this thread is getting too scientific - but I will add to the pot.
One of the reasons man (or woman, Cindy) can survive (at least for a while) at extreme altitude (like Everest) is because of the mystery of acclimatization. These changes are far more pronounced than those that occur in well-acclimatized Whitney hikers.
Acclimatization involves multiple physiological changes that include these and more: altered ventilation, making more red blood cells, incompletely understood metabolic changes at the intracellular level, and shifts in the oxyhemoglobin dissociation curve. That is how extremely low pressure inspired air at extreme altitude can yet result in a higher useable level of O2 in the blood, and just as importantly, release of O2 from the blood into the tissues. Said another way, if all your blood did was pick up ambient low pressure gaseous O2 then you would be long before dead. The miracle of hemoglobin makes life possible over a wide range of O2 pressures.
This graph is the famous oxyhemoglobin dissociation curve. If standing on the top of Everest ( without using bottled O2) the horizontal PaO2 would be 25-30 range, incompatible with life. But on the vertical axis you see that with a high-altitude left-shifted curve, ones hemoglobin saturation % ("O2 sat") is a just barely life-possible 60%. I can tell you that it really doesn't feel good unless it is 75, and even that would be low enough to put you in the ICU back home.
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