Question about respiratory physiology

Question: Assume if you have flown in a commercial airliner, you have heard the flight attendants instruct you that in case of a suIDen loss of cabin pressure, oxygen masks will be deployed. At a cruising altitude of 30,000 feet the “oxygen” arrives in your mask at the surrounding atmospheric pressure, which is substantially lower than the atmospheric pressure at sea level. Why does breathing an oxygen-enriched gas mixture help keep you alive at high altitude, when the pressure is so low overall? Can you tell if you’re getting enough oxygen, and why is it necessary to put on your own oxygen mask first before helping someone who needs assistance?

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