Simply stated… We inhale the exact same proportion of oxygen at high altitude as you do at sea level, but because the gas is at a lower pressure, the oxygen molecules are not pushed into our bloodstream with enough force to adequately oxygenate your tissue. Whether you are at sea level or standing on the summit of Mount Everest, the atmosphere remains roughly 20.9% oxygen. It is entirely the lack of overall atmospheric pressure—specifically the resulting drop in the partial pressure of oxygen—that makes it harder to breathe. Here is the mechanical breakdown of why this happens: