I'm confused about the relationship between partial pressure and concentration of a gas, in relation to the biology of respiration. If I have a planet with high atmospheric pressure - maybe like 10 atm - but the O2 concentration is lower, maybe around 10%, what effect would this have on oxygen-breathing life? On Earth, oxygen makes up 21% of our atmosphere, with a partial pressure of 0.21 atm. In a 10 atm atmosphere, with oxygen at 10%, this would be 1 atm, which is higher than on Earth, obviously. Would an atmosphere like this provide higher or lower access to oxygen than on Earth? I guess what I'm wondering is, is partial pressure or concentration of oxygen more biologically significant?
Take a look at the Hyperbaric chambers for both medical purposes and treating decompression sickness. Some of the hard chambers go up to 8 atmospheres and include 100% oxygen , but with scheduled air breaks where the oxygen percentage is brought down to 21% to reduce oxygen toxicity.
The Navy dive manual is interesting reading and there is quite a bit of we know this happens but maybe we don’t know why when it comes to oxygen toxicity. As mentioned in the comments a lot of that uncertainty is suspected to be the role of the inert gasses. The tolerance to oxygen is higher when dry than wet for example. Why? Not sure, but you treat wet suits different than dry suits. Take someone off oxygen, and they go into convulsions why? Shrug, it doesn’t seem to be that bad and is like a “vigorous workout”. What is the role of CO2, it matters…. So it is pretty empirical document, but it has lead to a field of hyperbaric medicine. Some claims are probably unrealistic.
Heliox (mixtures of helium and oxygen) are used for two reasons medically there is less resistance when breathing so that helps with breathing difficulty in a medical context. It also prevents nitrogen narcosis or rapture of the deep and the anesthetic nature of inert gas.
Trimix is cheaper, and has enough helium to minimize the chance for narcosis.
So for your planet, it seems that you have some license to have some realistic leeway with the oxygen percentages and the effects on people and how they respond.
With the partial pressure you also have some influence on how flammable things are. But around 21 percent that shouldn’t be that big a problem.
It's the partial pressure that matters for biological processes. There is a minimum pressure of oxygen you need to live. There is a somewhat higher total pressure you need to live because at the minimum oxygen pressure you get too much water evaporation in the lungs.
Otherwise, all that matters is toxicity and stability. (You can, for example, breathe perfectly well in 20kPa O2 and 80 kPa H2 but one spark and everything goes boom.) Everything you could breathe has a point where it becomes dangerous. Deep sea diving reaches a depth limit not because of what the pressure does to a person, but what you can breathe.