So from my quick googling it looks like oxygen absorption lowers with pressure (~ altitude), so the higher you go the more oxygenated air you have to breathe to

So from my quick googling it looks like oxygen absorption lowers with pressure (~ altitude), so the higher you go the more oxygenated air you have to breathe to maintain proper oxygen levels in blood. Source: https://www.inkling.com/read/guyton-hall-textbook-of-medical-physiology-12th/chapter-43/effects-of-low-oxygen-pressure https://www.inkling.com/read/guyton-hall-textbook-of-medical...