general characteristics of the thermal environment and mechanisms of thermal regulation

GENERAL CHARACTERISTICS OF GENERAL CHARACTERISTICS OF THE THERMAL ENVIRONMENT AND THE THERMAL ENVIRONMENT AND

MECHANISMS OF THERMAL MECHANISMS OF THERMAL REGULATIONREGULATION

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 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.

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.

CONVECTIONCONVECTION 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.

CONVECTIONCONVECTION

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 CONVECTIONTWO 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.


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.


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.


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 agreater thermal conductivity that air, and water temperature is usually lower than air temperature.


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.


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.


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).


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.


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.



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.


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 BIOPHYSICS OF HEAT TRANSFER AND TRANSFER AND

CLOTHING CLOTHING CONSIDERATIONSCONSIDERATIONS

HEAT TRANSFERHEAT TRANSFER Heat transfer is the analysis of the Heat transfer is the analysis of the rate of heat transfer, flow, or rate of heat transfer, flow, or exchange in a system, which is exchange in a system, which is governed by the laws of governed by the laws of thermodynamics; the modes of heat thermodynamics; the modes of heat transfer in a system are radiation, transfer in a system are radiation, convection, conduction, and convection, conduction, and evaporation; the combined evaporation; the combined interaction of these mechanisms interaction of these mechanisms results in the overall heat transfer results in the overall heat transfer within a system and consequently, within a system and consequently, heat storage, heat loss, or thermal heat storage, heat loss, or thermal balance.balance.

HEAT FLOW AND FLUXHEAT FLOW AND FLUX

Heat always flows from the Heat always flows from the region of high temperature to region of high temperature to a region of low temperature. a region of low temperature.

Heat flux is a term used to Heat flux is a term used to summarize the amount of heat summarize the amount of heat transferred per unit of time.transferred per unit of time.

HEAT BALANCE EQUATIONHEAT BALANCE EQUATION

Remember: S = M - (Remember: S = M - (++ W) W) ++ K K ++ C C ++ R - R - E; In this equation M is equal to E; In this equation M is equal to metabolic heat production (resting metabolic heat production (resting metabolic rate = 3.5 ml/kg/min or 50 metabolic rate = 3.5 ml/kg/min or 50 kcal/hr/m2; for every L of VO2, kcal/hr/m2; for every L of VO2, approximately 5.0 kcal are expended); W approximately 5.0 kcal are expended); W is equal to work, which is either is equal to work, which is either positive work representing energy positive work representing energy leaving the system or work against leaving the system or work against internal forces internal forces OROR negative or negative or eccentric work or work against external eccentric work or work against external forces; K, C, R, & E represent the forces; K, C, R, & E represent the mechanisms of heat transfer.mechanisms of heat transfer.

HEAT TRANSFERHEAT TRANSFER

•• In addition to previously discussed In addition to previously discussed information, information, insulation from air insulation from air and clothing and clothing are factors which need are factors which need to be takento be taken into consideration into consideration when when understanding the total understanding the total impact of heat transfer.impact of heat transfer.

Total Insulation = ITotal Insulation = Iclothing clothing + I + Iambient airambient air

Thermal insulation Thermal insulation is the resistance offered is the resistance offered to the flow of heat between two surfaces and to the flow of heat between two surfaces and is determined by:is determined by:

(T(T1 1 - T- T22)/Flow of heat per unit of surface )/Flow of heat per unit of surface area.area.

Note: The slower (i.e., lower) the flow of heat per unit of Note: The slower (i.e., lower) the flow of heat per unit of

surface area or the smaller the difference between the surface area or the smaller the difference between the

temperatures of two surfaces, the greater the thermal temperatures of two surfaces, the greater the thermal

insulation.insulation.

Insulation of ClothingInsulation of Clothing

1 CLO = unit of clothing thermal 1 CLO = unit of clothing thermal insulation; the clothing necessary insulation; the clothing necessary to insult in comfort to insult in comfort (thermoneutrality) a resting (thermoneutrality) a resting subject at 21 Csubject at 21 Co o (70(70oo F), air F), air movement of 10 cm/s or 20 fpm movement of 10 cm/s or 20 fpm (normal ventilation rate of a (normal ventilation rate of a room), and a relative humidity of room), and a relative humidity of less than 50%.less than 50%.

Factors Affecting the Insulative Factors Affecting the Insulative Value of ClothingValue of Clothing

Fabric's thermal conductionFabric's thermal conduction, which is a , which is a function of the thicknessfunction of the thickness of the clothing and of the clothing and extend of trapped air layersextend of trapped air layers; the greater the air ; the greater the air trapped and/or the thicker the clothing, the greater trapped and/or the thicker the clothing, the greater the insulation.the insulation.

Fabric's dispersion over the skin surface Fabric's dispersion over the skin surface areaarea, which extends the total potential surface , which extends the total potential surface area open to the environment; the greater the area open to the environment; the greater the dispersion of clothing over the skin surface area, dispersion of clothing over the skin surface area, the greater the insulation.the greater the insulation.


Variations in skin Variations in skin temperature temperature distribution and distribution and heat flow at various sites.heat flow at various sites.


Variations in clothing surface covering the Variations in clothing surface covering the skin and skin blood flow:skin and skin blood flow: none of the hands none of the hands and face, presence of arterial-venous anastomosis and face, presence of arterial-venous anastomosis in the extremities, and vasodilatory activities in in the extremities, and vasodilatory activities in the face.the face.

Air layer next to skinAir layer next to skin; an increase in movement ; an increase in movement will decrease the air layer and insulation around will decrease the air layer and insulation around the skin.the skin.

Exercise increases air movementExercise increases air movement, particularly , particularly if garment is not wind resistant.if garment is not wind resistant.


Wet clothing Wet clothing will decrease the will decrease the insulation of clothing to 30%. insulation of clothing to 30%. Sweating Sweating (30 ml/hr at rest, up to 1.5-(30 ml/hr at rest, up to 1.5-2.0 L/hr during exercise) and r2.0 L/hr during exercise) and rain or ain or snowsnow if the garment is not water if the garment is not water repellent will decrease the insulation of repellent will decrease the insulation of clothing.clothing.


Compression of clothing material. Compression of clothing material. Particularly Particularly true to the feet where compression of boots as a true to the feet where compression of boots as a person stands on a stone floor has been reported person stands on a stone floor has been reported to reduce insulation to that comparable to standing to reduce insulation to that comparable to standing with naked feet. Also with the hands, the gripping with naked feet. Also with the hands, the gripping of ski poles or bike handlebars will cause of ski poles or bike handlebars will cause compression of the gloves and therefore, reduce compression of the gloves and therefore, reduce the insulation of the gloves. Finally, water has the insulation of the gloves. Finally, water has been reported to decrease insulation of been reported to decrease insulation of compressed clothes by up to 50%.compressed clothes by up to 50%.


Air temperatureAir temperature. As air temperature . As air temperature increases above skin temperature, insulation increases above skin temperature, insulation increases which may lead to a hyperthermic increases which may lead to a hyperthermic (i.e., heat gain) response; as air temperature (i.e., heat gain) response; as air temperature decreases below skin temperature, insulation decreases below skin temperature, insulation decreases which may lead to a hypothermic decreases which may lead to a hypothermic (i.e., heat loss) response. (i.e., heat loss) response.

General Clothing RecommendationsGeneral Clothing Recommendations

Use multiple layers.Use multiple layers. Outer layer should be wind and Outer layer should be wind and

water resistant.water resistant. Middle layer should trap air.Middle layer should trap air.

Goose downGoose downWoolWoolPolyesterPolyesterPolyolefrinPolyolefrin

General Clothing RecommendationsGeneral Clothing Recommendations

Inner layer should also wick away Inner layer should also wick away moisture from the skin to prevent moisture from the skin to prevent evaporative heat loss.evaporative heat loss.

PolypropylenePolypropyleneCotton FishnetCotton Fishnet

Most important to cover trunk and Most important to cover trunk and head during prolonged exposure to head during prolonged exposure to cold.cold.

Efficiency Factor of ClothingEfficiency Factor of Clothing

• • Ratio of:Ratio of:

Thermal resistance between clothing surface and Thermal resistance between clothing surface and airair

Resistance between skin surface and airResistance between skin surface and air

•• Higher the ratio the greater the efficiency Higher the ratio the greater the efficiency factor of clothing or insulation and vice-factor of clothing or insulation and vice-versa.versa.

Insulation of Ambient AirInsulation of Ambient Air

AA function of temperature, air function of temperature, air velocity, altitude, relative velocity, altitude, relative humidity, and precipitation. humidity, and precipitation.

FACTORS THAT DECREASE THE FACTORS THAT DECREASE THE INSULATORY VALUE OF AIRINSULATORY VALUE OF AIR

Decrease in air temperature Decrease in air temperature below skin temperature (Tbelow skin temperature (Ta- a- TTsksk).).

Increase in air velocity Increase in air velocity (exercise will increase air (exercise will increase air velocity).velocity).

Decrease in relative humidity Decrease in relative humidity and/or precipitation.and/or precipitation.

Decrease in elevation (i.e., low Decrease in elevation (i.e., low elevation). Air at high altitude elevation). Air at high altitude provides better insulation.provides better insulation.

Altitude and Insulation of Altitude and Insulation of Ambient AirAmbient Air

Since altitude decreases convective Since altitude decreases convective heat loss and increases radiant heat heat loss and increases radiant heat gain and evaporative heat loss, the gain and evaporative heat loss, the increase in the insulatory value of air increase in the insulatory value of air at high altitudes suggests that the at high altitudes suggests that the decrease in convective heat loss and decrease in convective heat loss and increase in radiant heat gain is increase in radiant heat gain is greater than the increase in greater than the increase in evaporative heat loss at high altitude.evaporative heat loss at high altitude.

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general characteristics of the thermal environment and mechanisms of thermal regulation

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