How do Tibetans avoid altitude sickness?

In 2005, a group of researchers from Case Western University in Cleveland, Ohio, traveled to Tibet to test a hypothesis of why the Tibetan people aren't afflicted with high blood pressure and other maladies resulting from living at such high altitudes. They found the answer within the Tibetans' breath.

The researchers discovered that Tibetans exhale much less nitric oxide (NO) than a control group living at sea level did. What's more, the Tibetans' lungs transferred twice the amount of nitric oxide from their lung walls into their bloodstreams than their sea-level-dwelling counterparts'. Nitric oxide is believed to aid in the expansion of blood vessels. Blood flows more easily, which allows the heart to work at a normal pace, due to the decrease in blood pressure from vessel expansion.

That means that the Tibetans' hearts can deliver more of the lower ambient oxygen available in the air to their bodies. With the dilated blood vessels, Tibetans can achieve this with less effort than a person at the same altitude whose cardiopulmonary system is used to near-sea level pressure.

This represents a strong example of humans evolving to adapt to their environment. Humans living at high altitudes have adapted to the unusual atmospheric conditions, and it stands to reason that this would be found throughout the world wherever humans live at elevations similar to those in Tibet. Except it isn't.

The first study of people living at high altitudes came in 1890, when Frenchman Francois Viault studied the red blood cell count of people living in the Andes Mountains of South America. Red blood cells carry hemoglobin, the part of the blood that carries oxygen. So Viault theorized that the Andean people would have a high red blood cell count. He was right. Andeans developed a process that compensates for the lack of available oxygen in the thin mountain air. But this trait -- or phenotype -- is not found in Tibetans. Conversely, Tibetans' higher use of nitric oxide isn't found in Andeans.

A third group, the people of the Ethiopian highlands, don't have either of these traits. In fact, the Ethiopian highlanders don't appear to have any special traits to compensate for life at higher altitudes. The characteristics of their cardiopulmonary systems -- like oxygen saturation and hemoglobin count -- are virtually identical to those found in people living at sea level.

It's possible that the Ethiopians do possess a trait that has yet to be discovered; the highlanders have only been studied once, while the Andeans have been studied for more than a century and the Tibetans for decades. But the differences that have been found between Andeans, Ethiopians and Tibetans represent human biodiversity. This is significant, because it is through diversity that a species can thrive on Earth.

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