in on each other. And so, it becomes less efficient, there's less surface area exposed to the available oxygen, and they essentially asphyxiate in the air, but yeah, that's how it does it.

Chris - Because human lungs don't collapse like that because they have a surfactant which reduces the surface tension in the water on the surface of these little tiny air sacs, so they don't collapse, but fish don't have that surfactant because obviously, it all would be washing away and they don't need it...