HEAT AND SOLAR INJURIES - Medicine for Mountaineering and Other Wilderness Activities - page 413

CHAPTER 27
HEAT AND SOLAR INJURIES
James A. Wilkerson, M.D.
Principal Contributor
HEAT ILLNESS
Heat illnesses cover a spectrum that ranges from mild disability that is more of a nuisance
than a health threat to lethal heat strokes.
Normal Heat Loss
Normal human body temperature is maintained within a narrow range by sensitive tem-
perature control centers inthehypothalamus. Heat producedwithin thebodyisdissipated to
theenvironmentsothatatemperaturebetween97°and100°F(36°and38°C)ismaintained.
Largely through muscular activity, most individuals generate 2000 to 5000 kilocalories
of heat per day depending upon their size, physical activity, and state of nutrition. The body
must get rid of this heat to prevent a devastating increase in temperature. If no heat were
lost,thebodytemperatureofasedentaryindividualweighing154pounds(70kg)producing
only 2000 kilocalories per day would climb approximately 62°F (34°C) to 160°F (71.4°C)
in twenty-four hours. (The temperature would climb at that rate as long as the person was
alive!)
Humansloseheatlargelythroughtheirskin.Thelungsaretheprincipalrouteofheatloss
(panting)forhairyanimals.Athighaltitudesdeeper,morerapidbreathingofcold,relatively
dry air causes significant heat loss in humans, but in temperate or hot climates much less
heat is lost through the lungs. The skin acts much like the radiator ofa liquid-cooled engine.
Blood is warmed as it passes through exercising muscles and cooled as it circulates through
the skin. The thermostat on the engine increases the flow of coolant through the radiator
when the engine is hot; comparable mechanisms increase blood flow to the skin by dilating
cutaneous blood vessels when exercise heats the body.
The radiator for an engine is cooled only by air passing over it. Heat is lost from the skin
inthiswayalso(convection),butinahotclimatebyfarthelargestheatlossisthroughevap-
oration of perspiration. Evaporative cooling is highly effective because a large amount of
heat is required to change water from a liquid to a vapor. The evaporation of 1 cubic centi-
meter (1 ml) of water at a skin temperature of 95°F (35°C) requires 577 calories, enough to
reduce the temperature of 1 liter (1000 ml) of water approximately 1°F. Most of this heat is
extracted from the body.