Thermoregulation

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Thermoregulation Humans are able to regulate body
temperature within quite narrow deviations from a
resting body temperature of 370C. They are thus
classified as homeotherms.
Regulating homeotherms
Conforming poikilotherms
Internal state
Internal state
External state
External state
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Body temperature after 2 hours in 5, 10, 20, 30
35 0C
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cat
human
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Ornithorhynchus
Body temperature 0C
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Lizard
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10
20
30
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EXTERNAL TEMPERATURE 0C
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Core temperature is regulated by the
hypothalamusbehaves like a thermostat effecting
heat loss and heat gain responses. Preoptic
portion of anterior hypothalamus responds to heat
load signals Posterior portion of hypothalamus
responds to cold signals
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Response to a Heat Load
Thermal Receptors
Effectors
Cutaneous vasodilation
Core
Integration
HEAT LOAD
Hypothalamus (preoptic portion of anterior
Blood temperature
hypothalamus)
Skin
Sweating
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Response to Cold
Effectors
Thermal Receptors
Cutaneous vasoconstriction
Core
Integration
COLD
Shivering
Hypothalamus (posterior)
Blood temperature
Thyroxin release
Skin
Catecholamine release
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Set-point determined by the interaction of warm
and cold receptors
Cold sensors Warm sensors
Activity
Set-point
Temperature
Temperature inputs Hypothalamus
Effector response (compares sensed
information with set point)
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• Internal core temperature is a hypothetical
  average of all internal thermosensitive sites
• Sites used to provide an indication of core
  temperature
• tympaneum
• oesophagus
• oral (sublingual)
• rectum

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• Rates of temperature change at each measurement
  site vary
• -Oral temperature is lower than rectal
  temperature and this difference widens with
  increasing exercise intensity (difference is as
  large as 20C at 8METS)
• Oral temperature is more unreliable
• Oesophageal temperature is more rapid in response
  to heat storage with exercisebut influenced by
  fluid ingestion
• -Rectal temperature appears to be more reliable
  in diagnosing heat illness

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MODERATE INTENSITY CYCLE EXERCISE
Oesophageal temperature
CORE TEMPERATURE
Rectal temperature
20-40minutes
15-30minutes
TIME
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Mean skin temperature is estimated from a range
of skin sites with weightings assigned to take
account of the relative area the specific sites
represent. Mean body temperature is determined
as follows (0.33 x Tskin)
(0.67 x Trectal)
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Circadian Variation There are variations around
the normal resting core temperature due to
diurnal rhythms (daily fluctuations of 0.7 to
1.50C resting core temperature lower at
3.00am) and the menstrual cycle in women
(0.40C higher in the luteal phase).
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At temperatures above 440C
the parenchyma of cells
begins to deteriorate and denature At
temperatures below 340C Cellular metabolism
slows greatly leading to unconsciousness and
cardiac arrhythmias 410C reported in marathon
runners at end of race with no ill effects 17.70C
measured in WWII Concentration Camp Inmates
(Dachau-Germany)(respiration rate 3/min!)
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Deviations below normal core temperature are
better tolerated in survival terms than
elevations in core temperature. Upper limit for
survival is 440C, although many heat related
deaths have occurred at core temperatures lower
than this. Subjective sensations indicate acute
sensitivity to small changes in core temperature
(10C)
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Core Temperature
Upper limit for survival
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0C
Impaired thermoregulation, Heat stroke, brain
damage
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Extreme physical exercise and fever
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Resting core temperature 36.50C 37.50C
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Intense shivering and impaired coordination
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Violent shivering speech and thought impaired
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Decreased shivering erratic movements incoherent
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Muscular rigidity semiconscious
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Unconscious cardiac arrhythmias
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Thermoregulation absent
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• The thermal environment involves 6 fundamental
  factors
• Air temperature
• Radiant temperature
• Humidity
• Air movement
• Metabolic heat
• Clothing

Basic environmental parameters
Personal parameters
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• Environmental heat load is measured using
• Dry bulb temperature (Tdb)
• Wet bulb temperature (Twb)
• Black globe temperature (Tg)
• The wet bulb globe temperature (WBGT) index
• is a method for evaluating environmental stress
• WBGT 0.7 Tnwb 0.2 Tg 0.1 Tdb

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The WBGT when applied to the indoor environment
is based on two temperatures WBGT 0.7 Tnwb
0.3 Tg Despite being developed to reduce heat
illness casualties in the US army it is a poor
preictor of physiological stress and strain
Environments of the same WBGT do not represent
the same stress or strainwhen the humidity is
high greater strain is observed
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WBGT Guide for Outdoor Activities
WBGT (0C) Recommendations (ACSM) 26.5-28.8 Uncon
ditioned or un-acclimatised use
discretion 29.5-30.5 Avoid strenuous activity
in the sun gt31.2 Avoid exercise
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HEAT PRODUCTION Basal metabolic rate is defined
as heat production derived from cellular
metabolism oxidation of CHO, fats and
proteins. Calculate BMR from VO2 and RER. The
heat production value is usually expressed in
watts and normalised among individuals by
expressing heat production in terms of BSA.
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Clearly heat production is dependent on level of
physical activity(or VO2 consumed) Mechanical
efficiency ie. the conversion of energy into
mechanical work (W) in humans is 20 Need to
therefore subtract W from total energy
expenditure to accurately calculate total heat
production
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Temperature gradient illustrating heat loss
Exercising muscles ( 39.50C)
Muscles at rest 34-350C
Core temperature ( 38.50C)
36.5-37.50C
Skin temperature (290C)
32-340C
Air temperature (200C)
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Conductance of heat requires a temperature
gradient between the Muscle
Body core
Skin
Environment
The rate of heat flow is the product of fixed
conductance (passive cell to cell) and variable
conductance (convective transfer in the blood).
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• Avenues of heat dissipation
• Convection (C) movement of air about skin
  surface
• Heat is lost if Tskin gt Tair
• Heat is gained if Tskin lt Tair
• Conduction (K) transfer of heat between 2
  objects in direct contact very important heat
  loss mechanism for swimming
• Radiation (R) Heat exchange via electromagnetic
  waves
• Evaporation (E) of sweat from the skin

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Winslows heat balance equation M
E C R S M metabolic heat production E
evaporative heat loss C convective
conductive heat loss/gain R radiant heat
loss/gain S body heat storage/gain
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E, C R vary in their relative contribution to
heat balance according to the environmental heat
load and the metabolic heat load. eg. at rest in
a thermoneutral environment R is the greatest
form of heat loss - however during exercise under
the same environmental conditions E is the major
avenue of heat loss - C is the major avenue for
heat loss in water
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• During heat stress the body has two principal
  effector mechanisms for dissipating heat
• Increasing skin blood flow (peripheral
  vasodilation)
• Sweat production
• (1.0L of sweat evaporated 2436kJ)
• Behavioural strategies seek shade, immerse in
  water, take clothes off.

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Thermal balance versus environmental temperature
during moderate exercise (150W)
Heat production
96kJ
72kJ
evaporative
Heat loss rate
Convective plus radiative
24kJ
0
-24kJ

• 10 15 20 25
  30 35
• Ambient temperature (0C)

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• Factors determining the total thermal load
• Exercise intensity
• Environmental temperature
• Environmental evaporative capacity
• Clothing

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Delay in heat loss referred to as inertia or
temporal dissociation
plateau
300W
Core temperature 0C
200W
100W
time
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There is a linear relationship between core
temperature and the relative intensity of steady
state exercise (VO2max) Note that this
relationship appears to be curvi-linear at high
intensity
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Note Two individuals exercising at the same
absolute work load may have widely differing core
temperatures because of differences in the
relative exercise intensity ie they have very
different VO2maxs
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In a relatively dry environment, core temperature
is independent of environmental temperature (00C
200C) and directly related to exercise
intensity (VO2max).
80VO2max
Prescriptive zone
65VO2max
Core temperature (0C)
50VO2max
0 5 10 15
20 25
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• The prescriptive zone(effective
  thermoregulatory control) varies with
• Body size
• Body composition
• Fitness
• Acclimatisation
• Most individuals will have the same core
  temperature exercising at the same intensity
  (VO2max) across a wide range of ambient
  temperatures

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However when exercise intensity is high and/or
there is high humidity, heat storage within the
body can result in hyperthermia -under such
conditions there is a high metabolic heat load
and humid conditions restricts the effectiveness
of sweat (drips rather than evaporates) -no
longer operating within prescriptive zone
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Conflicts- Blood volume has to be partitioned
between the exercising muscles and skin during
exercise in the heat Blood flow to internal
organs is reduced and may result in damage to
these tissues if exercise is extended Sweat
production (if evaporated) cools the body but
leads to a dehydrated state
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Cardiovascular drift describes changes observed
in cardiovascular function when either the
metabolic and/or environmental heat load results
in Increasing HR Decreasing SV Reduced central
venous pressure Usually no change in CO
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C-V Drift in prolonged constant load exercise
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HR
VO2
10
of 10min value
CO
Art. BP
-10
SV
-20
time
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Rate of heat flow is governed by average
temperature gradient and
Overall conductance
Fixed conductance variable
conductance
Air velocity Temp.difference (skin environment)
Related to VO2 (rapid heat flow)
Body fat Skin temperature
Convection Radiation Evaporation
Increase in core temperature
Rate of skin blood flow
Ambient humidity -water vapour pressure gradient
between skin and environment independent of the
temperature gradient between skin and environment
Skin vascular resistance
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What do the terms threshold and
thermosensitivity mean in temperature
regulation? Threshold is that temperature above
which the effector response is different from the
baseline response at rest, in a neutral
environment Thermosensitivity (slope/gain) is the
difference in the effector response per unit
change in core temperature
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Threshold and thermosensitivity of the effector
response
R
R
Tcore Threshold
Tcore Thermosensitivity
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Acclimatisation Adaptation
Days HR decrease
3-6 Plasma volume increase 3-6
Trec 0C
5-8 RPE decrease
3-6 Sweat rate increase
8-14 Reduced sweat electrolytes 5-10
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Differences in classical and exertional heat
stroke Classical
Exertional age older younger physical
condition often chronically ill
fit onset subacute acute acid
base status respiratory alkalosis
metabolic acidosis rhabdomyolysis
occasionally frequently liver injury
occasionally frequently DIC
occasionally frequently disseminated
intravascular coagulation
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As a result of the ageing process a reduction in
heat tolerance and a decline in the efficiency of
thermoregulatory effector responses are reported
in most studies. At rest and during exercise,
older individuals demonstrate higher core
temperatures to heat stress compared with their
younger counterparts It is unknown whether the
decline in heat tolerance is a result of the
ageing process or a secondary occurrence to the
concomitant reduction in VO2max
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• In hot dry conditions, the observed
  thermoregulatory responses for older vs younger
  individuals are
• Higher core temperature (body heat storage)
• Less sweat produced and evaporated
• A reduction in sweating sensitivity ie. a slower
  response of sweating to changing exercise-heat
  stress conditions
• The reduction in sweating sensitivity is
  positively correlated to VO2max, therefore
  cardiorespiratory fitness may be involved in the
  sweating response

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• The differences between older and younger
  individuals are diminished in high humidity
  conditions
• During dehydration the reduction in intravascular
  fluid volumes are not defended as well by older
  adults
• Older subjects also demonstrate a reduced thirst
  sensitivity

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Pregnancy Increase in weight Decrease in
VO2max Increase in submaximal exercise VO2
cost Increase in resting body temperature Availab
le evidence indicates that the maintenance of
exercise during pregnancy has no adverse
consequences on maternal or foetal
health. However maternal core temperature should
not exceed 38OC
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Overheating during pregnancyparticularly during
the first 4-6weeks may have harmful effects for
the foetus (prior to closure of the neural tube).
This has ramifications for women exercising in
environments such as a hydrotherapy pool eg.
physiotherapists Studies on pregnant hamsters
subjected to exercise-heat exposure have found
gross malformations in the offspring. However
there have been no such reports in humans
although no specific studies have been
undertaken.