The body's blood is circulated closer to the skin's surface, and excess heat drains off into the cooler atmosphere. At the same time, water diffuses through the skin as perspiration. The skin handles about 90 percent of the body's heat dissipating function. When heat gain exceeds the level the body can remove, or when the body cannot compensate for fluids and salt lost through perspiration, the temperature of the body's inner core begins to rise and heat-related illness may develop. Sunburn, with its ultraviolet radiation burns, can significantly retard the skin's ability to shed excess heat.
Acclimatization has to do with adjusting sweat-salt concentration, among other things.
The idea is to lose enough water to regulate body temperature, with the least possible chemical disturbance.