The altitude problem for your body is the shortage of oxygen. As you climb higher, the air gets thinner. Each lungful gives you less oxygen the higher you go, so your body has to work harder to maintain the supply.

Acclimatization in this context means the deliberate, temporary modification of your physiology to cope with increased altitude. There is a lot more to it than just breathing, but that remains the major problem. There are practical limits to altitude acclimatization, and it is simply not possible to become permanently acclimatized to much above 5,000m, as the body inevitably deteriorates. Short term acclimatization to about 6,500m is the best that a fit climber can hope for.

The demand from your muscles depends on their activity level, but your brain needs a surprising amount of oxygen. Despite being only 2% of your body weight, it needs around 15% of its oxygen. If your brain is deprived of oxygen, your judgement declines, movement control suffers and speech becomes confused.

Your body responds in various ways to needing more oxygen. In simple terms:

• you breathe faster and more deeply

• your heart beats faster in order to maintain the oxygen to your tissues

• your body excretes bicarbonate in the urine and creates more red blood cells, making the blood thicker.

You start to breathe faster right away, and your heart rate rises within minutes. It can take several days before your blood levels change: if you suddenly find yourself urinating a lot that may be a sign that your body is acclimatising well. Making more red blood cells is a much longer process that gets under way within a week or two: on trips such as Kilimanjaro, this won't be in time to make a difference.

At altitude, breathe deeply and freely as much as possible. Sleep is an important time for the body's adjustment: avoid sleeping pills and alcohol, which depress breathing while asleep.

Be aware that some people have episodes of' periodic breathing: a pattern in which the sleeper's breathing becomes faster and louder for a minute or two, then decreases or perhaps even stops. The cycle repeats itself, but if the sleeper wakes up with a start, they may be prone to panic. Simply reassure them that all is well and try to get back to sleep. Periodic breathing is normal for some people even at sea level, but it becomes more obvious at altitude; with acclimatisation, it diminishes.