I'm a big fan of soaking in hot water and have noticed that cardiac function seems to have a massive effect on heat tolerance as measured vs body temperature.

For example, if I've been totally sedentary for the whole day (and my feet are chilly+blue), a body temperature as low as 101F is unbearable. But if I've been actively moving around all day (and my feet are warm and pink), I only start getting uncomfortable at a body temperature around 103.5F-104F.

This also seems to correlate over a longer timespan re: exercise habits, consumption habits, sickness, etc.