I participated in a trek in Nepal, the high point of which was ascending to the peak of Kala Patthar (chosen because of the great views of Everest it offers; we

I participated in a trek in Nepal, the high point of which was ascending to the peak of Kala Patthar (chosen because of the great views of Everest it offers; we got great photos of an avalanche in progress there). It's 5,644.5 m (18,519 ft). We absolutely felt the lack of oxygen. It was a strange feeling, like nothing else I've experienced. I definitely wouldn't have had the competence to do much else than mindlessly climb up and mindlessly climb down again on the day we did the peak (other than snapping some photos)! My wife is a retinal surgeon. She was interested in the effects high altitude on the retina, since the retina can, in some ways, almost be almost considered to be part of the brain. At a lower elevation than the top of Kala Patthar, she took out a magnification instrument and could spot some mild hemorrhaging on my retina. Not enough to worry about but enough that she could see it. It was interesting and a bit scary to think what could be going on in our brains. I can't imagine how people climb Everest without oxygen. But one factor is that most of them probably take more time to acclimatize than we did. (It took us 2 weeks to trek to the peak.) https://en.wikipedia.org/wiki/Kala_Patthar https://en.wikipedia.org/wiki/Kala_Patthar