Academic Reading — Test 94

Reading passage

For most lowland visitors, arriving in the high Himalaya is a physiological shock. Above roughly 2,500 metres, the air contains the same proportion of oxygen as at sea level, but the falling atmospheric pressure means that each breath delivers fewer oxygen molecules to the lungs. The result is a condition known as hypoxia, in which the body's tissues are starved of the oxygen they need. Newcomers who ascend too quickly may suffer headaches, nausea, breathlessness and disturbed sleep, a cluster of symptoms collectively termed acute mountain sickness. In its most severe forms, fluid can accumulate in the lungs or the brain, and without descent the outcome may be fatal. Against this background, the apparent ease with which the Sherpa people of Nepal live and labour at extreme elevations has long fascinated researchers. When a lowlander spends several weeks at altitude, the body begins a process of adjustment called acclimatisation. The most immediate response is an increase in the rate and depth of breathing, which helps to draw more oxygen into the bloodstream. Over subsequent days the kidneys excrete more bicarbonate, allowing the blood chemistry to compensate for the changes brought about by faster breathing. A slower but well-known adaptation is a rise in the number of red blood cells, the cells that transport oxygen around the body. This thickening of the blood was once regarded as straightforwardly beneficial, yet it carries a hidden cost: blood that is too rich in red cells becomes more viscous and harder for the heart to pump, raising the risk of clots and of a chronic illness sometimes seen in long-term high-altitude residents. Crucially, the Sherpa do not rely on the same strategy. Studies comparing Sherpa with acclimatised lowlanders have repeatedly found that the Sherpa maintain relatively modest concentrations of red blood cells, closer to sea-level values than might be expected. Rather than carrying more oxygen-bearing cells, their bodies appear to use the available oxygen far more efficiently. Measurements taken during exercise suggest that Sherpa muscles extract a greater fraction of the oxygen delivered to them and waste less energy in doing so. This pattern, refined over many generations of life at altitude, allows the Sherpa to avoid the dangers associated with excessively thick blood while still meeting the heavy physical demands of mountain life. The roots of these differences lie partly in the Sherpa's genetic inheritance. Researchers have identified particular variants of genes that influence how the body senses and responds to low oxygen. One gene that has attracted special attention helps to regulate the production of red blood cells and the dilation of blood vessels. The Sherpa version of this gene tends to keep red-cell production in check, which may explain their comparatively thin blood. Several of these advantageous variants are thought to have entered the Sherpa lineage through an archaic human population, the Denisovans, with whom the ancestors of modern Himalayan peoples interbred tens of thousands of years ago. Natural selection subsequently favoured these variants, so that today they are far more common in the Sherpa than in populations of lowland origin. Genes, however, tell only part of the story, for adaptation also operates at the level of individual cells. The Sherpa appear to possess unusually efficient mitochondria, the tiny structures within cells that convert nutrients and oxygen into usable energy. In addition, their tissues seem to manage the harmful by-products of energy production, known as free radicals, more effectively than those of lowlanders. Some investigations have noted that Sherpa muscles rely more heavily on glucose, a fuel that yields slightly more energy per unit of oxygen than fat does. Together these cellular features mean that, breath for breath, the Sherpa wring more useful work from the thin mountain air. Such findings have implications well beyond mountaineering, since hypoxia also lies at the heart of many medical emergencies. Patients in intensive care, and those recovering from heart attacks or strokes, all face the problem of tissues deprived of oxygen, and a better understanding of how the Sherpa cope may eventually inform their treatment. It would be a mistake, however, to imagine that the Sherpa are wholly immune to the hazards of altitude. They are not invulnerable, and serious illness can still strike, particularly during very rapid ascents or at the most extreme elevations. What distinguishes them is not the absence of risk but a markedly greater margin of safety, the product of biology shaped over thousands of years. For the visiting climber, by contrast, caution remains essential: a gradual ascent, adequate rest and attentiveness to early warning signs offer the surest protection, since no amount of short-term acclimatisation can replicate what the Sherpa have inherited.