Surviving Everest: Despite Technological Innovations, Climbing Everest Remains One of the World's Most Challenging Physical Endeavours. Dr Charles Clarke, Who Accompanied Chris Bonington on His 1975 and 1982 Expeditions, Discusses the Problems That Mountaineers Face at High Altitude and the Ways in Which Their Bodies React

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THE HOWLING WIND OF EVEREST is one of my clearest memories from 20 years ago--those sudden gusts exploding and echoing like thunderclaps off the rock walls of the Rongbuk Valley in Tibet. Even at 6,000 metres, it blasted our faces with showers of ice and gravel and tore at the tents.

Temperatures far below -40[degrees]C, icy winds howling past at more than 160km/h and sudden changes in weather, coupled with the lack of oxygen and its medical complications, make climbing Everest one of our greatest physical endeavours. But what happens to the body at such high altitudes?

There are two main physical components that make climbing Everest so hazardous--low temperature and a lack of oxygen. We all know that it gets colder the higher you go. So, for example, in Denver, Colorado, which lies at 1,500 metres, summer temperatures average 23[degrees]C. But on nearby Pike's Peak (4,300m), the figure is 5[degrees]C. The chilling power of high winds increases the effect, so that a wind speed of 40km/h at 0[degree]C causes an `equivalent chill temperature' of -20[degrees]C. In such conditions, frostbite and hypothermia are never far away.

More importantly, the amount of atmospheric oxygen diminishes at altitude as the pressure falls. The key to adapting to this is gradual acclimatisation--it's best to spend a fortnight getting your body used to the thin air before venturing above 5,000 metres if starting from sea level. An unacclimatised person would soon lapse into a coma if he or she were lifted from Kathmandu (1,500m) and dropped on Everest's summit (8,848m).

In 1953, John Hunt was rigorous about acclimatisation, insisting that his team spent a month at 4,000 metres, climbing peaks of around 6,000 metres. Reading his book The Ascent of Everest, one can't help noticing that there is little mention of the fatigue, headaches or sense of torpor that is now known as acute mountain sickness. This is an unpleasant, but self-limiting illness. It usually lasts for several days and happens to most people above 3,000 metres. Usually it just means feeling rotten.

We don't really know what is actually happening physiologically as we acclimatise, except that with each day below 5,000 metres, acute mountain sickness usually subsides and we feel stronger and more `normal'. We do know that as the body acclimatises, changes occur over several weeks, particularly in the mitochondria, the energy generators in our cells, Levels of haemoglobin, which transports oxygen in red blood cells, increase as the bone marrow is stimulated by hypoxia (a lack of oxygen). Breathing becomes deeper and more rapid. There are changes in the way blood flows to many organs, and the kidneys shift the body's acid-base balance. The brain and lungs take the brunt of the problem, where increased blood flow, coupled with leaking capillaries, leads to potentially fatal medical problems.

Dehydration is also an issue. At altitude, the increase in ventilation (breathing out moist air) and the lack of water except from melting snow and ice make the situation worse.

The body also reacts in other, almost spiteful ways. Appetite falls off above 5,500 metres, just when one needs extra energy. There is intolerance to fatty, high-energy foods. Simple, familiar carbohydrates are preferred and cooking becomes a time-consuming chore.

Above 5,000 metres, the blood flow to the brain increases markedly--you can often hear blood pounding through your head at night--but this rarely causes a major problem It also affects the eyes, and tiny haemorrhages frequently appear in the retina. Usually they aren't a problem, but occasionally they spread across the retina, which can impair vision quite severely. This typically recovers with prompt descent and no further treatment is necessary.

Occasionally, the acclimatisation process fails, especially if ascent is too rapid, and this can cause two potentially fatal illnesses. …