How do fish gills get oxygen from water, and can it be done mechanically? — Peadar, Lurgan, Ireland
In lots of science fiction movies you see people breathing underwater like a fish. It happens in Waterworld (Kevin Costner has gills). It happens in Star Wars (Jedi Knights carry tiny mouth pieces in their pockets that let them breathe underwater).
Meanwhile, real human beings seem so be stuck with bulky compressed air tanks or rebreathers.
The gills of a fish work a lot like your lungs do. In your lungs, blood in capillaries flows near the surface of tiny air sacks. Oxygen transfuses from the air in the sacks into the blood, and carbon dioxide transfuses from the blood to the air in the sacks. A fish’s gills work exactly the same way, except that it is water instead of air and the gill structures that handle the transfusion are visible (and actually dangling in the open water) rather than hidden inside the lungs.
There is such a thing as “artificial gills” that mechanically extract oxygen from water for a human to breathe. There is a nice interview with the inventor (Alon Bodner) and a description of the mechanism here: Like a fish – underwater breathing system. See also Breathing in oceans full of air. As mentioned in the first article, it would be possible to separate water into oxygen and hydrogen with electrolysis. Instead the apparatus uses a centrifuge to lower the pressure of water and release the air that is dissolved in it. The air rises our of the water like bubbles in a carbonated beverage.
For more info see: How closed circuit rebreathers work