The researchers hypothesize that Tibetans have a genetic mutation

The low barometric pressure of high altitudes generally causes low arterial oxygen content among Tibetans, yet the researchers have found that Tibetans consume oxygen at normal rates.
How can some people live at high altitudes and thrive while others struggle to obtain enough oxygen to function?The answer for Tibetans who live at altitudes around 14,000 feet is increased nitric oxide (NO) levels. High levels of NO circulate in various forms in the blood and produce the physiological mechanisms that cause the increased blood flow that maintains oxygen delivery despite hypoxia -- low levels of oxygen in the ambient air and the bloodstream.
In this research, blood flow is determined by the length, number and width of the diameter of blood vessels.
(www.case.edu)