What you can't do when the pressure gets too low is breathe. Don't try to climb Everest without supplied air. Your diaphragm can't do the job by itself as breathing also depends on diffusion/osmosis, which won't work if there isn't enough pressure pushing air into your lungs, aveoli, then on out into the blood. (I don't remember the exact mechanics as it has been a long time since I was in school - - which explains why I put it as diffusion/osmosis instead of just one or the other.)

As for low pressure effects on the body: Astronauts/Cosmonauts visit outside the atmosphere and seem to do fine. It's the lack of gravity that starts to damage them.