The effects of heat exhaustion on the body and the pathology of resultant depleted physical output. (A fairly accurate scientific waffle-fest by some fell runner).

So why does getting too hot screw you up? What's going on, and why did I turn into an 18st jogger when I got very hot at alderman's?

As far as I can tell it's down to Hypovolemia which is decreased blood volume, a decrease that is down to water loss due to sweating. The big hitter for fell runners encountering this condition is that vascular dilation occurs. The body shuts down blood supply to extremeties in order to preserve the vital organs, which is remarkably similar to cold shock, and cell osmosis is also impaired. There is a whole gamut of other nasty effects, but I've singled out Hypovolemia because it slows you right down to a stagger.

The reason you can't get up a hill in hot conditions is basically because your body is shutting down blood supply to your legs, and when exactly this happens is different for everyone due to heat acclimation and your body's thermal efficiency. So big lads carry more heat than racing snakes in other words. The issue of salt loss is to me relatively insignificant compared to the onset of progressively deminishing cardiovascular performance. As soon as you take water on board, Hypovolemia decreases and you get your legs back as a direct result of blood vessels opening up.

This is a process that soldiering on will intensify very quickly, to the point of syncope (fainting) when your body stops you running by impairing your brain function and cognitive ability. At this level, all blood is directed to major organs so that multiple organ failure does not occur, and you end up bollocksed on the floor.

The only way to save yourself this prognosis is to either stop for water or carry it with you. Everyone's Hypovolemic trigger point will be different.