An average person at sea level has about 13-14 kilopascals (kPa) of oxygen in their bloodstream, University College of London medical researcher Dan Martin says. When his intensive care patients drop to around 8 kPa he gets very worried, and a normal person with 6 kPa of oxygen faces almost certain death. Imagine his shock, then, when he and three colleagues on the top of Mount Everest measured their own blood oxygen level to be between 2.5 and 4 kPa, the lowest ever measured in live people.
I tracked down the original article at the New England Journal of Medicine. Here's a link to the abstract:
and an excerpt:
PaO2 fell with increasing altitude, whereas SaO2 was relatively stable. The hemoglobin concentration increased such that the oxygen content of arterial blood was maintained at or above sea-level values until the climbers reached an elevation of 7100 m (23,294 ft). In four samples taken at 8400 m (27,559 ft) — at which altitude the barometric pressure was 272 mm Hg (36.3 kPa) — the mean PaO2 in subjects breathing ambient air was 24.6 mm Hg (3.28 kPa), with a range of 19.1 to 29.5 mm Hg (2.55 to 3.93 kPa). The mean PaCO2 was 13.3 mm Hg (1.77 kPa), with a range of 10.3 to 15.7 mm Hg (1.37 to 2.09 kPa). At 8400 m, the mean arterial oxygen content was 26% lower than it was at 7100 m (145.8 ml per liter as compared with 197.1 ml per liter). The mean calculated alveolar–arterial oxygen difference was 5.4 mm Hg (0.72 kPa).
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According to the article, the scientists also took tiny biopsy samples of their muscles while they were there and froze them; now they are studying them, primarily the mitochondria. I'm not a doctor and I don't really understand the implications and ramifications of all this, however, this stuff could perhaps be useful to runners one day. As it is, the benefits of training at high altitude are already well-known. Though I doubt you could, would, or should really run at 29,035 feet, maybe this study might provide us with a better insight that could make runners stronger and faster.