What is the pathophysiology of hyperthermia?

Updated: Oct 14, 2020
  • Author: Erik D Schraga, MD; Chief Editor: Sage W Wiener, MD  more...
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Answer

Answer

Effective thermoregulation, controlled by the hypothalamus, is critical for proper function of the human body, with normal temperature exhibiting diurnal variation between 36-37.5°C. Heat is both produced endogenously and acquired from the environment. Metabolic reactions in human bodies are exothermic, contributing 50-60 kcal/h/m2 of body surface area, or 100 kcal/h for a 70-kg person. During strenuous exercise, heat production increases 10- to 20-fold. [3] Environmental heat transfer involves the following 4 mechanisms [3] :

  • Conduction: Direct physical contact transfers heat from a warmer object to a cooler object. Water is about 25 times more conductive (more effective at conducting heat) than air.

  • Convection: Heat is transferred through air and water vapor molecules surrounding the body. Convective heat transfer depends on wind velocity and explains the effect of wearing loose-fitting clothing in warm climates to keep cool.

  • Radiation: Heat is transferred by electromagnetic waves. Radiation is the major source of heat gain in hot ambient climates; up to 300 kcal/h can be gained on a hot summer day.

  • Phase change: The conversion of a solid to a liquid (melting) or a liquid to a gas (evaporation) results in heat transfer. Evaporation of 1 L of sweat from the body results in a loss of 580 kcal of heat.


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