I'd guess there is also some efficiency in muscle tissue taking in oxygen over fat tissue? Also a bit better lungs, for runners and such? Though, I also don't

I'd guess there is also some efficiency in muscle tissue taking in oxygen over fat tissue? Also a bit better lungs, for runners and such? Though, I also don't know the relationship between lung efficiency and heart rate. If you have links on any of this, it feels like it would be a very fascinating read.