In 1978, Reinhold Messner, along with Peter Habeler, made the first ascent of Mount Everest without supplemental oxygen.
He also was the first climber to ascend all fourteen peaks over 8,000 metres.
If you travel quickly from sea level to elevations above 2,400 metres, where atmospheric pressure and partial pressure of oxygen are lower, your body responds with symptoms of acute mountain sickness (headaches, shortness of breath, nausea, and dizziness. In severe cases of AMS, lethal pulmonary and cerebral edema may occur.
When you move on a long-term basis from sea level to the mountains, your body makes respiratory and hematopoietic (the formation of blood cellular components) adjustments via an adaptive response called acclimatization.
High altitude conditions always result in lower than normal hemoglobin saturation levels because less oxygen is available to be loaded.
Although body tissues at high altitude receive adequate oxygen under normal conditions, problems arise when all-out efforts are demanded of the cardiovascular and respiratory systems. Unless a person is fully acclimatized, such conditions almost guarantee that body tissues will become severely hypoxic (deficiency in the amount of oxygen reaching the tissues).