When the human body is exposed to very hot conditions, one result can be heatstroke. This is often the case for athletes and people who have to work outside in summer.
Heatstroke is a sudden, uncontrolled rise in body temperature. It is reactions that result from not being able to replace fluid lost-through respiration. If the lost fluids are not replaced then dehydration occurs and this leads to a decrease in blood.
In this situation, the body must decide whether to give the blood to the main organs [livers, kidneys, and so on] or to skin. Because the main organs are more important, they will receive the blood. Also, as a consequence of the drop in fluids, the body loses its ability to sweat. The situation becomes critical. The body now cannot produce sweat; therefore, it cannot cool itself. Excess heat cannot be released through the skin as a result of the loss of blood supply to that part of the body to overheat.
Heatstroke can cause permanent injury if not treated properly. It is one of how the body can react to heat.