Death zone
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The death zone is a term that refers to high altitudes, encountered by mountain climbers, where the amount of oxygen present cannot sustain human life.
The human body functions at its best at sea level (0 meters) when the air pressure is measured at 1 atmosphere. This is because the hemoglobin (the red pigment in red blood cells) is saturated with oxygen (nearly 100%) at that air pressure. Oxygen is required for every bodily process.
As humans go higher, the air pressure drops and so does the amount of available oxygen. At 5,000m (height of Everest base camp), the amount of oxygen is only half that of sea level's availability. At 8,850m (summit of Mt. Everest), only one third is available. When the amount of oxygen pressure drops, the human body tries to compensate by a process known as altitude acclimatization. Additional red blood cells are manufactured, the heart beats faster, non essential body functions are temporarily shut down, and you breathe deeper and more frequently. However, acclimatisation cannot take place immediately - in fact, it takes place over a period of days or even weeks. Failure to acclimatize may result in altitude sickness, including pulmonary edema or cerebral edema.
At extreme altitudes (above 7,500m), breathing bottled oxygen becomes almost mandatory for 99% of the climbers. This is because at that height, available oxygen becomes so low you can hardly function without supplementary oxygen. Sleeping becomes very difficult, digesting food is impossible (the body shuts the digestive system down), and you get hosts of other problems without additional oxygen.
Finally, at the "death zone", 8,000m and higher, no human body can acclimatize. Staying longer than necessary will result in deterioration of body functions, loss of consciousness and ultimately, death.