gums.
Right about the partial pressure. Perhaps another way to explain it to the uninitiated. (Aeromed lectures a long time ago.)
Call the pressure of air at sea level 100. Oxygen then has a partial pressure of 20, sufficient to force it into the bloodstream. At about 35,000' the pressure of the air in total is only now about 20. 100% oxygen from an economy (non-pressure) mask is still 20, so OK. Above this it drops with altitude, necessitating pressure breathing to get it back up again. As I understand it anyway.