Part of my anti-COVID toolkit includes an SPO2 meter for measuring changes in blood oxygenation, and out of curiosity I decided this little meter would accompany me on a high-altitude hike. At 12,000 feet my blood oxygenation was at 84-89 (as opposed to a normal 100). If this reading had happened at sea level, I would be running to a hospital.
Curiously, I found that I could get my blood oxygenation back up to 97 simply by hyperventilating for half a minute. I ran across this strategy a few years ago when going up Mt Whitney. I found that I could amble along at 14,000 feet at a normal walking pace as long as my breathing sounded like a steam engine. I got some strange looks, but I was also passing everybody.
This got me wondering why my body's lovely homeostatic systems don't do this for me automatically? Why should I have to consciously force myself to breathe at the rate that lets me get things done? It turns out that the body is managing something else -- respiratory alkalosis. If you breathe too hard for too long, you cause your circulatory system's CO2 levels to drop, which causes a chemical reaction that pulls hydrogen ions out of your blood, causing your blood pH to rise. Eventually bad things happen. It seems strange that we wouldn't have a secondary mechanism for managing blood pH so we can more rapidly adapt to high altitudes, but it's unlikely that people regularly journeyed from sea level to high altitudes over the course of a day in the ancestral environment. In any case, it's nice to discover yet another system keeping me alive that I didn't know existed.
As a side note, I've noticed that I'm moderately dumber at high altitude; I've regularly noticed small lapses in planning ability and working memory over the last few days.
(in the picture, the meter's circuitry refresh rate is so slow that the shutter only captured some of the digits)