Heat production and absorption are balanced by heat loss via
four mechanisms. Conduction carries heat between
a body and a contacting surface along a temperature gradient. This
is the primary mechanism of cooling during ice or water immersion. Convection transfers
heat from the body surface to or from a gas or fluid circulating around
the body. With conduction, heat transfer stops when the contacting
surface temperature reaches body temperature, whereas with convection,
circulation of fresh gas or fluid around the body preserves the
temperature gradient between the body and the circulating gas or
fluid thus increasing the efficiency of heat transfer. Radiation transfers
heat from a warmer to a colder body via electromagnetic waves. Evaporation removes
heat by promoting a phase transition from liquid to a gas. Of these
four mechanisms, radiation is the principal mechanism of heat elimination
in temperate environments. Heat loss by convection and radiation
are increased by cutaneous vasodilation, a neurally mediated response
to hyperthermia. As either heat production or environmental temperature increases,
evaporation of sweat becomes the principal mechanism for heat elimination.
It is important to recognize that sweating is the only mechanism
for heat loss at environmental temperatures above body temperature
because for heat to be lost by conduction, convection, or irradiation,
body temperature must be warmer than the gas, fluid, or objects
to which heat is dissipated. In fact, conduction, convection, and
irradiation lead to heat gain when environmental temperatures exceed
body temperature. Under these conditions, the adaptation to heat
stress relies on both a greater rate of sweating at a lower degree
of exercise and a lower electrolyte content of sweat. Sweating and
subsequent evaporation are very effective mechanisms of temperature
control, provided that the environment’s humidity is low
enough to permit evaporation. This does not occur when the air becomes saturated
with water vapor at humidity levels of 90% to 95% and
is reduced at humidity levels greater than 75%. In addition,
sweating can cause a substantial water and electrolyte loss occurring
at the rate of 1 L/hr/m2 of body surface area.
This may contribute to the clinical manifestations found in a child
exposed to excessive heat.