Because if it all stays in then your body will keep in this poisons gases. http://wiki.answers.com/Why_does_air_move_in_our_out_of_your_respiratory_system#ixzz1ZCdvdlbk
To begin a breath, your diaphragm contracts. This causes a reduced pressure area in your chest. Since the outside atmospheric pressure is now higher than that in your chest, it causes the air to be...