How do fish get their oxygen under water?
Not so differently from the way we get it from air. Water contains dissolved
oxygen, enough of it to support life. It's just a question of extracting it. Gills are
specialized structures in which the blood comes into contact with the water over a very
large surface area, just like in the lungs, blood comes into contact with the air over a
large surface area.
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Update: June 2012