High altitude is considered to be an elevation between
1,500–3,500 metres (4,900–11,500 ft) above sea level.  Very high altitude is an elevation between 3,500–5,500
metres (11,500–18,000 ft) while extreme altitude is considered to
be an elevation above  above 5,500 metres (18,000 ft) (International
Society for Mountain Medicine.  Retrieved 22 December 2005).
 High altitude regions include Tibetan
plateau in the Himalayas (with long term resident populations of Tibetans,
Ladakhis and Sherpas), the Andean altiplano (with resident populations of
Quechua and Aymara), the Semien Plateau of North Africa (with resident
population of Ethiopians), Tien-Shan and Pamir mountains in Asia (populated by
the Kyrgyz).  More than 140 million
people live at elevation more than 2500 m above sea level, of these 80 million
live in Asia, where the major population density is at elevations exceeding
3500m (ref ).  Apart from residential people, millions of
sojourners visit to the high altitude regions for recreational activity,
travelling, mountaineering, climbing, trekking, personnel duty and many others

            At high altitude, the atmospheric or
barometric pressure, and thus the partial pressure of oxygen, is considerably
less than at sea level.  Due to decrease
in pressure, the air expands rises up and become less dense.  Therefore, although the percentage of oxygen
in air  remains constant at 20.93%  up to an altitude of 100,000 meters  (328,083 feet) (Clausen, 1977), but the
number of oxygen molecules per breath is reduced, causing diminished oxygen
diffusion from the alveoli to blood.  This condition is called hypobaric hypoxia.  This
reduced inspired partial pressure of oxygen results in marked declines in
maximal oxygen consumption (VO2 max), element that need to be
compensated in order to meet the body’s energy requirements.  Hypoxia at high altitude triggers a cascade of
 favorable  physiological changes that try to minimize
the decrements to an lowland individual’s physical and cognitive work performance
level and health under these extreme conditions ((Fulco et al 2000; Banderet et
al 2002).  This process is known as altitude acclimatization.  Several physiological changes that permit this
acclimatization, including  hyperventilation,
increased in red blood cell production , decreased plasma volume, increased hematocrit (polycythemia), increased myoglobin content  and  a
higher concentration of capillaries in skeletal
muscle, increased mitochondria, increased aerobic enzyme concentration, hypoxic pulmonary vasoconstriction, and right ventricular hypertrophy (Young et al 2002, West 2006). These
changes tend to decrease the gradient of oxygen partial pressure from ambient
air to tissues. Nevertheless, even after initial acclimatization, there is
reduction in physical work capacity with fall in arterial oxygen saturation,
decrease in maximal heart rate and reduction in maximum oxygen uptake capacity
(VO2max) (Fulco et al 1998, Muza et al 2004, McSherry PE 2007).  

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