One of the aspects of climbing Mt.Everest is that it requires oxygen tanks. Then, once at the top, climbers look up and wonder how flocks of birds can fly past at that altitude.
How do birds do that, as flying requires so much energy and thus oxygen?
It is because they are descended from dinosaurs, an animal that evolved at the end of the Permian extinction when there had been an oxygen collapse.
Birds, and dinosaurs, have a physiology that traps air inside their bodies.
Apparently peeple fall into that category. They have a physiology that traps air inside their heads.