High-altitude adaptation in humans is an instance of evolutionary modification in certain human populations, including those of Tibet in Asia, the Andes of the Americas, and Ethiopia in Africa, who have acquired the ability to survive at extremely high altitudes.
People living in high altitude can inhale more air with each breath and breathe more rapidly than either sea level populations or Andeans. Tibetans have better oxygenation at birth, enlarged lung volumes throughout life, and a higher capacity for exercise. They show a sustained increase in cerebral blood flow, and less susceptibility to chronic mountain sickness than other populations, due to their longer history of high-altitude habitation
They can expands their blood vessels, allowing them to deliver oxygen throughout their bodies more effectively than sea-level people do. For example Tibetans’ lungs synthesize larger amounts of a gas called nitric oxide from the air they breathe. One effect of nitric oxide is to increase the diameter of blood vessels, which suggests that Tibetans may offset low oxygen content in their blood with increased blood flow.