Let's say you accidentally walk into a room pressurized with pure nitrogen (or you're jettisoned into space). Within a couple of seconds, the partial pressure of oxygen within your lungs drops to 0. But there's still a (small?) reserve within your bloodstream.
Now, will the oxygenated blood travel to your lungs? If so, will the usual gas exchange reverse, with oxygen going from your blood to the oxygen-poor environment?