How does the body prevent itself from overheating in exercise?

Sweating: At least two mechanisms. The body first reacts by dilating (enlarging the diameter) of the vessels near the skin surface, thus allowing for more surface area heat exchange to occur. Sweating is additive to this process, cooling the skin by evaporation. Together, these processes help to prevent overheating. Normally, we sweat as much as a liter throughout the entire day.