Page 235 - Color_Atlas_of_Physiology_5th_Ed._-_A._Despopoulos_2003

9 Thermal Balance and Thermoregulation

Heat loss occurs by the physical processes of
Thermal Balance
radiation, conduction, convection, and evapo-
The body temperature of humans remains ration (! B).
relatively constant despite changes in the en- 1. Radiation (! B1, C). The amount of heat lost by
vironmental temperature. This homeothermy radiation from the skin is chiefly determined by the
applies only to the core temperature (! 37 !C) temperature of the radiator (fourth power of its ab-
of the body. The extremities and skin (“shell”) solute temperature). Heat net–radiates from the
exhibit poikilothermy, i.e., their temperature body surface to objects or individuals when they are
varies to some extent with environmental cooler than the skin, and net–radiates to the body
temperature. In order to maintain a constant from objects (sun) that are warmer than the skin.
core temperature, the body must balance the Heat radiates from the body into the environment
amount of heat it produces and absorbs with when no radiating object is present (night sky). Heat
radiation does not require the aid of any vehicle and
the amount it loses; this is thermoregulation is hardly affected by the air temperature (air itself is a
(! p. 224). poor radiator). Therefore, the body loses heat to a
Heat production. The amount of heat pro- cold wall (despite warm air in between) and absorbs
duced is determined by energy metabolism radiation from the sun or an infrared radiator without
(! p. 228). At rest, approximately 56% of total air (space) or cold air, respectively, in between.
heat production occurs in the internal organs 2. Conduction and convection (! B2, C). These
and about 18% in the muscles and skin (! A2, processes involve the transfer of heat from the
top). During physical exercise, heat production skin to cooler air or a cooler object (e.g. sitting
increases several-fold and the percentage of on rock) in contact with the body (conduction).
heat produced by muscular work can rise to as The amount of heat lost by conduction to air
much as 90% (! A2, bottom). To keep warm, increases greatly when the warmed air moves
the body may have to generate additional vol- away from the body by natural convection
untary (limb movement) and involuntary (heated air rises) or forced convection (wind).
(shivering) muscle contractions. Newborns 3. Evaporation (! B3, C). The first two
also have tissue known as brown fat, which mechanisms alone are unable to maintain ade-
enables them to produce additional heat quate temperature homeostasis at high en-
without shivering (! p. 225). Cold stimulates a vironmental temperatures or during strenu-
reflex pathway resulting in norepinephrine re- ous physical activity. Evaporation is the means
lease (! 3-adrenergic receptors) in fatty tissues, by which the body copes with the additional
which in turn stimulates (1) lipolysis and (2) heat. The water lost by evaporation reaches the
the expression of lipoprotein lipase (LPL) and skin surface by diffusion (insensible perspira-
thermogenin. LPL increases the supply of free tion) and by neuron-activated sweat glands
fatty acids ( ! p. 254). Thermogenin localized (! B3, pp. 73ff. and 225 D). About 2428 kJ
in the inner mitochondrial membrane is an un- (580 kcal) of heat are lost for each liter of water
coupling protein that functions as an H uni- evaporating and thereby cooling the skin. At
+
porter (UCP1, ! p. 230). It short-circuits the H + temperatures above 36 !C or so, heat loss oc-
gradient across the inner mitochondrial mem- curs by evaporation only (! C, right). At even
brane (! p. 17/B2), thereby uncoupling the higher environmental temperatures, heat is
(heat-producing) respiratory chain of ATP pro- absorbed by radiation and conduction/convec-
duction. tion. The body must lose larger amounts of
Heat produced in the body is absorbed by heat by evaporation to make up for this. The
the bloodstream and conveyed to the body surrounding air must be relatively dry in order
surface. In order for this internal flow of heat to for heat loss by evaporation to occur. Humid air
occur, the temperature of the body surface retards evaporation. When the air is extremely
must be lower than that of the body interior. humid (e.g., in a tropical rain forest), the aver-
The blood supply to the skin is the chief deter- age person cannot tolerate temperatures
minant of heat transport to the skin (! p. 224).
222 above 33 !C, even under resting conditions.

Despopoulos, Color Atlas of Physiology © 2003 Thieme
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