The lowest ever recorded arterial oxygen tension (pO₂) was just one result of a series of remarkable physiological investigations conducted on Mount Everest that included arterial blood gas analysis at an altitude of 8,400 m (27,599 ft) above sea level, just 484 m short of the summit. At this high altitude the partial pressure of oxygen in inspired air (FO₂(I)) is just 47 mmHg, less than a third of the FO₂(I) at sea level. Prior to this study blood gas values at this high altitude could only be predicted by indirect means, but now there is reliable data that will inform understanding of the physiological process involved in acclimatization to an oxygen-deprived environment and hypoxia in the critically ill. Arterial blood was sampled for blood gas analysis from between four and ten climbers in London UK (altitude 246 ft), at Everest base camp (17,388 ft) and at three further altitudes (20,997 ft, 23,294 ft and 27,559 ft) during ascent towards the summit (29,029 ft). Samples obtained in London and at base camp were obtained via an indwelling radial arterial catheter and analyzed immediately. All other samples were obtained via the femoral artery using needle and syringe. These were transported on ice by Sherpa for analysis at a "laboratory" sited at camp 2 (20,997 ft). All samples were analyzed within 2 hours of collection. Blood was sampled from four climbers at the highest altitude. Mean pO₂ at this altitude was 3.3 kPa (24.6 mmHg); range 2.5-3.9 kPa. pH ranged from 7.45 to 7.65 and pCO₂ ranged from 1.4 to 2.1 kPa (10.3-15.7 mmHg). Although pO₂ decreased progressively with each increase in altitude, oxygen content of blood (ctO₂) was maintained up to an altitude of 20,997 ft by increasing hemoglobin production. At the highest altitude the mean Hb was 19.3 g/dL (range 18.7-20.2 g/dL).