Fever (pyrexia) is defined as an elevation of core body temperature above the level normally maintained by the individual. Under normal circumstances, core body temperature (the temperature of blood in the right atrium) is tightly regulated, with circadian variations over a range that usually does not exceed 1� F (0.6� C) and a mean value of 98.6� F (37� C) (the normal "set-point"). An array of thermoregulatory mechanisms, described in detail here, ensure that this temperature is maintained. During episodes of fever, the thermoregulatory set-point is shifted such that the same thermoregulatory mechanisms are used to maintain an abnormally elevated temperature.
It is important to realize that fever is not equivalent to an elevated core temperature but to an elevated set-point. Under many circumstances, ranging from intense physical exertion to immersion in hot liquids, core temperature may be elevated yet fever does not exist because the body is attempting to cope with the departure from homeostasis. Failure of thermoregulation may also be associated with elevated core temperature; this problem too (which occurs in malignant hyperthermia) is distinct from fever.
Core body temperature is determined by two opposing processes, each of which is regulated by the central nervous system. On the one hand, energy in the form of heat is generated by living tissues ("thermogenesis"). Energy may be passively absorbed from the environment as well. On the other hand, energy is inevitably lost to the environment, chiefly through the emission of infrared radiation and through transfer of energy to the surrounding medium. The temperature at which tissues are maintained is related to heat capacity (e.g., to the amount of energy required to elevate temperature by a defined increment) and to the quantity of energy lost or gained by the system.
Metabolic reactions proceed more rapidly at an elevated temperature....