Academic Reading — Test 37

Reading passage

For most people, a rapid ascent to high altitude is an uncomfortable and occasionally dangerous experience. Above roughly 2,500 metres, the proportion of oxygen in the air remains unchanged at about twenty-one per cent, but the falling atmospheric pressure means that each breath delivers far fewer oxygen molecules to the lungs. The body's tissues are starved of the oxygen they need, a condition known as hypoxia. Visitors who climb too quickly may suffer headaches, nausea, breathlessness and disturbed sleep, a cluster of symptoms collectively termed acute mountain sickness. In severe cases, fluid can accumulate in the lungs or the brain, and without descent or treatment the outcome may be fatal. When a lowlander travels to the mountains and stays for several weeks, a series of gradual physiological adjustments begins. This process is called acclimatisation, and it represents the body's attempt to compensate for the thinner air. The first response is an increase in the rate and depth of breathing, which helps draw more oxygen into the lungs. Over the following days the kidneys excrete more alkaline urine to counteract the chemical imbalance that rapid breathing creates in the blood. Within a week or two, the bone marrow is stimulated to manufacture additional red blood cells, the carriers of oxygen, so that the blood is able to transport more oxygen to the tissues despite the shortage in the air. These changes are temporary; once the individual returns to sea level, the body soon reverts to its former state. The Sherpa of Nepal, together with other long-established highland peoples such as the Tibetans, present a striking contrast to these visitors. Having lived on the Tibetan plateau for many thousands of years, they thrive at elevations that leave newcomers gasping. Curiously, the Sherpa do not rely on the most obvious strategy of producing extra red blood cells. Lowlanders who remain at altitude for long periods, and indeed the inhabitants of the high Andes in South America, often develop very thick blood crammed with red cells. While this carries more oxygen, it also makes the blood more viscous and harder for the heart to pump, raising the risk of clots and other complications. The Sherpa, by contrast, maintain a relatively modest concentration of red blood cells, and their blood flows more freely. Instead of thickening their blood, the Sherpa have evolved a suite of subtler adaptations that allow their bodies to use the available oxygen with remarkable efficiency. They tend to breathe faster than lowlanders even at rest, and the small vessels supplying their muscles are more numerous, delivering blood to the tissues more effectively. At the cellular level, research suggests that Sherpa muscles are better at generating energy without consuming large quantities of oxygen, partly because they manage their mitochondria, the tiny structures that power each cell, in a more economical way. One widely studied difference involves a gene known as EPAS1, sometimes called the "super-athlete gene". A particular version of this gene, which appears to have entered the human population through interbreeding with an archaic group called the Denisovans, prevents the Tibetans and Sherpa from over-producing red blood cells in response to low oxygen. It is a genuine genetic inheritance rather than something acquired during a single lifetime. The advantages of these adaptations extend beyond mere survival. Sherpa women experience fewer complications in pregnancy at altitude than lowland women would, and their babies tend to be born at a healthier weight. Such reproductive success is one reason the beneficial gene variants have spread so widely through the highland population over the generations, a clear example of natural selection operating in our own species within a comparatively short span of time. Understanding how the Sherpa cope so well is not merely of academic interest. Hypoxia is not confined to the mountains; it also occurs in patients with severe lung or heart disease, and in those undergoing major surgery. By studying populations that flourish where oxygen is scarce, medical researchers hope to develop better treatments for the many people at sea level whose tissues are, for very different reasons, deprived of oxygen. The quiet endurance of the Sherpa, refined over countless generations on the roof of the world, may therefore hold lessons that reach far beyond the Himalaya.