For Relax Max: Pressure Breathing

Relax Max asked: Finally, is this the same reason high altitude fighter pilots have to concentrate on forcing the air out of their lungs when they are breathing pressurized oxygen/air, or is that an old-wives tale?

Nope, not an old's wives tale - it's a real necessity. Depending on the type of oxygen mask, aviators may need to learn to pressure-breathe.

You see, in normal pressure, we breathe in and out by expanding and contracting our diaphragm, increasing/decreasing the size of a pocket of air inside our bodies, creating a high pressure (exhale)/low pressure (inhale) condition relative to the outside air. Are bodies tend to be tolerant of quite a range of air pressures as long as the partial pressure of oxygen doesn't drop below certain levels or get above certain levels. We can condition our bodies to be even more tolerant., but those tolerance levels have limitations.

Once the outside pressure drops below a certain pressure we need higher concentrations of oxygen, up to and including 100%. This situation is made worse by the fact that a certain portion of air in our lungs doesn't get expelled. As the pressure drops, that portion that doesn't get expelled (and therefore is new air that doesn't come in) takes up more and more volume until, even with 100% oxygen, there isn't enough "new" air for me to survive.

In that case, pressurized oxygen is used. For the type of mask in question, the ambient pressure is higher than ambient, making inhalation easy; however, unless the "used" gas is expelled, you can't inhale again. In this case, you're pushing against a greater pressure than the rest of the body is exposed to. Hence the difficulty.

Fun stuff.