[evan_au (OP)]

I hear that many patients admitted to hospital with COVID-19 have abnormally low blood oxygen saturation.

In some cases the patients themselves were unaware of it - because the drop in oxygen saturation is slow, their body has time to adapt. This is just like mountain climbers preparing to climb mount Everest - they spend a week or more at base camp acclimatizing to low oxygen levels.

Acclimatization effects include increase in red blood cell count, increases in breathing and heart rates, the blood pH becomes more alkaline, etc etc.
See: https://en.wikipedia.org/wiki/Effects_of_high_altitude_on_humans#Acclimatization

There has also been talk of tissue damage in COVID patients due to blood clots.
This was one of the potential risks to athletes taking EPO to increase their red blood cell count - it makes the blood more viscous.
See: https://en.wikipedia.org/wiki/Blood_doping#Erythropoietin

[alancalverd]

One of my flying buddies was recently discharged after 10 days in intensive care with COVID. He had been recovering at home from a  mild dose and tracking his O2SAT with a pulse oximeter (a handy piece of kit if you fly unpressurised light aircraft: the weather is so much nicer above the clouds, but it's easy to get suckered into hypoxia!) when he noticed a rapid decline, so called for an ambulance and survived.

As I understand it, acclimatisation takes several days even if you are fit and eating well, but the loss of lung function with COVID infection can be very rapid and progressive. One of the surprising findings, IIRC, is that smokers do not feature particularly strongly among the COVID dead - they seem to have compensated over many years for limited oxygen uptake. Likewise women, having a lower basal metabolic rate than men, presumably have a lower resting oxygen demand and thus better resilience against compromised lung function.