There are a number of challenges a climber faces during a high-altitude mountain expedition. Extreme weather conditions, sleep deprivation, and the extraordinary energy demands placed on the body combine to push even the fittest mountaineers to their physical limits. One environmental factor stands out, however, as perhaps the greatest challenge to high-altitude climbers: lack of oxygen.

No matter where on earth you are, the concentration of oxygen in the air is about 21 percent. Depending on the altitude, however, the density of air can vary widely. The density of a gas is the measure of how many molecules there are in a given volume. The less dense the air, the fewer molecules -- including oxygen molecules -- that air contains. On Mount Everest, the highest point on earth, for example, the air is about one-third less dense than the air at sea level, and thus contains about one-third fewer oxygen molecules.

In addition, the lower air pressure at high altitudes makes the transfer of oxygen from lung tissue into lung capillaries a less efficient process. There is simply less pressure to push the air through the capillary walls. To compound this problem, as a climber works harder at extremely high altitudes, his or her heart beats faster. This causes blood to pass more rapidly through the capillaries, giving the oxygen less time to move from the lung tissue into the bloodstream.

The human body's capacity to compensate for moderate gains in altitude is quite good. Most people have little difficulty going from sea level to 8,000 feet above sea level. Climbing to this altitude may cause a person's breathing rate to increase, and they may feel fatigued as the body struggles to acquire the oxygen it needs, but few people suffer more severe effects unless they continue to ascend.

Serious mountaineers, of course, climb much higher than 8,000 feet -- 20,000 feet higher in the case of Mount Everest. In so doing they risk suffering the effects of altitude sickness, which ranges from headache, dizziness, and loss of appetite to more severe symptoms, including vomiting and loss of consciousness. Nearly all climbers experience at least mild symptoms of altitude sickness. Fatigue, headache, and lack of appetite are all part of the experience for some. The more extreme symptoms, however, can be life threatening, and most climbers go to great lengths to avoid them.

The most common cause of altitude sickness is ascending too high, too quickly. While the body responds immediately to a lack of oxygen by quickening its breathing rate, there are limits to this response. Other ways in which the body compensates -- by increasing blood pressure and the flow of blood in the lungs, and by increasing the production of red blood cells -- take several days to several weeks.

This is part of the reason mountaineers are unable to climb from 10,000 feet to the 28,000-foot summit of Everest in a single day or even several days. Not only do they need to rest along the way, but their bodies also need time to adapt physiologically to the low-oxygen conditions. This process, called acclimatization, involves climbing a mountain in stages. Climbers will ascend several thousand feet and then wait for their bodies to adapt to the lack of oxygen before continuing higher. Most climbers do this several times en route to a high-altitude summit.