FYI whilst looking up other things I happened upon an interesting scientific observation : that because Tibetans in the himalayas have such a long history at high altitude they have physiologically evolved extra blood flow in their brains.
from wikipedia- Tibetans demonstrate a sustained increase in cerebral blood flow, lower hemoglobin concentration, and less susceptibility to chronic mountain sickness (CMS) than other high altitude populations. These adaptations may reflect their longer history of high altitude habitation.[19][20]