At high-altitude - above about 30,000 feet - you need to have pressure, and not just funny diaphragm things going on, or you'll die. You don't breathe by atmospheric pressure - not at sea level and not higher up. Given sea level atmospheric pressure is equivalent to a water column of 10m, even 5% of that is quite adequate to fill your lungs.
Pressure and oxygen content both need to be adequate: if the pressure drops quickly while you're full of nitrogen, you can get the bends too.