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Simplified control circuit of human thermoregulation. [8]The core temperature of a human is regulated and stabilized primarily by the hypothalamus, a region of the brain linking the endocrine system to the nervous system, [9] and more specifically by the anterior hypothalamic nucleus and the adjacent preoptic area regions of the hypothalamus.
Thermoregulation is the ability of an organism to keep its body temperature within certain boundaries, even when the surrounding temperature is very different. A thermoconforming organism, by contrast, simply adopts the surrounding temperature as its own body temperature, thus avoiding the need for internal thermoregulation.
Other circumstances also affect the body's temperature. The core body temperature of an individual tends to have the lowest value in the second half of the sleep cycle; the lowest point, called the nadir, is one of the primary markers for circadian rhythms. The body temperature also changes when a person is hungry, sleepy, sick, or cold.
The human body always works to remain in homeostasis. One form of homeostasis is thermoregulation. Body temperature varies in every individual, but the average internal temperature is 37.0 °C (98.6 °F). [1] Sufficient stress from extreme external temperature may cause injury or death if it exceeds the ability of the body to thermoregulate.
In humans, hyperthermia is defined as a temperature greater than 37.5–38.3 °C (99.5–100.9 °F), depending on the reference used, that occurs without a change in the body's temperature set point. [3] [10] The normal human body temperature can be as high as 37.7 °C (99.9 °F) in the late afternoon. [2]
Aural and skin temperature measurements require special devices designed to measure temperature from these locations. [11] While 37 °C (99 °F) is considered "normal" body temperature, there is some variance between individuals. Most have a normal body temperature set point that falls within the range of 36.0 to 37.5 °C (96.8 to 99.5 °F). [13]
The cause of postmortem caloricity varies depending on the cause of death: Postmortem glycogenolysis – a phenomenon beginning soon after death observed in nearly all cadavers. In an average adult, postmortem glycogenolysis can produce up to 140 calories of heat which can raise the temperature of the body by up to 2 °C.
During these multi-day torpor bouts, body temperature drops to ~1 °C above ambient temperature and metabolism may drop to about 1% of the normal endothermic metabolic rate. Even in these deep hibernators, the long periods of torpor is interrupted by bouts of endothermic metabolism, called arousals (typically lasting between 4–20 hours).