A trip to Tibet can be a transcendent experience. The Dalai Lama’s homeland is known as the “roof of the world” — a high, windswept place with roaring rivers, snowcapped peaks, and pastoral villages. But a Tibet visit can also mean a nasty case of altitude sickness for people who are used to the atmospheric pressure found at lower elevations. This illness, also known as hypoxia, is the result of a lack of oxygen to the tissue in the body. Hypoxia’s symptoms include nausea, vomiting, lethargy, confusion, and breathlessness. It can also be fatal.
A person struck by altitude sickness may wonder how the people of Tibet can live at such high altitudes without suffering hypoxia themselves. Researchers have studied Tibetans to find out why they can live in settlements averaging 16,000 feet above sea level. (By contrast, the city of Denver, Colo. is 5,280 feet above sea level.) As it turns out, Tibetans actually are hypoxic; however, their bodies have simply developed a unique way of dealing with the lower atmospheric pressure. But how? To understand the answer to that question, we first have to understand why people shouldn’t be able to thrive at such high elevations.
The human body’s oxygen-distribution system has developed over hundreds of millions of years. You inhale oxygen into your lungs. Oxygen is then transferred to blood, which then distributes the oxygen via hemoglobin, the part of your blood that carries oxygen.
There is about the same amount of oxygen (around 21 percent) in the air regardless of elevation, but because of a lack of atmospheric pressure at high altitudes, it’s harder for the human lungs to absorb. Our cardiopulmonary system (the heart and lungs working together to get oxygen to the body) has to work overtime at high altitudes to get the oxygen we need. This leads to hypertension — or high blood pressure.
Anthropologists believe the reason people have trouble coping with the thin air at high altitudes is that the human body developed at or near sea level. The Tibetans are an anomaly: They shouldn’t have been able to thrive on the roof of the world for thousands of years.
So how did they adapt?
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 Vault 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.
- Beall, Cynthia M. “Andean, Tibetan and Ethiopian patterns of adaptation to high altitude hypoxia.” Journal of Integrative and Comparative Biology. February 2006. http://icb.oxfordjournals.org/cgi/reprint/46/1/18?maxtoshow=&HITS=10 &hits=10&RESULTFORMAT=1&author1=beall&andorexacttitle=and& andorexacttitleabs=and&andorexactfulltext=and&searchid=1&FIRST INDEX=0&sortspec=relevance&fdate=2/1/2005&resourcetype= HWCIT
- Cachel, Susan Ph.D. Physical Anthropologist, Rutgers University. Personal correspondence. December 12, 2007.
- Easen, Nick. “Man set foot in Ice-Age Tibet.” CNN April 18, 2002. http://edition.cnn.com/2002/WORLD/asiapcf/east/04/17/tibet.iceage/
- Hoit, Brian D. et al. “Nitric oxide and cardiopulmonary hemodynamics in Tibetan highlanders.” Journal of Applied Physiology. July 14, 2005. http://jap.physiology.org/cgi/reprint/99/5/1796?maxtoshow=&HITS=10&hits =10&RESULTFORMAT=&fulltext=Nitric+Oxide+and+Cardio-Pulmonary+ Hemodynamics+in+Tibetan+Highlanders.&searchid=1&FIRSTINDEX=0& sortspec=relevance&resourcetype=HWCIT
- Umer, Mohammed. “Biographical information.” University of Addis Ababa. 2005. http://www.pages2005.org/mediaroom/umer.html
- “Altitude illness.” Centers For Disease Control. June 18, 2007. http://wwwn.cdc.gov/travel/yellowBookCh6-AltitudeIllness.aspx