FEVER is an elevation of body temperature that exceeds thenonnal daily variation and occurs in conjunction with an increase in the hypothalamic set point-for example, from 37°C to 39°C. This shift of the set point from "normothermic" to febrile levels very much resembles the resetting of the home thermostat to a higher level in order to raise the ambient temperature in a room. Once the hypothalamic setpoint is raised, neurons in the vasomotor center are activated and vaso constriction commences. The individual first in fever notices vaso constriction in the hands and feet. Shunting of blood away from the periphery to the internal organs essentially decreases heat loss from the skin, and the person feels cold.
For most fevers, body temperature increases by 1 to 2°C. Shivering, which ncreases heat production from the muscles, may begin at this time; however, shivering is not required if heat conservation mechanisms raise blood temperature sufficiently. Heat production from the liver also increases. In humans, behavioral instinct (e.g.,putting on more clothing or bedding) lead to a reduction of exposed surfaces, which helps raise body temperature.
A fever of > 41.5°C (106.7°P) is called hyperpyrexia. This extraordinarily high fever can develop in patients with severe infections but most commonly occurs in patients with central nervous system hemorrhages. In the preantibiotic era, fever due to a variety of infectious diseases rarely exceded 106°P, and there has been speculation that this natural "thennal ceiling" is mediated by neuropeptides functioning ascentral antipyretics.
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