Tibet is the highest region in the world, with an average altitude of 4000 meters. Yet Tibetans seem to do fine despite the low oxygen levels at these dizzying heights. Now researchers have figured out how: Tibetans compensate for the lack of oxygen by boosting the supply of blood to their tissues. The findings may aid efforts to develop treatments for oxygen deprivation after heart attacks and organ transplants.
A lowlander exposed to the low oxygen concentrations of high elevations can quickly fall victim to altitude sickness, whose symptoms include nausea, dizziness, and fatigue. But Tibetans do not suffer from altitude sickness. Previous studies have shown that they have lower oxygen concentrations in the blood of their arteries (ScienceNOW, 16 September 2004). Yet the body tissues of Tibetans consume just as much oxygen as those of people who live at sea level. A group of researchers led by physical anthropologist Cynthia Beall of Case Western Reserve University in Cleveland, Ohio, hypothesized that Tibetans might offset low oxygen levels by increasing blood flow.
Beall and her co-workers studied 88 healthy volunteers from the Tibetan district of Panam Xiang, where the elevation is 4200 meters. They compared the Tibetans to a group of 50 volunteers from the Cleveland area, which is about 200 meters above sea level. The team measured the subjects' blood flow noninvasively using a rubber gauge fitted around their forearms. Twice as much blood coursed through the arms of the Tibetans as through the arms of the lowlanders, even though heart rates and blood pressures were no different between the two groups, the researchers report online this week in the Proceedings of the National Academy of Sciences.
To figure out what accounts for this dramatic increase, the researchers gauged nitric oxide levels in each group. Nitric oxide and its metabolites cause blood vessels to expand and thus carry more blood. The team found that the blood of Tibetans had more than 10 times the concentration of biologically active nitric oxide breakdown products, such as nitrate and nitrite, than did blood from people who lived at sea level, signifying a high level of nitric oxide in their bloodstreams. That's "unprecedented for healthy people," the authors say, although the Tibetans do not seem to suffer any ill effects.
"These remarkable data in Tibetans provide a beautiful demonstration of how nature has evidently exploited" nitric oxide levels to offset the effects of high altitude, says Jonathan Stamler, a cardiovascular disease researcher at Duke University Medical Center in Durham, North Carolina. Mark Gladwin, a researcher in pulmonary medicine at the National Heart, Lung, and Blood Institute (NHLBI) in Bethesda, Maryland, says that the nitrite levels in the Tibetans' blood are similar to those NHLBI researchers are using to treat heart and liver injuries caused when blood is restored to oxygen-deprived tissues in experimental animals. This correspondence provides additional evidence that these concentrations might have therapeutic value, he says. Gladwin adds that NHLBI is now planning a clinical trial that would use similar levels of nitrite to treat heart-attack victims.