Most of the problems are caused by lack of oxygen. Because the atmospheric pressure is lower the higher you go, at 2,000 meters above sea level, a given volume of air contains some 20 percent less oxygen, and at 4,000 meters, air contains 40 percent less oxygen.

Lack of oxygen affects most of your bodily functions. Your muscles can do less work, your nervous system can take less stress, and your digestive system cannot handle fat as well.

Normally when your body needs more oxygen, you automatically breathe more heavily and fill the need. Then why doesn’t this happen when you arrive at a high altitude?

Just how your body controls your rate of breathing is a wonder that is not completely understood. But when you exert yourself, heavy breathing is not triggered simply by lack of oxygen.

Rather, the carbon dioxide buildup in the blood produced by the muscle activity seems to be a key factor in making you breathe more.

You do breathe more heavily when at a higher altitude but not enough to compensate for the persistent oxygen shortage.

What causes the headaches? A speaker at the First World Congress of High Altitude Medicine and Physiology, held in Las Paz, Bolivia, explained that many of the symptoms of mountain sickness result from an accumulation of fluid in the brain.

 In some people this causes pressure inside the head. Apparently, because of the size of their cranium, some people do not experience these effects.
Nevertheless, in rare cases a life-threatening condition can develop. Loss of muscular control, blurred vision, hallucinations, and mental confusion are signs that warn you to seek medical help immediately and get down to a lower altitude.

The effects of high altitude reach their peak about the second or third day, so a few days before and after arrival, it is best to take only light meals, especially at night.

After arrival, you should eat carbohydrates, such as rice, oats, and potatoes, rather than fatty foods. You may do well to pay attention to the advice. “Eat breakfast like a king, but eat supper like a beggar.”

Also, avoid physical exertion, as it can bring on a bad attack of mountain sickness. Perhaps because young people tend to disregard this advice, they are often the ones who suffer most.

“Slip on a hat, and slop on some sunscreen cream” is good advice here too, since there is less atmosphere to protect you from the dangerous rays of the sun.

  Those rays can irritate or even damage your eyes, so use good sunglasses. The thin mountain air also dries up your tears, causing further eye irritation. The advice is to drink plenty of fluids.

Doctors have warned people who are seriously overweight or who have such conditions as high blood pressure, sickle-cell anemia, or heart or lung disease to have a careful medical evaluation before deciding on a trip above the clouds.

Some doctors prescribe acetazolamide to stimulate breathing at very high altitudes. Other drugs for mountain sickness are advertised, but not all doctors recommend them.

If you have a bad cold, bronchitis, or pneumonia, it may be wise to delay your trip, since high altitude together with a respiratory infection or heavy physical exercise can sometimes cause a dangerous buildup of fluid in the lungs.

Respiratory complaints can cause even lifelong highlanders to become oxygen starved and experience serious health problems.

On the other hand, asthmatics often feel better living higher up. In fact, a group of Russian doctors reported to the First World Congress of High Altitude Medicine and Physiology that take patients with certain complaints to a high altitude clinic as therapy.