Sweating cools you by evaporating sweat. When body temperature rises, or when you are exposed to high temperatures, the blood vessels dilate and more fluid is lost from the skin. As this extra fluid evaporates it raises your temperature slightly. However, if you continue to exercise at a higher rate of speed than normal then your body temperature will continue to rise until there is no longer enough water in the skin for evaporation to keep pace with heat loss. At that point any added exertion will only accelerate heat production further increasing your core temperature – sweating does not combat heat-exhaustion!