Humans and higher animals have a certain body temperature. The production of body temperature is the result of the body's constant metabolism. At the same time, body temperature is an important condition for the normal conduct of the body's functional activities.

With the evolution of animals, the thermoregulation function is more and more perfect. The body temperature of lower animals changes with the change of environmental temperature, and cannot keep its body temperature relatively constant, so it is called a thermoregulated animal. Human beings and higher animals are able to maintain the relative stability of the ambient temperature in the body through the regulation of the heat production and dissipation processes in the body at different ambient temperatures, and improve the ability to adapt to changes in ambient temperature, and are therefore called thermostats.

In a healthy state, such as normal diet, appropriate clothing, the body temperature is generally relatively constant, that is, to remain in the 37 ℃ up and down (between 36.2 ~ 37.3 ℃), and not due to changes in the external environmental temperature and change. But the normal body temperature does not mean a specific temperature, but a temperature range. For example, for most normal people, the oral temperature range of 36.7 ~ 37.7 ℃ (and 37.19 ℃ is only an average), the armpit temperature range of 36.0 ~ 37.4 ℃, rectal temperature range of 36.9 ~ 37.9 ℃. Although human body temperature is relatively constant, there are some differences in body temperature between human individuals, and the standard body temperature of a few people can be lower than 36.2℃ or higher than 37.3℃. Even if the same person body temperature in a day is not exactly the same, the fluctuation of body temperature between day and night can be up to about 1 ℃.