Dear student,
The air that we breathe in consists of many gases like nitrogen - 78%, oxygen - 21%, carbon dioxide - .03 - .04%, hydrogen - traces etc. Nitrogen and other gases have no effect on the workings of our body and so it is exhaled back when we breathe out.
The only gas in the air which is used by human body is oxygen. The oxygen from lungs enters into blood where from it is supplied to all cells through heart.
Regards