A Life high up in the mountains
is harsh, but it's surprising how many people make their homes there. Worldwide,
around 140 million people live above 2,500 metres, and that's set to rise as
resident populations grow and more people move into mountainous areas. It's a
testament to human adaptability that we can survive in this alien environment.
Despite evolving in the African lowlands we have the resourcefulness to cope
with most of the challenges imposed by a life at altitude. Human ingenuity
allows us to overcome extremes in temperature, a natural scarcity of resources
such as wood and food, and exposure to damaging solar radiation. But at high
altitude one challenge dwarfs all others: lack of oxygen. Many Tibetans, for
example, breathe air that contains little more than half the oxygen of that at
sea level. B How can humans survive when deprived of this
essential life-giving gas? How we adapt depends on our ancestry. Native
lowlanders who've moved to the mountains show a range of anatomical and
metabolic changes that help them get by. People whose families have lived at
altitude for generations, however, have evolved to cope with the low oxygen
levels. Their genes give them a head start for mountain life. And, intriguingly,
women seem to be better adapted to the high life than men.
C Overall, genetics seems to account for about 25 per cent of the
variability in physical fitness between people at altitude. Developmental
influences contribute a similar amount and lifestyle makes up the rest. One of
the most obvious physical adaptations is a larger, deeper chest with greater
lung capacity. Whatever your ancestry, this will develop simply as a result of
growing up at altitude. Other adaptations are a higher breathing rate and more
efficient lungs, which together increase the amount of oxygen in the blood. Once
again, there seems to be a gradient, with people whose ancestors have lived at
altitude for longest having the highest oxygen levels. D
On average, there's an 11 per cent reduction in VO2 max, a measure of
maximum exercise capacity, for every 1,000-metre increase in height. What's also
clear-as any mountaineer who has climbed with a Sherpa knows-is that
acclimatized newcomers suffer most and native highlanders least. Indeed,
populations that have been mountain dwellers for the longest, such as native
Tibetans and Bolivians, have VO2 max measurements similar to those
found in people living at sea level-and are apparently immune to mountain
sickness. E Acute mountain sickness (AMS) affects over
half of all lowlanders who spend more than a few hours above 3,500 metres. It
makes little difference whether you're an ultra-fit climber with dozens of
Himalayan summits under your belt or a couch potato who has never climbed higher
than the top shelf of the larder. What's more, on one visit to altitude you may
be fine, but on the next you could be struck down with the headaches, nausea,
disorientation, and lethargy that are the hallmarks of AMS. Why is it so
unpredictable? F The jury is still out, but theories
abound. "The received wisdom is that fitness is irrelevant to developing acute
mountain sickness," says Jo Bradwell from Birmingham University. But he doesn't
buy that. And this summer he and his team took an exercise bike up a mountain in
the Bolivian Andes to test their paradoxical theory that fitter people are more
susceptible. G The root cause of AMS is a lack of oxygen
in the blood—hypoxia—which somehow triggers fluid to leak from blood vessels
into the brain. The body normally tries to compensate for hypoxia by stepping up
the heart rate and breathing rate. But at altitude this can be
counterproductive, because the faster blood flows through the lungs, the less
time it has to become fully oxygenated. Bradwell suspects that the fitter you
are, the more likely it is this will happen when you exercise at altitude.
That's because those who are fit tend to have bigger muscles, which require more
oxygen. That leads to more severe hypoxia and a stronger attempt by the body to
compensate. H And that's where the exercise bike comes
in. Bradwell and his team had 20 climbers furiously pedalling away at 3,600,
4,700 and 5,250 metres on a mountain called Huayna Potosi near La Paz. The
researchers measured the oxygen in their brains at varying levels of exertion.
They are still analysing the results, but Bradwell is upbeat. "I suspect that
the fitter individuals will be the ones with lower brain-blood oxygen counts."
he says. I "Exertion does make everything worse," agrees
Charles Houston from the University of Vermont in Burlington. But after two
decades studying AMS, he believes that the best place to look for differences in
susceptibility is our genes. "Of the several dozen influences that affect us at
altitude, I think many of them will turn out to have a genetic basis," he
says. J Some recent research supports Houston's
prediction. In July this year, Masayuki Hanaoka from the Shinshu University
School of Medicine in Matsumoto, Japan, announced a possible genetic link to
high-altitude pulmonary oedema (HAPE)-a potentially fatal side effect of hypoxia
in which fluid builds up in the lung. He found that two variations in a gene
called eNOS occur more often in people who suffered HAPE. They are bad news for
anyone wanting to climb to altitude, according to Nicholas Morrell from
Cambridge University, who has identified a similar genetic variation in the
people of mountainous Kyrgyzstan. "Put simply, you're more likely to get to the
summit of Everest if you don't have these variations." But Houston believes that
psychology may be almost as important as physiology. "A lot of tolerance to
altitude is due to motivation," he says. "If you expect to get sick at altitude
then you probably will." Questions 27-31
Reading Passage 3 has ten paragraphs, A-J. Which paragraph
contains the following information? Write the appropriate
letter, A-J, in boxes 27-31 on your answer sheet.
填空题
The symptoms of acute mountain sickness
填空题
A description of how an experiment was carried out
填空题
Gender differences in the ability to cope with living at altitude
填空题
The increasing numbers of inhabitants of high-altitude parts of the world
填空题
Changes in the body shape of people who live at high altitude