Eli5: If water is 2 thirds oxygen, why do we need lungs?
In: Chemistry
Regardless of what breathing structures you use, gills or lungs, you are taking advantage of a process much like a magnet pulls on pieces of metal.
Imagine this, you have a smallish magnet that is ready to pull on some tiny metal fragments (blood cell with no bound oxygen), the metal floating around the magnet eventually gets pulled into it (oxygen pulled into res blood cell), the magnet moves in the blood until it finds an even bigger magnet (the cells in the body, combined with chemicals such as 23-bpg pull off the oxygen with more strength than that blood cell has), this then repeats.
(Outside of ELI5 material below)
Now that above scenario was done using very little energy (23bpg is a byproduct of regular metabolism and red blood cells can be reused and require very little energy to stay alive). Now imagine you wanted to use oxygen from water particles alone, you would first need to split the hydrogen from the water, a process which takes 260kj/mol, the equivalent of about one third of the energy you get from a single glucose molecule.
If a person needs about 1kg of oxygen per day (roughly 2E25 molecules/day of O2) that equates to about 16,000 kj/day of energy required, which requires 3-4 kg of glucose needed to fuel it.
In other words, you would need 5 cups of straight table sugar to be able to separate oxygen from water to use instead. Not to mention all of the heat generated in the process.
It is muchhh easier to just take the oxygen already in the air, already separate from any hydrogen.
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