Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. This process is one aspect of homeostasis: the keeping of a constant internal environment. If the body is unable to maintain a normal temperature and it increases significantly above normal, a condition known as hyperthermia occurs. This occurs when the body is exposed to temperatures of approximately 55° C; any exposure longer than a few hours at this temperature or up to around 70° C kills. The opposite condition, when body temperature decreases below normal levels, is known as hypothermia.
The human body has automatic responses to help regulate temperature. When the external environment heats up, arterioles leading to the capillary loops in the dermis dilate (widen). This increases blood flow to the surface of the skin where its heat can more easily radiate away. This process is called vasodilation. Sweat glands also produce greater amounts of sweat. This liquid is secreted onto the surface of the skin. Sweat needs energy to turn from a liquid into a gas and evaporate. This energy is called the latent heat of vaporisation. The body supplies this heat and so it cools down as the sweat evaporates.