Originally Posted by
surak
The Real Science of Sport podcast covered this. Pros are getting the right hydration and will do heat acclimatization training, but that's not what separates them from amateurs. You'll still see pros dunking water over their heads and putting ice packs behind their necks, but their fitness gives them a lot more headroom on the thermometer before they suffer as much as amateurs do.
Excellent point. I can’t find a quick number for how much the heat response expands the vascular space that the heart has to pump blood through, but it’s big and you need big reserves to perform under those conditions, especially with the inevitable volume loss due to sweating.