Artificial gills (human)

From Freepedia

Artificial gills are devices that exist in science fiction only to extract oxygen dissolved in water allowing a human to survive underwater of extended periods.

It is generally thought that in the real world they would have to be unwieldily bulky because they would have to process an unrealistically massive amount of water to extract enough oxygen to supply an active diver, as an alternative to a scuba set.

According to this link, "In sea water, the average amount of oxygen is 7 ppm". Roughly, this means that 1,000,000 kg (1000 metric tons, a 10 m×10 m×10 m cube) of sea water holds 7 kg or roughly 5,000 litres (7 kg/1.42 kg/m³) of oxygen at atmospheric pressure. The most economic diving equipment for oxygen consumption is a fully closed-circuit rebreather, where divers typically consume 1 litre of oxygen per minute. So, the diver would need to pass at least 200 litres (200 kg) per minute (3.3 liters = about 0.66666 of a gallon per second) of normally oxygenated sea water through 100% efficient gills to stay alive, and the gill equipment would be unwieldily bulky. And it would not work in anoxic water.

Natural gills work because all or most animals with gills are cold-blooded and so use much less oxygen than a warm-blooded animal the same size.

But see Like-A-Fish for an ongoing attempt to develop such a system in the real world.