GENERAL CHARACTERISTICS OF
THE THERMAL ENVIRONMENT
AND MECHANISMS OF THERMAL
REGULATION

Humans tend to control their internal
environment at about 37o C (98.6o F)
although temperatures as high as 42o C
(108o F) and as low as 18o C (64o F)have
been reported in extreme cases.

The hypothalamus, human thermostat, controls
thermal regulation in humans by acting as a
thermal sensor, an integrator of information from
other locations in the body, and as a controller of
various effector mechanisms which are ready to
either increase or decrease the body's ability to
conserve or dissipate heat.

Anterior hypothalamus is the heat
dissipation (loss) center.

Posterior area of the hypothalamus is the
heat conservation (gain) center.

The two controller areas are reciprocally
innervated, stimulation of one results in
inhibition of the other.
Set Point of Hypothalamic Thermostat Is
Affected By A Variety Of Factors Such As:
1. Fever - a pyrogen (viral or bacterial)
elevates the set point.
2. Antipyrogenic agents (e.g., aspirin) lower set point.
3. Circadian (24 h) rhythms - low set point
in the early morning and high lateafternoon set point which corresponds to the
usual light-dark cycle and usual
pattern of
metabolic activity.
4.
Gender - women have a higher set point
during the second half of the monthly
menstrual cycle, which may be due to the
anabolic effect of progesterone.


Heat Balance Equation is derived from
the First Law of Thermodynamics (energy
is neither created or destroyed):

S = M - (+ Wk) - E + R + C + K

S = Heat Storage; S = 0 at thermal
equilibrium.
S = M - (+ Wk) - E + R + C + K


M = Metabolism or metabolic heat production;
total energy released by both the aerobic
and anaerobic processes (VO2 X
approximately 5 kcal/L of VO2; a little
higher if CHO rather than fat is the fuel
source).
Note: 1 kcal = amount of heat required to
raise 1 kg of water 1o C.
1 MET = 3.5 ml/kg/min
Wk = Work: where + is positive work
representing energy leaving the system or
work against internal forces and - is
negative or eccentric work or work against
external forces; at rest, W = 0.
+
+
S = M - (+ Wk) - E R + C K




E = Evaporation: insensible exchange
of heat via vaporizing moisture.
R = Radiation: sensible exchange of
heat via electromagnetic waves.
C = Convection: sensible exchange of
heat via a circulating medium.
K = Conduction: sensible exchange of
heat via a static medium.
+
+
S = M - (+ Wk) - E R + C K

Thermal Equilibrium exists when S = 0.

The ability of an individual to maintain thermal
equilibrium with the environment is a net result of
the interaction of physics (e.g., clothing insulation
or absorptivity) and physiology (esp., hydration
levels).
+ flow or S = hyperthermia as an individual can not
transfer excess body heat to environment.
- flow or S = hypothermia as a individual can not
effectively retain body heat as excessive amounts
are being transferred to the environment.



Sensible or Dry Heat Exchange - it is a
function of the measurable difference in
temperature between an organism and the
environment; includes convection,
conduction, and radiation.

Insensible or Moist Heat Exchange - it is
a result of evaporation of water (sweat or
perspiration) from the surface of the body.
CONVECTION

Heat is transported by a stream of molecules
from a warm object toward a cooler objective.
The most common exchange of body heat by
convection begins with heat loss from a warm
body to a surrounding fluid (air or water). The
heated fluid expands, becomes less dense, and
rises taking heat with it. The area immediately
adjacent to the skin is then replaced by a cooler,
dense fluid, and the process is repeated. Note
that heat gain can also occur through the
opposite or reverse process.
CONVECTION


Also occurs within the body in which
warmed blood is cooled by cooler tissue
and cooled blood is warmed by warmer,
more metabolically active tissue; this is
known as countercurrent heat exchange.
Convective heat loss is greater for water
than for air because water is more dense
than air.
TWO TYPES OF CONVECTION

Free convection - function of fluid density
(decrease temperature, increase density) and is
important in static or very slow flow rate. The
concepts of free convection are most closely
associated with the medium of water.

Forced convection - function of fluid velocity
and becomes increasing important at higher fluid
speeds (i.e., fast wind speeds). Forced
convection results in greater heat loss per unit of
time than free convection.
Forced Convection and Laminar vs
Turbulent Flow

Laminar flow results in faster velocity; creates
layers of increasing velocity flow above the surface.

Turbulent Flow, which may be caused by rough
surfaces, disrupts layers of flow bringing more
opposing/diverse fluids of differing temperature in
contact with the surface; increases in turbulence
increases the potential for heat exchange by
convection.
Factors that Increase Convective Heat
Loss






Increase in the difference between T1 and T2
(i.e., air or water temperature is lower than skin
temperature).
Decrease in temperature of circulating medium.
Increase in surface area, which is related to
dimension and shape of body.
Decrease in clothing covering the body
increases the surface area exposed.
Increase in thermal conductivity of the circulating
medium.
Increase in density of circulating medium.
Factors that Increase Convective
Heat Loss



Thermal conductivity is greater for water than
air.
Decrease in temperature of circulating medium
increases the density of the circulating medium
and its thermal conductivity.
Increase in precipitation would increase free
convective heat loss, but may increase or
decrease forced convective heat loss depending
on how air temperature is changed relative to
skin temperature.
Factors that Increase Convective
Heat Loss



Decrease in the insulation of clothing when wet
as the insulatory layer of air is replaced by the
higher conductive medium of water.
Decrease in altitude (i.e., convective heat loss is
greater at lower elevations due to a higher air
density; at altitude air density decreases and
hence convective heat loss decreases).
Increase in turbulent flow and/or a decrease in
laminar flow of circulating medium.
Factors that Increase Convective
Heat Loss



Increase in velocity of circulating medium
increases the heat loss per unit of time.
Increase in air pollution due to an increase in the
density of air.
Increase in hyperbaria (i.e., underwater diving)
as an increase in barometric pressure increases
the density of water, water has a
greater thermal conductivity that air, and water
temperature is usually lower than air
temperature.
Factors that Increase Convective
Heat Loss

Exercise and the associated increase in
core temperature.

In general, the opposite changes in the
factors listed above would have just the
opposite effect by decreasing convective
loss or perhaps increasing convective heat
gain.
Conduction (K)

Conduction (K) of heat occurs whenever two
surfaces with differing temperatures are in direct
contact.

Conductors are substances that conduct heat
readily, such as metals, water, & muscle tissue;
where as insulators are substances that do not
conduct heat readily, such as still air, nonmetals,
and fat tissue.
Conduction (K)

Note: Trapped still air in clothing makes an
excellent insulator due to low conductivity
and the fact that it increases the thickness
(distance) through which heat must be
transferred in order to be lost.
Conduction (K)


Generally, conductive heat loss represents only a
minor percentage of total heat exchange between the
body and environment as the skin surface area in direct
contact with external objects is usually minimal and
people usually avoid contact with highly conductive
materials. However, body heat is conducted from the
skin to clothing where it is dissipated from the outer
surfaces of the clothing via evaporation, convection, or
radiation depending on the vapor pressure (i.e. relative
humidity), air movement, and the skin-clothing-ambient
temperature gradients.
Also, conductive heat transfer also occurs within the
body from one area to another as well as from the core
and muscle shell to the skin surface.
Factors that Increase Conductive
Heat Exchange




Increase in the difference between T1 and T2.
Increase in surface area, which is related to
dimension and shape of body.
Decrease in clothing covering the body
increases the surface area exposed.
Increase in the thermal conductivity of tissue,
clothing, or surfaces that contacts the body
(e.g., metals, water, and muscle tissue have
greater thermal conductivity than fat, air, and
non-metals).
Factors that Increase Conductive
Heat Exchange



Decrease in the thickness or distance between
two surfaces, areas, or static mediums.
Exercise and the associated increase in core
temperature.
In general, the opposite changes in the factors
listed above would have just the opposite effect
by decreasing conductive heat exchange.
Radiation (R)

Radiation (R) is the exchange of
electromagnetic energy waves emitted from one
object and absorbed by another; it is a complex
term which represents the net effective radiation
balance of an individual.

The human body absorbs nearly all the radiation
that falls upon it.
Understanding Radiation

In understanding radiation, heat can be
considered as photons or light particles emitted or
absorbed by the body. An atom is like a miniature
solar system. At the heart of the solar system is
the nucleus of the atom with one or more electrons
orbiting around the nucleus. The orbital path of
the electrons can change as absorption of photons
or light particles cause the electrons to move to an
outer orbit and emitted photons cause the
electrons to move to a closer, inner orbit around
the nucleus.
Understanding Radiation

Molecules absorb and emit radiation in different
ways than atoms; they increase or decrease their
vibration due to changes in the atom. Photons
coming from the sun at 186,000 miles per second
are absorbed in the skin thereby increasing
molecular vibrations (as absorption of photons or
light particles cause the electrons to move to an
outer orbit in the atoms) and warming the body.
Heat is lost from molecules when the amount of
molecular vibrations decreases (emitted photons
cause the electrons to move to a closer, inner orbit
around the nucleus in the atoms).
Understanding Radiation

The wavelength of radiation determines whether
we can see it or feel it. Long wavelength
radiation is invisible and can only be perceived
as heat. For example, you can feel the radiation
emitted from the body as heat or the infrared
radiation from a fire that has stopped glowing.
Shorter wavelengths can be seen. The color
shifts through dull red through yellow to white as
the wavelength becomes shorter.
Radiation (R)

Six Factors Affect Radiation

3 solar (sun) factors: direct, diffuse, & reflected
(ground).
2 thermal or heat factors (ground and sky).
1 radiation factor emitted from the body.


Factors that Increase Radiate
Heat Gain




Increase in the difference between T skin surface
temp and T environmental radiant temp.
Increase in the surface area exposed, which is
related to dimension and shape of body.
Decrease in clothing covering the body
increases the surface area exposed.
Increase in dark colors relative to light colors
that are exposed.
Factors that Increase Radiate
Heat Gain




Increase in smooth textured surfaces relative to
rough textured surfaces of skin and clothing.
Increase in altitude (i.e., higher elevations) due to a
decrease air mass that increases solar radiation
and an increase in snow, ice, and rocks that
increases reflected solar radiation
Decrease in air pollution which decreases the
density of air.
In general, the opposite changes in the factors
listed above would have just the opposite effect by
decreasing radiant heat gain.
Insensible or Evaporative Heat
Exchange

Insensible or Evaporative Heat Exchange is
the result of evaporation or condensation of
water on the body surface as water is changed
from a liquid to gas; this process requires
heat which is extracted from the immediate
surroundings (i.e., skin) which results in cooling;
the amount or degree of evaporation is
determined by the water concentration
gradient between the body surface area and
the environment.
Sources of Evaporative Heat Loss
1. Insensible perspiration (diffusion of water through
the skin).
2. Thermal and nonthermal (nervous) sweating.
3. Water losses from the respiratory tract during
respiration.



Rest - 30 ml/hr of water loss.
High Environmental Temperatures and/or Strenuous
Exercise sweating rates may be as high as 1.5 to 2.0 L/hr
Evaporative Heat Losses from the Respiratory Tract
(Eres) are usually minor, but may become physiological
significant at high altitude and/or extremely cold and dry air,
particularly during exercising conditions.
Evaporative Heat Loss

Note: (1) the cooling of air decreases the capacity of air for
moisture and therefore the concentration gradient for
evaporation; (2) however, when cold air comes in contact
with the body it's temperature increases thereby
increasing it's moisture capacity and hence, dehydration
can occur even during cold temperatures; (3) also, if
clothing is not properly ventilated so that moisture can not
pass directly into the air from the skin for evaporation, the
warm skin air will be cooled and moisture will condense
in the clothing thereby decreasing the insulatory effects of
the clothing which may result in combined dehydration and
hypothermia.
Factors That Increase
Evaporative Heat Loss




Increase in the difference between the vapor
pressure in the air and the vapor pressure at the
skin.
Decrease in relative humidity decreases the
vapor pressure in the air thereby increasing the
gradient for evaporation.
Increase in the surface area exposed, which is
related to dimension and shape of body.
Decrease in clothing covering the body
increases the surface area exposed.
Factors That Increase
Evaporative Heat Loss







Increase in sweat rate.
Increase in thermal conductivity of sweat; decrease in
osmolarity of sweat (i.e., more dilute sweat) increases
thermal conductivity of sweat.
Increase in the surface area that is wetted.
Increase in altitude (i.e., higher elevations) due to an
increase in the capacity of the air for moisture.
Increase in air velocity (i.e., wind speed).
Exercise.
Increase in core temperature increase the latent heat
available to vaporize sweat from a liquid into a gas.
Factors That Increase
Evaporative Heat Loss





Increase in ventilation rate which increases heat loss by
respiration.
No precipitation as precipitation decreases evaporation
as the air becomes completely saturated with moisture.
Increase in air temperature increases the capacity of air
for moisture.
Hyperbaria (i.e., underwater diving) completely eliminates
evaporative heat loss.
In general, the opposite changes in the factors listed
above would have just the opposite effect by decreasing
evaporative heat loss.
Partitioning of Actual Heat Loss
to the Environment
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BIOPHYSICS OF HEAT
TRANSFER AND
CLOTHING
CONSIDERATIONS
HEAT TRANSFER

Heat transfer is the analysis of the rate of heat
transfer, flow, or exchange in a system, which
is governed by the laws of thermodynamics;
the modes of heat transfer in a system are
radiation, convection, conduction, and
evaporation; the combined interaction of
these mechanisms results in the overall heat
transfer within a system and consequently,
heat storage, heat loss, or thermal balance.
HEAT FLOW AND FLUX

Heat always flows from the region of
high temperature to a region of low
temperature.

Heat flux is a term used to summarize
the amount of heat transferred per unit of
time.
HEAT BALANCE EQUATION

Remember: S = M - (+ W) + K + C + R - E; In
this equation M is equal to metabolic heat
production (resting metabolic rate = 3.5
ml/kg/min or 50 kcal/hr/m2; for every L of
VO2, approximately 5.0 kcal are expended);
W is equal to work, which is either positive
work representing energy leaving the system
or work against internal forces OR negative or
eccentric work or work against external
forces; K, C, R, & E represent the
mechanisms of heat transfer.
HEAT TRANSFER
• In addition to previously discussed
information, insulation from air and
clothing are factors which need to be
taken into consideration when
understanding the total impact of heat
transfer.
Total Insulation = Iclothing + Iambient air

Thermal insulation is the resistance offered to the
flow of heat between two surfaces and is determined
by:

(T1 - T2)/Flow of heat per unit of surface area.
Note: The slower (i.e., lower) the flow of heat per unit of
surface area or the smaller the difference between the
temperatures of two surfaces, the greater the thermal
insulation.
Insulation of Clothing
1 CLO = unit of clothing thermal
insulation; the clothing necessary to
insult in comfort (thermoneutrality) a
resting subject at 21 Co (70o F), air
movement of 10 cm/s or 20 fpm (normal
ventilation rate of a room), and a relative
humidity of less than 50%.
Factors Affecting the Insulative Value
of Clothing


Fabric's thermal conduction, which is a function of the
thickness of the clothing and extend of trapped air
layers; the greater the air trapped and/or the thicker the
clothing, the greater the insulation.
Fabric's dispersion over the skin surface area, which
extends the total potential surface area open to the
environment; the greater the dispersion of clothing over
the skin surface area, the greater the insulation.
Factors Affecting the Insulative Value
of Clothing

Variations in skin temperature
distribution and heat flow at various
sites.
Factors Affecting the Insulative Value
of Clothing



Variations in clothing surface covering the skin and
skin blood flow: none of the hands and face, presence
of arterial-venous anastomosis in the extremities, and
vasodilatory activities in the face.
Air layer next to skin; an increase in movement will
decrease the air layer and insulation around the skin.
Exercise increases air movement, particularly if
garment is not wind resistant.
Factors Affecting the Insulative Value
of Clothing

Wet clothing will decrease the insulation of
clothing to 30%. Sweating (30 ml/hr at rest,
up to 1.5-2.0 L/hr during exercise) and rain or
snow if the garment is not water repellent will
decrease the insulation of clothing.
Factors Affecting the Insulative Value
of Clothing

Compression of clothing material. Particularly true to
the feet where compression of boots as a person stands
on a stone floor has been reported to reduce insulation to
that comparable to standing with naked feet. Also with the
hands, the gripping of ski poles or bike handlebars will
cause compression of the gloves and therefore, reduce
the insulation of the gloves. Finally, water has been
reported to decrease insulation of compressed clothes by
up to 50%.
Factors Affecting the Insulative Value
of Clothing

Air temperature. As air temperature increases
above skin temperature, insulation increases which
may lead to a hyperthermic (i.e., heat gain) response;
as air temperature decreases below skin
temperature, insulation decreases which may lead to
a hypothermic (i.e., heat loss) response.
General Clothing Recommendations



Use multiple layers.
Outer layer should be wind and water
resistant.
Middle layer should trap air.
Goose down
Wool
Polyester
Polyolefrin
General Clothing Recommendations


Inner layer should also wick away moisture
from the skin to prevent evaporative heat loss.
Polypropylene
Cotton Fishnet
Most important to cover trunk and head during
prolonged exposure to cold.
Efficiency Factor of Clothing
• Ratio of:
Thermal resistance between clothing surface and air
Resistance between skin surface and air
• Higher the ratio the greater the efficiency factor
of clothing or insulation and vice-versa.
Insulation of Ambient Air

A function of temperature, air velocity,
altitude, relative humidity, and
precipitation.
FACTORS THAT DECREASE THE
INSULATORY VALUE OF AIR




Decrease in air temperature below
skin temperature (Ta- Tsk).
Increase in air velocity (exercise will
increase air velocity).
Decrease in relative humidity and/or
precipitation.
Decrease in elevation (i.e., low
elevation). Air at high altitude
provides better insulation.
Altitude and Insulation of
Ambient Air

Since altitude decreases convective heat
loss and increases radiant heat gain and
evaporative heat loss, the increase in the
insulatory value of air at high altitudes
suggests that the decrease in convective
heat loss and increase in radiant heat
gain is greater than the increase in
evaporative heat loss at high altitude.
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