Question: why can fish breathe under water?

Ben Brilot answered on 14 Mar 2013:

Fish need oxygen just like we do. The difference is that they extract oxygen out of water while we extract it out of the air. Effectively they’re doing exactly the same thing as us: getting oxygen molecules to pass from the environment into their blood through a very thin layer of cells (a membrane). In people that happens deep inside your lungs. In fish it happens in a specialised organ called the gills. These are just like lungs: they’re made of lots of very small blood vessels that come very close to the surface of the skin. But in fish they’re on the side of the ‘neck’ and they have an opening to the water. This means that as they swim, water passes in through the mouth, over the gills and then out into the sea or river. As it passes over the gills, oxygen passes from the water into the fish’s blood. The major difference is that fish lungs are much more efficient than ours because there’s much less oxygen in water than there is in air (which is why we can’t breathe in water). Fish can’t breathe in air because they need water passing over their gills. Once you take them out of the water, the gills collapse and the fish can’t breathe (although there are some fish that can breathe in air, but they don’t use gills: they have a type of lung, surprisingly enough they’re called lungfish). Hope that explains things.