At atmospheric pressure and body temperature the lungs contain nitrogen, oxygen, carbon dioxide, and water vapor where pn, -573 mm hg, po, 100 mm hg, and ph, o-47 mm hg. a) what is the partial pressue of co2? b) what is the percent composition of the air inside the lungs? c) why is your number given above a percent by volume? d) the percent composition of air does not change appreciably with altitude. why, then, does one experience difficulty in breathing at high elevations?