Now, scientists at the Case Western Reserve University and Cleveland Clinic have found that people who live in such high altitudes carry elevated levels of nitric oxide in blood.
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, the researchers said.
During the course of their study, the scientists found that Tibetans had 10 times more NO and had more than double the forearm blood flow of low-altitude dwellers.
They said while the low barometric pressure of high altitudes generally caused low arterial oxygen content, yet the Tibetans were found to consume oxygen at normal rates.
As part of her study, Cynthia Beall, the S. Idell Pyle Professor of Anthropology at Case Western Reserve University, collected blood samples and blood flow readings from the forearms of 88 Tibetans during a 2002 National Science Foundation funded research trip.
For comparison, the scientists collected the same information from 50 near sea-level dwellers from the United States.
According to Prof. Beall, the combined increase in NO and blood flow levels resulted in nearly double the amount of oxygen being delivered to the capillary beds in the Tibetans' arms.
During the study, the scientists also recognized another population difference: Tibetan women had higher nitrite and lower nitrate levels than those of Tibetan men, whereas no such gender differences were found among sea-level dwellers.
Prof. Beall and her team have now hypothesized that Tibetans possess a genetic mutation that allows high NO production.
But, genetic studies and comparable data on sea-level populations living at high altitude would be needed to test that hypothesis, she said.
The study, "Higher Blood Flow and Circulating NO Products Offset High-altitude Hypoxia among Tibetans," appears in the current Proceedings of the National Academy of Sciences (PNAS).