Explain the amount of various gases in an inhaled and exhaled air?

Dunno, but when a lad at school my phsysics teacher, one Jock Noble, told us that you breath in so many billions of atoms in each breath that it is reasonable to ume that when you breath, you could be breathing in an atom which came forth when Julius Ceasar uttered 'et tu Brutus' all those years ago...................