With regard to body temperature, animals fall into two classes: poikilotherms (cold-blooded animals), which have the same temperature as their surroundings, and homeotherms (warm-blooded animals), which have an approximately constant temperature maintained by a "thermostat" in the brain. The normal temperature for most homeotherms lies between 35°C (95°F) and 40°C (104°F); it is greatly reduced during hibernation.
For humans, the normal mouth temperature usually lies between 36°C (97°C) and 37.2°C (99°F). It fluctuates daily, and in women monthly. The temperature setting is higher than normal in fever.
When the body is too hot, the blood vessels near the skin expand to carry more blood and to lose heat by radiation and convection, and the sweat glands produce perspiration which cools by evaporation. When the body is too cold, the blood vessels near the skin contract, the metabolic rate increases, and shivering occurs to produce more heat. Fat under the skin, and body hair (fur in other animals), help to keep heat in. If these defenses against cold prove inadequate, hypothermia results: body temperature falls, functions become sluggish, and death may result. Controlled cooling may be used in surgery to reduce the need for oxygen.