Define Decreased oxygen availability at high altitude (Hypobaric hypoxia)?

The governing biophysical factor at high altitude is decrease in barometric pressure with increase in altitude. Although atmospheric concentration of oxygen remains at  a constant 20.93% at al.1 terrestrial altitudes, the partial pressure of oxygen falls along  with decline in barometric pressure (PO₂ = 0.2093 x barometric pressure). As the  altitude increases, the lowered oxygen pressure (PO₂) in pulmonary alveoli causes  a decline in saturation of haemoglobin in arterial blood, and a lower oxygen pressure  gradient throughout the body, especially at the level of capillaries, where PO₂maY be close to zero.

With low PO₂, the blood flow is too rapid to allow appropriate gaseous exchange, resulting in unfavourable oxyhaemoglobin dissociation. The CO₂ that is  produced metabolically exerts a tension of 40 mm Hg at sea level while at high  altitude, it is' decreased to a minimum of about 24-27 mm Hg while water vapours  exert a tension of 47 mm Hg at all altitudes. Thus, PCO₂₊ PH₂O, that is, 71 mm Hg is always to be deducted from total available gas pressure in alveoli at a given altitude. When breathing pure oxygen, the nitrogen present in alveolar air can be replaced by oxygen (but not CO₂ and H₂O vapour) and alveolar PO₂ can be increased. The barometric pressure, PO₂ in the air and alveoli at different altitudes are given in Table.