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SECTION III
Central & Peripheral Neurophysiology
HEAT LOSS
The processes by which heat is lost from the body when the
environmental temperature is below body temperature are
listed in Table 18–3.
Conduction
is heat exchange between
objects or substances at different temperatures that are in
contact with one another. A basic characteristic of matter is
that its molecules are in motion, with the amount of motion
proportionate to the temperature. These molecules collide
with the molecules in cooler objects, transferring thermal en-
ergy to them. The amount of heat transferred is proportionate
to the temperature difference between the objects in contact
(thermal gradient).
Conduction is aided by
convection,
the
movement of molecules away from the area of contact. Thus,
for example, an object in contact with air at a different tem-
perature changes the specific gravity of the air, and because
warm air rises and cool air falls, a new supply of air is brought
into contact with the object. Of course, convection is greatly
aided if the object moves about in the medium or the medium
moves past the object, for example, if a subject swims through
water or a fan blows air through a room.
Radiation
is the
transfer of heat by infrared electromagnetic radiation from
one object to another at a different temperature with which it
is not in contact. When an individual is in a cold environ-
ment, heat is lost by conduction to the surrounding air and by
radiation to cool objects in the vicinity. Conversely, of course,
heat is transferred to an individual and the heat load is in-
creased by these processes when the environmental tempera-
ture is above body temperature. Note that because of
radiation, an individual can feel chilly in a room with cold
walls even though the room is relatively warm. On a cold but
sunny day, the heat of the sun reflected off bright objects ex-
erts an appreciable warming effect. It is the heat reflected
from the snow, for example, that makes it possible to ski in
fairly light clothes even though the air temperature is below
freezing.
Because conduction occurs from the surface of one object
to the surface of another, the temperature of the skin deter-
mines to a large extent the degree to which body heat is lost or
gained. The amount of heat reaching the skin from the deep
tissues can be varied by changing the blood flow to the skin.
When the cutaneous vessels are dilated, warm blood wells
into the skin, whereas in the maximally vasoconstricted state,
heat is held centrally in the body. The rate at which heat is
transferred from the deep tissues to the skin is called the
tis-
sue conductance.
Birds have a layer of feathers next to the
skin, and most mammals have a significant layer of hair or fur.
Heat is conducted from the skin to the air trapped in this layer
and from the trapped air to the exterior. When the thickness
of the trapped layer is increased by fluffing the feathers or
erection of the hairs
(horripilation),
heat transfer across the
layer is reduced and heat losses (or, in a hot environment, heat
gains) are decreased. “Goose pimples” are the result of horrip-
ilation in humans; they are the visible manifestation of cold-
induced contraction of the piloerector muscles attached to the
rather meager hair supply. Humans usually supplement this
layer of hair with one or more layers of clothes. Heat is con-
ducted from the skin to the layer of air trapped by the clothes,
from the inside of the clothes to the outside, and from the out-
side of the clothes to the exterior. The magnitude of the heat
transfer across the clothing, a function of its texture and
thickness, is the most important determinant of how warm or
cool the clothes feel, but other factors, especially the size of
the trapped layer of warm air, are important also. Dark clothes
absorb radiated heat and light-colored clothes reflect it back
to the exterior.
The other major process transferring heat from the body in
humans and other animals that sweat is vaporization of water
on the skin and mucous membranes of the mouth and respira-
tory passages. Vaporization of 1 g of water removes about 0.6
kcal of heat. A certain amount of water is vaporized at all
times. This
insensible water loss
amounts to 50 mL/h in
humans. When sweat secretion is increased, the degree to
which the sweat vaporizes depends on the humidity of the
environment. It is common knowledge that one feels hotter on
a humid day. This is due in part to the decreased vaporization
of sweat, but even under conditions in which vaporization of
sweat is complete, an individual in a humid environment feels
warmer than an individual in a dry environment. The reason
for this difference is unknown, but it seems related to the fact
that in the humid environment sweat spreads over a greater
area of skin before it evaporates. During muscular exertion in a
hot environment, sweat secretion reaches values as high as
1600 mL/h, and in a dry atmosphere, most of this sweat is
vaporized. Heat loss by vaporization of water therefore varies
from 30 to over 900 kcal/h.
Some mammals lose heat by
panting.
This rapid, shallow
breathing greatly increases the amount of water vaporization
in the mouth and respiratory passages and therefore the
amount of heat lost. Because the breathing is shallow, it pro-
duces relatively little change in the composition of alveolar air
(see Chapter 35).
The relative contribution of each of the processes that
transfer heat away from the body (Table 18–3) varies with theTABLE 18–3 Body heat production and heat loss.
Body heat is produced by:
Basic metabolic processes
Food intake (specific dynamic action)
Muscular activity
Body heat is lost by: Percentage of heat lost at 21 °C
Radiation and conduction 70
Vaporization of sweat 27
Respiration 2
Urination and defecation 1