If the air is 78% nitrogen and 21% oxygen, why do we say we breathe in oxygen but not nitrogen ?

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Thanks for this question, over to the scientists in the house. I'm a science educator

niyidenrele:
If the air is 78% nitrogen and 21% oxygen, why do we say we breathe in oxygen but not nitrogen ?

simple na. You breath in everything, it's what the body use which is the oxygen because of the hemoglobin in the blood they are referring to. The nitrogen is inactive

While inhaling, the composition of air that goes in is-(78% Nitrogen, 21% Oxygen, 0.04% CO2, 0.1%argon and some other rare gases) just like it is in the atmosphere
But
While exhaling, the composition of air that goes out is - (78%Nitrogen, 16% Oxygen, 5% CO2, 1% argon, H2O...)
.
There's is...
- no change in %composition of Nitrogen because it doesn't react in human body/hemoglobin in the blood (in short it's of no use)
-decrease in oxygen because part of it would've reacted with haemoglobin
-increase in CO2 as CO2 from the process of metabolism( ie digestion) would have been added to it
So we say we breath in Oxygen and not Nitrogen because, Oxygen is what the Body uses
I hope this answers your question

The two posters above me are correct. Also, if you prefer, you can say that we breathe air instead of oxygen.