breathing water

[boobesque]

just watched the film 'the abyss' and it raised a question for me.
in the film the navy seals have a fluid which you can breath. Now, when we breath air, our lungs expand, drawing in the air and the oxygen is absorbed from the air into the blood stream. the excess waste air is then expelled and the process repeats.
So why cant we breath water? Why cant we extract the oxygen from water.
One thing i thought of was the concentration of oxygen in air as opposed to water but surely water with excess oxygen (one water company sells water with 8X more oxygen) which is used as sports drinks would provide enough oxygen.

[Dr.Mario]

Because your lungs would fill with liquid and become unoperable , i'd guess.

[wildwood]

Our bodies need a lot more chemicals besides oxygen. Carbon dioxide being a very common one, this 'hangs' in the bottom of out lungs and it is crucial to have a proper oxygen/CD balance.

When we hyperventilate through stress or fear, our lungs have a shortage of carbon dioxide, not oxygen. That's why breathing in an paper bag works because we inhale the previously exhaled CD. Breathing fresh air deeply in those situation only makes matters worse.

Fish have different blood to mammals and use a lot less oxygen so they can utlilise the oxygen in the water.

[SteveD]

Have a read here:

http://en.wikipedia.org/wiki/Liquid_breathing

[Kuin]

Because water is made of H2O ... the oxygen is attatched to the hydrogen molecule. What our bodies need is pure oxygen, to be carried by the red blood cells through the body, which cannot be taken off of the H2O.

Something like that anyway. :)