The people of the Tibetan Plateau survive and thrive on the roof of the world, a region averaging 14,763 feet (4,500 meters), or nearly three miles, above sea level. The air at that elevation is not the rich soup of oxygen that humans enjoy at lower elevations. Instead, as many would-be mountain climbers have discovered to their chagrin, it is difficult to get enough of the life-enabling element into their lungs and blood as they ascend, which often results in debilitating symptoms including nausea and dizziness, and can even be fatal. According to new research, Tibetans avoid altitude sickness because they have broader arteries and capillaries delivering oxygen to their muscles and organs.
"At the same time that [Tibetans] are extremely hypoxic at high altitude, they consume the same amount of oxygen that we do at sea level," says anthropologist Cynthia Beall of Case Western Reserve University in Cleveland. "One of the ways they do that is to have very high blood flow—delivering blood to tissue at twice the rate that we are."
The Tibetans increase their blood flow by producing prodigious amounts of nitric oxide in the linings of the blood vessels. This gas diffuses into the blood and forms nitrite and nitrate, which cause the arteries and capillaries to expand and deliver oxygen-bearing blood to the rest of the body more rapidly than normal. By measuring blood flow in the forearm, Beall and her colleagues report today in Proceedings of the National Academy of Sciences that 88 Tibetan men and women move twice as much blood as 50 Clevelanders, the latter of whom reside a mere 675 feet (205 meters) above sea level. Also, the nitric oxide by-products circulating in Tibetan blood are 10 times greater.
In fact, the Tibetan levels of these nitrites and nitrates are higher than those in patients suffering from a bacterial blood infection—septic shock—and the blood flows are typical of people suffering from high blood pressure. Yet, they have no ill effects in Tibetans. "We don't see an increase in vascular resistance," Beall says. The Tibetans also appear to have higher levels of antioxidants in their bodies, perhaps to help reduce the risk of putting so much nitric oxide—a free radical—into their bloodstreams.
Tibetans breathe a lot, too, averaging more breaths per minute than lowlanders or even their peers in other highland regions, such as the Andes of South America, the latter of whom boast larger lungs than the average human. Also, giving Tibetans pure oxygen actually slows their heart rates by 16 percent. But these scientists say that Tibetans' ability to produce higher levels of nitric oxide is the key to their ability to thrive among the world's tallest peaks.
That raises the question of whether this is evidence of evolutionary functional adaptation in humans. "You have to identify the gene and identify the gene variants that are different," Beall notes. "But it sure seems like a reasonable hypothesis at the moment."
The Tibetans have lived for an estimated 20,000 years on the plateau that bears their name and, in addition to conquering Mount Everest—along with other mountains that number among the planet's highest peaks—on a regular basis, they have managed to build and sustain great societies under challenging conditions. The secret to that success may be encoded in their genes. "This is an example of adaptation to high-altitude hypoxia," Beall says. "Tibetans know they are special because they live at high altitude without getting sick."