KEEPING WARM 
evaporative, convective, conductive, and 
radiative heat losses through skin 
changes and the circulatory system is 
internal. But the body’s heat loss can 
also be controlled by changing the 
amount of body surface area exposed to 
the environment. 
You may not be aware of the way you 
unconsciously spread your limbs or raise 
your arms on a hot day in order to cool 
off. Cats offer excellent examples of this 
behavior. On a cold day, a cat will curl 
up not only to minimize its total exposed 
surface area to the air but also to shield 
its stomach from the environment. As 
the temperature rises, the cat will un- 
curl, stretch out, and in a truly warm 
environment, may lie on its back or side 
to deliberately expose its relatively 
warm stomach area, all in order to cool 
off. While there are limits to how 
humans can copy this feline “presented 
area” approach to keeping warm, it at 
least reminds us that the various por- 
tions of the body differ in their ability to 
control the body’s heat loss. For exam- 
ple, although the blood flow to the ex- 
tremities can be clearly demonstrated to 
vary in volume and temperature and 
thereby to control the body’s heat loss, 
the same is not true of the head. There is 
no significant automatic control of blood 
flow in the head. The amount of heat 
that can be transferred from the head 
does not change significantly because 
the blood flow supplied to the brain is 
fairly rich, relatively constant, and close 
to the surface. In addition, the skin on 
the head does not dilate or perspire as 
much as the skin on the limbs. Finally, 
the head is relatively free of the subcuta- 
neous fat layer of other portions of the 
body; hence there is little insulation be- 
tween blood in the head and the environ- 
ment. 
Although blood flow to the brain is 
more or less constant, the rate of blood 
flow as well as the blood pressure can 
rise in the cold. When this happens, heat 
is conserved in the head and torso at the 
expense of the extremities. The cold 
body has an interesting effect on the 
bladder. As blood accumulates in the 
torso, the additional fluid pressure gen- 
erates the signal to urinate. This is 
caused by the constriction of blood ves- 
sels when 'the body is cold, although 
being cold is not usually thought of as 
the cause of increased excretion of 
urine. 
All of the foregoing suggests that in 
attempting to protect ourselves from the 
cold, we might profit from taking a 
different approach to protecting our 
limbs and extremities from that used to 
protect the head. One might be tempted 
to concentrate on insulating the extrem- 
ities because they are the areas through 
which considerable heat loss can occur. 
If the body’s trunk is amply protected, 
however, heat, loss in lightly protected 
hands and feet may be tolerable. Some 
individuals feel quite comfortable work- 
ing with their bare hands even at moder- 
ately cold temperatures because they 
have a greater blood flow to their ex- 
tremities than other people have. The 
head, however, is another matter. Even 
though it represents only about 5 per- 
cent of the body area, the head can 
“dump” from 7 to as much as 50 per- 
cent of the body heat, depending on the 
conditions involved. This strongly sug- 
gests that no attempt to protect the body 
from cold should omit the head. A full 
head of natural hair is an indisputable 
asset in a cold environment, but at least 
for many males, a head covering would 
seem to be in order. Equally important, 
the ears represent prime areas through 
which body heat can escape. 
Looking at the clothing we wear to 
keep warm reveals our limited choices: 
we can select clothing on the basis of 
materials or design or some combination 
of the two. Clothing materials depend 
on trapped air to provide insulation. The 
insulation value of trapped air is consid- 
erable, since in the absence of turbu- 
lence — the replacement of the warmed 
air by the cooler air — the heat loss is 
significantly reduced. Clearly, any ma- 
terial that has the capacity to trap air or 
that can in some other way provide a 
still-air layer is a candidate for insula- 
tion in clothing. 
The principle of creating a still-air 
layer for insulation can be applied to any 
article of clothing, which seems to sug- 
gest that a single insulated garment 
would be sufficient to maintain the body 
at a comfortable temperature. Unfortu- 
nately, this is not the case. A single 
garment would not accommodate the 
tremendous range of human metabolic 
activity. It might keep the body warm 
under moderate working conditions but 
would overheat it during strenuous ac- 
tivity. This problem has led clothing 
designers to the concept of layered 
clothing. Individual layers can be 
opened to permit a greater flow of air 
between the layers, or the layers can be 
removed, as needed, to control body 
temperature. 
As people become overheated in a 
cold environment, they usually begin to 
open, loosen, or remove their torso cloth- 
ing while keeping their headgear on. 
Given the body’s ability to remove heat 
through the head, a more logical heat- 
regulation mechanism would be to first 
remove the hat. A multilayered hat sys- 
tem might also provide enhanced con- 
trol over the body’s heat-regulation 
mechanism in a cold environment. 
In attempting to create the best bal- 
ance between materials and design, we 
confront the premise that to be useful, 
clothing materials must be compress- 
ible, flexible, and bendable. And they 
must be able to “breathe” in order to 
relieve some of the body’s moisture. At 
the knees, elbows, and in the shoulder 
area, however, the insulation material 
will be under compression, reducing the 
volume of trapped air. When the cloth- 
ing is fully compressed, much of the still 
air will be forced out and the thermal 
conductivity of the compressed insula- 
tion material will be the factor govern- 
ing heat loss, along with such radiation 
as can occur through the material. Body 
movement is another factor that must 
be considered in clothing design. The 
motion of the body and its garments can 
set up a “pumping” action that intro- 
duces some of the ambient air in be- 
tween the layers of insulation. In many 
tropical countries, men wear their shirts 
outside their trouser tops instead of 
tucked in, as a way of increasing the 
convective airflow over their torsos by 
means of the pumping action. 
Many new clothing materials that 
may keep us warmer in the future have 
emerged in the marketplace. Advances 
have also been made in the scientific 
tools to help the engineers, physiologists, 
and designers working on clothing. Flat- 
plate calorimeters, which measure the 
resistance to heat transfer, are available 
to determine the insulation value of ma- 
terials both in compressed and uncom- 
pressed states. When moisture is added 
to the plate calorimeter a more realistic 
simulation of the body is achieved, at 
least insofar as temperature and mois- 
ture are concerned. Researchers, recog- 
nizing that the distribution of clothing 
on the body and the convective currents 
set up by the body’s own heat will influ- 
ence the heat and moisture absorbed by 
the clothing, also utilize conductive rep- 
licas of the body, usually made of cop- 
per. Precise amounts of heat can be sent 
through these copper bodies, and by 
measuring the amount of energy re- 
quired to maintain them at a given tern- 
If 
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