Based upon the wikipedia article, heat injury is very complex. The straightforward dehydration/heat exhaustion is only one piece of the puzzle. Elevated body temp damages the intestines ability to contain contain endotoxins, portions of bacterial cells that are toxic in themselves as distinct from toxins produced by bacteria, resulting in toxic shock and damage to diverse organs. High body temp resulting from exertion in high heat environments can start a cascade of devastating physiological events that can kill very healthy people very quickly: Michael Popov's last run.. This seems to be simple physics-generating more heat than can be dissipated, but the wiki article suggests that high heat/high humidity environments may result in scenarios that are fatal to people resting in front of a fan in the shade. Scary stuff.