Determinants of body temperature

Body temperature itself is determined by a number of physical and physiological factors. At a purely physical level, it is a fundamental tenet that for any body, be it animate or inanimate, heat production must equal heat loss plus heat retention. Thus at steady state, i.e. zero heat retention, heat production and heat loss must be equal. If heat production exceeds loss, body temperature will rise. This may be the consequence of either increased heat production or reduced heat loss. Such a rise in body temperature will normally result in increased heat loss, by either active or passive mechanisms, and one of two consequences will follow: either normothermia will be restored or the balance between production and loss may only be regained at a new elevated temperature.

These thermodynamic considerations have two important implications. Firstly, rising temperature is not necessarily the consequence of increased heat production but rather of an imbalance between production and loss. Secondly, in the febrile state, if body temperature is elevated but stable, i.e unchanging, heat production and heat loss must once again be equal and are not necessarily abnormal. What is abnormal is the new set-point. Fever does not necessarily imply increased heat production.

The thermoregulatory center is located in the preoptic region of the hypothalamus near the floor of the third ventricle. This area contains heat-sensitive neurons as well as receiving neural inputs from other thermoreceptors. Humoral signals from circulating factors act primarily via the organum vasculosum of the lamina terminalis, an area of fenestrated capillaries which permits access for larger molecules, such as cytokines, to neuronal receptors. Also found in this area are fixed reticuloendothelial cells analogous to the Kupffer cells in liver, and these may have an important role to play in local cytokine synthesis and signaling ( S.§.p§.Land... Bie.d.er.19.94).

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