Thermoregulation

Thermoregulation is how the body holds its core temperature steady, close to 37 °C in humans, even when the surroundings are much hotter or colder. Endotherms such as mammals and birds make most of their heat through metabolism, while ectotherms such as most reptiles, fish and insects rely largely on outside heat and on behavior, like basking in the sun or seeking shade.

In humans, the hypothalamus works like a thermostat and controls temperature by negative feedback. When the body warms, it widens blood vessels in the skin so more heat radiates away, and it starts sweating, which cools the skin as the sweat evaporates. When the body cools, it narrows skin vessels to keep warm blood in the core, triggers shivering, in which rapid muscle contractions release heat, and raises heat production through thyroid hormone and epinephrine. Heat leaves the body by radiation, evaporation, convection and conduction; at rest radiation removes the most, while evaporation of sweat takes over during exercise or in hot weather.

Fever is a deliberate upward reset of the hypothalamic set point during infection. When thermoregulation is overwhelmed, the result is hypothermia or heat stroke.