Title: GENERAL CHARACTERISTICS OF THE THERMAL ENVIRONMENT AND MECHANISMS OF THERMAL REGULATION
1GENERAL CHARACTERISTICS OF THE THERMAL
ENVIRONMENT AND MECHANISMS OF THERMAL REGULATION
2- 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.
3- 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.
4- 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.
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7- 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 late- afternoon 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.
8- 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.
-
9S 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.
10S 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.
11S 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.
12- 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.
13CONVECTION
- 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.
14CONVECTION
- 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.
15TWO 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.
16Forced 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.
17Factors 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.
18Factors 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.
19Factors 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.
20Factors 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.
21Factors 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.
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25Conduction (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.
26Conduction (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.
27Conduction (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.
28Factors 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).
29Factors 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.
30Radiation (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.
31Understanding 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.
32Understanding 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).
33Understanding 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.
34Radiation (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.
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36Factors 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.
37Factors 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.
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39Insensible 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.
40Sources 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.
41Evaporative 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.
42Factors 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.
43Factors 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.
44Factors 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.
45Partitioning of Actual Heat Loss to the
Environment
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47QUESTIONS?
48BIOPHYSICS OF HEAT TRANSFER AND CLOTHING
CONSIDERATIONS
49HEAT 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.
50HEAT 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.
51HEAT 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.
52HEAT 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.
53Total 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.
54Insulation 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.
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56Factors 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.
57Factors Affecting the Insulative Value of Clothing
- Variations in skin temperature distribution and
heat flow at various sites.
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59Factors 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.
60Factors 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.
61Factors 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. -
62Factors 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. -
63General Clothing Recommendations
- Use multiple layers.
- Outer layer should be wind and water resistant.
- Middle layer should trap air.
- Goose down
- Wool
- Polyester
- Polyolefrin
64General 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.
65Efficiency 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.
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67Insulation of Ambient Air
- A function of temperature, air velocity,
altitude, relative humidity, and precipitation.
68FACTORS 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.
69Altitude 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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71QUESTIONS?