Every breath we inhale brings oxygen into our bodies that our cells need to sustain metabolism. Normally we can supply enough oxygen to fuel metabolism,but at high altitude this can become a challenge. As barometric pressure declines with elevation, the air gets thinner and reduces the amount of oxygen we can load into our blood. Consequently, human populations indigenous to the high altitude plateaus of Tibet have evolved to cope with this problem. Tibetans breathe more than low altitude humans at elevation, which helps load oxygen into their blood. However, less is known about how the circulatory system works in Tibetan highlanders, so Serpil Erzurum and colleagues from the Cleveland Clinic, USA, wondered whether oxygen delivery throughout the body is higher in Tibetans. To investigate this, they compared how blood flow is controlled in highlanders from Panam Xiang, a rural district of Tibet that is 4200 m above sea level, with a sea level reference group of humans from the USA.
To get an idea of blood flow levels in the whole body, Erzurum and colleagues measured blood flow to the forearm of 88 Tibetans and 20 Americans,including both male and female subjects. They found that forearm blood flow at rest was twice as high in Tibetan highlanders as in low altitude humans. Furthermore, the Tibetans could increase their blood flow more than the sea level subjects during forearm exercise, better meeting the increased oxygen demands of muscle work. As a consequence of these higher flows, oxygen delivery to the forearm was always much greater in Tibetans. But what caused the higher blood flows in the highlanders?
It is already known that nitric oxide, or NO, is a gaseous substance released by the endothelial cells lining blood vessels, and that it causes blood vessels to dilate, increasing blood flow and oxygen delivery to tissues. However, NO is only active for a short time after release, because it is quickly converted to products such as nitrite, nitrate, and nitroso-protein compounds. The authors wondered whether the Tibetans' higher blood flow could result from higher NO levels, so they measured the amount of NO products in the blood of Tibetans and in the sea level reference group. As they suspected,the highlanders had more circulating NO products than sea level humans,explaining their higher blood flow.
The results of Erzurum and colleagues demonstrate how human physiology can evolve in different populations from diverse environments. They show for the first time that indigenous Tibetan highlanders not only breathe more than their low altitude counterparts, but also circulate oxygen around the body at a higher rate. Simply increasing the production of NO probably causes this difference, emphasizing how a little extra gas can go a long way!
