DAvMed 44
Thermoregulation
KCL 2011
Human physiology & thermal environment
Introduction
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Humans have endeavoured through building, engineering and clothing, to maintain the same
conditions around the body as applied in their ancestral home.
Mean skin temperature in all humans is 33°C.
For an aviator, the consequences of being unable to cope with a given thermal environment in a
cockpit or survicval situation can range from subtle decrements in performance to death.
Thermal stress
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Heat & Temperature
Celsius scale with 100 degrees between the freezing and the boiling points of water at sea level.
Various other scales:
o Celsius (C)
o Fahrenheit (F)
o Kelvin (K) — 0°K = -273.15°C
o SI (Joule J)
Heat energy is associated with the movement of individual atoms or molecules.
Rates of heat exchange are measured in Watts (1W = 1J/s).
Heat & temperature are related by the specific heat capacity of a substance (cp)
Volume-specific heat capacity = cp×d
Air & Water: a comparison of two fluids
 @ 37°C, the volume-specific heat capacity of water is 3431 times of that of air.
o Thermo-neutral temperature of water is 35°C (@ rest)
o Thermo-neutral temperature of air is 26°C (@ rest)
o Body core temperature cools 2 to 5 times faster during immersion in cold water than with
in air at the same temperature.
o Cold water produces more profound responses earlier and at higher temperatures, in
comparison with air.
o On average, water temperature can be 11°C higher than air and produce an equivalent
physiological response.
 When immerged upright, a negative transthoracic pressure of 14.7 mmHg is established, which
results in negative pressure breathing.
o Increase of central blood volume by ≈ 700 ml
Heat (energy) balance
The first 2 thermodynamic laws state:
o Energy cannot be created or destroyed, but is transformed from one form to another.
o Heat flows down a thermal gradient.
 To remain in heat balance, the heat gained by a body must match that being lost from it.
 Four physical routes by which heat may be exchanged:
o Conduction K
o Convection C
o Radiation R
o Evaporation E
 The human heat-balance equation always contains terms for the generation of heat within the
body, heat transfer with the environment and heat storage:
M  W   R  C  K  E  S
(in Watts)
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Thermoregulation
KCL 2011
 To remain in heat balance, S must be equal to 0
(in Watts)
S  M  (W )  R  C  K  E
o W/m2
o M is the metabolic energy utilization
o W is measurable external work
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o If the body doesn't remain in heat balance, then S will change and will be reflected in a
change in mean body temperature (Tb):
Tb 
S  cpm
 t 
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Metabolism
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M is the total metabolic energy utilization
= energy liberated by sum of all catabolic chemical
reactions of the body
o Only 20-25% of the chemical energy used during muscle contraction is converted into
mechanical work.
o The remainder is liberated as heat.
o Normally, 67-85% of an individual's total daily energy expenditure is due to the activities
of the body that maintain vital functions.
o @ complete rest, the basic metabolic rate is about 1kcal/min (≈ 1500kcal/day)
Metabolic heat production can be measured by:
o Direct calorimetry (difficult)
VCO2
o Indirect calorimetry based on the use of O2 ⇒ RQ 
VO2
 RQ = steady-state non-protein respiratory quotient.
o Isotope-based method that employs double-labelled water (with known concentrations of
deuterium ( 2 H ) and oxygen-18 ( 18O2 ).
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Radiation
Thermal radiation R is part of the electromagnetic spectrum (from γ rays to radio waves)
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Ultraviolet portion is divided in:
o UV-A (320-400nm)
o UV-B (290-320nm)
o UV-C (200-290nm)
o UV-B is very efficient at causing sunburn (+ cancer)
o White skin absorbs ≈ 60% of the incident radiation
o Dark skin absorbs ≈ 80% of the incident radiation, but is less susceptible to sunburn.
All objects possessing heat emit thermal radiation from their surfaces in the form of a wave of
energy containing particles (photons) within the IR range.
o The peak wave-length of the emitted radiation is inversely proportional to the absolute
temperature of the emitting surface.
No medium is required for the transfer of heat by radiation.
The temperature of any air through which heat radiates has little effect on the heat transferred.
Radiant heat is responsible for high cockpit temperatures (by green-house effect)
The quantity of heat transferred from one object to another by radiation depends on:
o Effective radiating surface area
o Difference between the mean surface temperatures of both objects
o Emissivity (ε) of the surface
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Thermoregulation
KCL 2011
Convection
Convection C is the exchange of heat by molecular mass transfer within a fluid medium, during
which molecules retain their heat energy but move within the confines of the medium.
It is often the most significant route of heat loss from the body in cold environment.
The rate of heat exchange between a body and its environment through convection depends on:
o Temperature gradient between the two
o Density, pattern and relative movement of the fluid in which the body is placed
o Surface area exposed.
o Can vary from 1-4W/m2/°C in still air to 60-100 W/ m2/°C in still water and up to
400W/m2/°C in water with a velocity of 0.5m/s
Free convection is important only in environments with very low fluid flow rates.
Boundary layer around a naked individual standing in cold still air.
⇒Stream of molecules possessing thermal energy and rising away from the heat source.
o Boundary layer of about 180mm thick at the face for a naked person in 25°C
o Velocity of 0.5m/s ⇒10l/s of air passing over the head.
o Boundary layer destroyed by moving fluid
o Convectional heat exchange is increased through forced convection ⇒ hc  8.3V 0.5
Conduction
 contact or in solid-fluid
Conduction K is the heat exchange between two solid surfaces in direct
interface.
There is no physical movement of material within the respective objects.
The rate of conductance depends on:
o Temperature gradient between the two surfaces
o Surface area in contact
o Thermal conductivity and specific heat capacity of each object
o Distance through which the heat is conducted.
Usually, the amount of heat exchange of the human body by conduction is very small, but can
increase if the individual lies down.
The inverse of conductance is thermal insulation.
o The most common units associated with insulation are:
 the Clo
 the tog (1Clo = 1.55tog = 0.155°C/m2/W)
Evaporation
Evaporation E is the process by which energy transforms a mass of liquid to a gas.
o The phase change from liquid to water vapour requires 2.4kJ/g of energy
o Is not associated with an increase in the temperature of the liquid
o Evaporative heat exchange occurs only within a gaseous medium and when liquid
evaporates from the surface of an object.
The rate of evaporation depends on:
o Skin surface area that is wet
o Air relative movement around the body
 Air movement prevents the boundary effect
 he  124V 0.5
o Difference between the vapour pressure at the sklin surface and that in the air.
There are no temperature terms in the evaporation heat loss equation.
Even
in cold environment, the body is losing water by evaporation from the lungs and the skin to
the surrounding environment. (1 l/day ≈ 24kcal/h = 28W)
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Thermoregulation
KCL 2011
The thermal environment & its assessment
Air temperature
Is measured using a thermometer.
With regard to thermal balance, the air temperature of most interest is that close to the surface of
the skin.
 There can be great gradient of temperature ⇒has to measured at different heights.
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Radiant temperature
 Radiation can represent a significant thermal load on an individual outdoors.
 The mean environmental radiant temperature is usually measured using a globe thermometer.
 The black globe provides a good approximation in most situations, but it may not be
representative for non-uniform (directional) radiation.
Altitude
 Air temperature falls by approximately 1.98°C per 1000ft increase in altitude.
 The reduction in air density with altitude reduces the rate of convective heat transfer.
 An increase in evaporative heat transfer at altitude is related to the change in thermal diffusivity.
Air movement
 Has an important influence on convective and evaporative heat exchange.
 Is measured using a cup or a vane anemometer.
 Humans can perceive air velocity below 0.1m/s
Combinations of measures and indices
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In order to assess some aspects of the environment (humidity…), two or more measures are taken
simultaneously.
Humidity
Humidity is the amount of water vapour present in a gaseous atmosphere.
It can be expressed in several ways:
o water vapour pressure
o dew-point temperature
o relative humidity.
Humidity is an important determinant of the capacity for heat loss (evaporation) and comfort in
both natural and artificial environments.
It is measured using a hygrometer.
Operative temperature
 Operative temperature is a means of expressing all the parameters of dry heat exchange in a
single variable for human subjects.
 Operative temperature is the uniform temperature of an imaginary isothermal "black" enclosure
in which an individual at thermal equilibrium would exchange the same amount of by radiation,
convection and conduction from the skin surface as they would in the real non-uniform
environment.
h T  hrTR 
To  c a
hc  hr 
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Thermoregulation
KCL 2011
Indices and models of heat and cold stress
 cf table 12.4 p.201-203
The response: the physiological response to thermal stress
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Behavioural responses such as removing clothing, move to shade and fanning will influence the
level of heat loss.
To achieve thermal homeostasis, the human thermoregulatory systems combines:
o peripheral sensors
o central sensors
o afferent pathways
o efferent pathways
o central integrating and controlling centres
Cutaneous thermosensors
 Surface of body well supplied with thermosensors extremely sensitive
o Cold sensors which increase firing when cooled
 Free nerve endings (C-fibres)
 Active between 10 & 40°C
 Static maxima @ 25°C
 Inhibited by warming
 3-4 times more cold sensors than warm sensors
 Are more superficial
o Warm sensors which increase firing when warmed
 Free nerve endings (C-fibres)
 Active between 30 & 50°C
 Static maxima @ 44°C
 Inhibited by cold
 3-4 times less warm sensors than cold sensors
 Are deeper situated
o Are not evenly distributed.
o Same pathways as pain.
Central reception and processing
 Several sites withion the body are capable of eliciting generalized thermoregulatory responses.
 Primary centre of thermoregulatory integration and control = Preoptic Nucleus-Anterior
Hypothalamus (POAH).
o Warm sensitivity is an inherent property of some POAH neurons.
o Half of the thermosensitive POAH neurons also respond to non-thermal stimulation.
Reference
Comparator - Controller
Detectors
Effectors
Controlled variable
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Thermoregulation
KCL 2011
With larger animals such as humans, the thermal drive from the deep body sites is greater than
that from the skin.
⇒Skin temperature has to change by a greater amount than deep body temperature to achieve the
same impact on the thermoregulatory effector response.
Effector responses
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The primary autonomic thermoregulatory responses of the body are:
o Vasomotion
o Sweating
o Shivering
More powerful and therefore more important are:
o Behavioural responses, driven by the conscious perception of temperature and comfort.
Behaviour
 Driven by the conscious perception of temperature and comfort
 Have enabled humans to move away from their equatorial origins
 A given sklin temperature can be perceived as either pleasant or unpleasant, depending on
whether it is assisting deep body temperature return towards normal values.
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Vasomotion
Microcirculatory structures influence cutaneous blood flow.
Neural control of blood flow achieved by active vasoconstriction (VC) and vasodilattion (VD).
When exposed to heat ⇒active cutaneous VD ⇒subsequent increase in skin blood flow.
Homeothermic animals without fur depend heavily for their heat regulation on control of their
skin circulation.
Exposed to cold, total skin blood flow may be reduced to 20ml/min ⇒➚ skin & fat insulation
Exposed to heat, total skin blood flow may be increased up to 3000ml/min
Thermoneutral skin blood flow = 160ml/min.
3 functionally different regions:
o Extremities: small mass/area ratio ⇒facilitate heat transfer with environment
o Trunk & proximal limbs
o Head & brow: lack significant VC input: ⇒can be responsible for high heat loss in cold
environment.
Vasomotor tone regulates heat loss.
Up to 70% of total tissue insulation can be provided by unperfused muscle.
o Source of insulation lost when exercise, including shivering (increased muscle blood flow).
o Adipose tissue = fixed insulation
VD bypasses the insulating tissues of the body and transports body heat by mass flow directly to
the skin.
High skin blood flow places a strain on the cardiovascular system: associated with pooling of
blood in compliant skin and subcutaneous vascular beds.
Shivering
Involuntary simultaneous asynchronous and rhythmic contractions of skeletal muscle motor
units.
o 9-20 Hz out of phase with other units
o ⇒Little external work is done and most of the energy consumed is converted to heat.
In the early phase, heavy shivering may be interspersed with periods of light shivering or rest.
Later, it becomes continuous before progressing into an almost tonic state.
Individuals are able to shiver at a maximum of 46% of their VO2max.
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Thermoregulation
 Shivering is attenuated if:
o Blood glucose level ➘
o ➘ O2 in the inspired air
o ➚ CO2
 Shivering and moderate levels of voluntary activity can coexist.
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KCL 2011
Sweating
Humans have about 2.5M sweat glands:
o 100-600 /cm2
o More on:
 Forehead
 Neck
 Trunk
 Hands & forearms
o Fewer on the thighs.
Activation:
o within seconds of the commencement of exercise or exposure to heat.
o Maximum output after about 30 minutes
Tonicity of sweat:
o Same tonicity as extracellular fluid when secreted by the sweat gland
o Reabsorption of Na and Cl by active ion-exchange pumps in the duct ⇒hypotonic.
Maximum sweat rate ≈ 2 litres per hour
Maximum sustained sweat rate ≈ 1 litre per hour
People acclimatized to heat:
o Produce a greater volume of sweat
o Contains less salt than those who are unacclimatized
o Start sweating at lower temperature
Factors influencing thermoregulation
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Heat exchange with the environment will be determined by:
o Environmental factors:
 Humidity
 Air movement
 Immersion in air or water
 Clothing
o Bodily factors:
 Fat content
 Muscle mass and its perfusion
 Fitness and the performance of exercise ➘ heat balance
o Non thermal factors:
 Ageing
 Gender: ♀have a smaller thermogenic response, more body fat, higher area/mass
ratio
 Anaesthesia (➘ shivering)
 Dehydration (➘ sweating)
 Hyperbaria: inert gas narcose ➘ shivering and thermal perception
 Hypoxia / hypobaria: impaired shivering response
 Hypercapnia: impaired shivering response
 Hypoglycaemia: delayed initiation of the shivering
 Trauma
 Motion illness: ➘ VC response to cold
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Acclimation:
 Cold: ➘ shivering response over body temperatures experienced during
acclimation
 Heat: earlier and ➚ production of more dilute sweat
Medical or genetic conditions
Drug intoxications:
 OH: ➘ thermal perception and VC response in cold air
 Marijuana: non particular influence
 Ecstasy: the thermoneutral zone is widened, and hypo/hyperthermia occur
more easily.
Measuring the body's temperature and thermoregulatory response
Body temperature
 Skin temperature can be measured using a variety of techniques
o thermistor
o thermocouple
o IR thermography
 Deep body temperature can be measured @ several different sites:
o Oral: ☹
o Rectum: ☺
o External auditory meatus / tympanic membrane:☹
o Oesophagus: ☺
o Urine: ☺but easily inaccurate
o Gastrointestinal tract: ☺for field studies
o Transcutaneous temperature: ☺
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Measuring effector responses
Peripheral blood flow can be measured and estimated by:
o Laser Doppler
o IR photoplethysmography
o Strain gauge
o Venous-occlusion plethysmography
The method chosen will depend on the specific circumstances.
Shivering is normally assessed by measuring oxygen consumption.
In naked individuals, sweat production and evaporation normally are assessed by the change in
naked body weight before and after exposure.
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