Question

Why we breath in oxygen but not nitrogen which is having big percentage​

Answer 1

Breathing an atmosphere of pure oxygen would damage the delicate tissues and blood vessels in our lungs, so it's a good thing that most of our atmosphere is nitrogen.Nitrogen is a fairly inert chemical (it does react, but not nearly as much as oxygen - this is why it is fairly stable in the atmosphere, whereas oxygen has to constantly be regenerated by photosynthesis).

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