Part 4
Thermoregulation and Fluid
Balance During Heat Stress
1
Chapter 10
Physical Activity:
Thermoregulation, Fluid Balance,
and Rehydration
2
Challenge of Environmental Stress
• Most individuals acclimatize to all earth’s environments
after 8- to 14-day exposure
• Loss of acclimatization occurs in 14 to 28 days
• Six factors impact individual differences in
accommodation and acclimatization:
1. Genetic characteristics
2. Available resources
3. Age
4. Nature and duration of previous exposures
5. Number of similar prior experiences
6. Emotional and psychological response (worry, fear, panic,
self-assurance) to environmental stress
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Mechanisms of Thermoregulation
Thermal Balance
• Deep tissue (core) body temp represents a
dynamic equilibrium that adds and subtracts body
heat
• Three integrating mechanisms
1. Following heat transfer to the periphery
2. Regulate evaporative cooling
3. Vary heat production rate
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Mechanisms of Thermoregulation
Thermal Balance
FIGURE 10.1 Factors contributing
to heat gain and heat loss to
regulate core temperature at about
37°C (98.6°F)
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Mechanisms of Thermoregulation
Mechanisms for Temperature Regulation
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Mechanisms of Thermoregulation
Heat-Regulating Mechanisms
• Become activated in two ways:
1. Blood perfusing the hypothalamus directly
stimulates its control center
2. Thermal receptors in skin provide input to
modulate hypothalamic activity
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Mechanisms of Thermoregulation
Body Temperature Measurement
• Thermal gradient exists within the body
– Core temp (Tcore) highest
– Shell temp (Tskin) lowest
o Mean body temp (Tbody) = average skin +
internal temp
• Measurement sites to estimate core temp (Tcore )
o Rectum, eardrum (tympanic), esophagus
(esophageal)
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Mechanisms of Thermoregulation
Hypothalamic Regulation of Core Temp
• Hypothalamus contains central neural coordinating
center to regulate temperature
• Specialized neurons at the brain floor serve as
“thermostat”
– Usually set and regulated at 37°C ± 1°C (98.6°F
± 1.8°F)
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Mechanisms of Thermoregulation
FIGURE 10.2 Illustration of skin and underlying structures. Skin surface (right) shows dynamics of
conduction, convection, and sweat evaporation. Each 1 liter of H2O evaporated from the skin transfers 580
kilocalories of heat energy to the environment
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Mechanisms of Thermoregulation
Thermoregulation During Heat Stress
• Thermoregulatory mechanisms protect against
overheating
• Four ways body heat loss occurs:
1. Radiation
2. Conduction
3. Convection
4. Evaporation
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Mechanisms of Thermoregulation
FIGURE 10.3 Heat
production within
active muscle and its
transfer from core to
skin. Excess body
heat dissipates to
regulate core temp
within a narrow range
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Mechanisms of Thermoregulation
Heat Loss by Radiation
• Objects emit electromagnetic heat waves
• Body temp warmer than environment
• Radiant heat energy leaves the body through air to
solid, cooler objects
• Body absorbs radiant heat energy when temp of
objects in environment ≥ skin temp
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Mechanisms of Thermoregulation
Heat Loss by Conduction
• Transfers heat directly through a liquid, solid, or
gas from one molecule to another
• Circulation transports body heat to the shell
• Small amount of heat moves through deep tissues
to cooler surfaces
• Involves warming of air molecules and cooler
surfaces in contact with the skin
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Mechanisms of Thermoregulation
Heat Loss by Convection
• Requires air movement near the skin
• Warm air next to the skin acts as zone of
insulation
• Heat loss increases when cool air replaces warmer
air surrounding the body
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Mechanisms of Thermoregulation
Heat Loss by Evaporation
• Major physiologic defense against overheating
• Water vaporization from respiratory passages and
skin surface continually transfers heat to the
environment
• 2 to 4 million sweat (eccrine) glands secrete large
quantities of hypotonic saline solution
• Cooling occurs when sweat evaporates from skin
surfaces
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Mechanisms of Thermoregulation
Heat Loss at High Ambient Temperatures
• Increased ambient temp reduces heat loss by
conduction, convection, and radiation
– When ambient temp ≥ body temp, evaporation,
convection, and conduction increase heat gain
• Sweat evaporation from skin and water
vaporization from respiratory tract provide only
avenues for heat dissipation
– Sweating rate increases directly with ambient
temperature
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Mechanisms of Thermoregulation
Heat Loss in High Humidity
• Three factors determine skin sweat loss
1. Surface area exposed to environment
2. Ambient air temp + relative humidity (RH)
o
RH = %H2O in air at particular temp
compared to total quantity of air moisture
possible
3. Convective air currents around the body
• RH exerts greatest impact on effectiveness of
evaporative heat loss
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Mechanisms of Thermoregulation
Heat Loss
in High
Humidity
FIGURE 10.4 Hourly
sweating rates related to
environmental conditions
and PA intensity
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Mechanisms of Thermoregulation
Integration of Heat-Dissipating Mechanisms
•
Circulatory, evaporative, + hormonal adjustments
contribute to integration
– Circulation
o Main “workhorse” to control thermal balance
o HR and cardiac output increase while superficial
arterial + venous blood vessels dilate
– Evaporation
o Large skin blood flow + evaporative cooling
produces effective thermal defense
– Hormonal
o Antidiuretic hormone + aldosterone
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Mechanisms of Thermoregulation
Evaluating Environmental Heat Stress
• Five factors (other than temp) determine heat
stress
1. Body size and fatness
2. Training level
3. Acclimatization
4. Hydration adequacy
5. External factors (convective air currents,
radiant heat gain, PA intensity)
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Mechanisms of Thermoregulation
Evaluating Environmental Heat Stress—Use of WBGT as an Index of Heat Stress
• Prevention the most effective way to minimize or
eliminate heat stress injuries
– Wet bulb-globe temperature (WB-GT) evaluates
thermal challenge
– WB–GT = 0.1 × DBT + 0.7 × WBT + 0.2 × GT
o DBT = air temperature
o GT = globe temp in direct sun recorded by a
thermometer with black metal sphere
surrounding bulb (measures radiant heat gain)
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Mechanisms of Thermoregulation
Evaluating Environmental Heat Stress—Use of WBGT as an index of heat stress
FIGURE 10.5 WB-GT for outdoor activities and wet-bulb temperature (WBT) guide
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Thermoregulation During PA in Heat
Core Temp During PA
• Heat generated by active muscles can raise core
temp to fever levels
• Distance runners show few ill effects from rectal
temp as high as 41°C (105.8°F)
• Increased core temp during PA does not reflect a
failure of the heat-loss mechanisms
• A well-regulated rise in core temp occurs even during
cold weather PA
– A modest rise in core temp reflects favorable
internal adjustments
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Mechanisms of Thermoregulation
FIGURE 10.6 Heat
index. How hot is too
hot?
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Thermoregulation During PA in the Heat
Water Loss in the Heat: Dehydration
• Any degree of dehydration impairs capacity of
circulatory and temperature-regulating mechanisms
to adjust to physiological demands
– Dehydration ≤2% body mass impairs physical
work capacity and physiologic function,
threatening heat injury
• Risk for dehydration increases during vigorous coldweather PA
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Thermoregulation During PA in the Heat
• Average H2O loss/h via
sweating at various air
temps during rest and
light/moderate PA
– H2O loss (sweating) in
acclimatization peaks
at 3 L·h−1 during
intense PA in heat and
nearly 12 liters (26 lb)
on a daily basis
– Several hours of
intense sweating can
cause sweat gland
fatigue that impairs
temp regulation
FIGURE 10.7 Average water loss per hour for a
typical adult caused by sweating at various air
temperatures during rest and light and
moderate physical activity
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Thermoregulation During PA in the Heat
Physiologic and Performance Consequences
• With dehydration and plasma volume decreases,
peripheral blood flow and sweating rate diminish and
thermoregulation becomes progressively more
difficult
– Contributes to larger increases in HR, RPE, and
core temperature than under normal hydration
and fatigue
– Fluid loss of 1% of BW increases rectal temp and
impairs circulatory and temp-regulating
mechanisms
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Thermoregulation During PA in the Heat
Physiologic and Performance Consequences
•
Dehydration of only 2% body mass impairs work capacity
and physiologic function and predisposes to heat injury
– Each liter sweat loss increases activity HR by 8 b·min−1
with a 1.0 L·min−1 decrease in CO
– Results in:
o Decreases plasma volume
o Reduces skin blood flow
o Reduces SV
o Increases HR
o Compromises circulation and thermoregulation
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Thermoregulation During PA in the Heat
Water Replacement: Rehydration
• Proper fluid replacement maintains plasma volume to
preserve circulation and sweating progress
• Well-hydrated persons function at a higher
physiologic and performance level than dehydrated
persons
• Hyperhydration before PA in a hot environment
protects against heat stress
– Delays dehydration
– Increases PA sweating
– Reduces core temp rise
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Thermoregulation During PA in the Heat
Pre-Activity Hydration
• Ingesting “extra” H2O (hyperhydration) before PA in
hot environment protects against heat stress
because
1. Delays dehydration
2. Increases PA sweating
3. Minimizes core temp increase
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Thermoregulation During PA in the Heat
Recommended Fluid Intake
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Thermoregulation During PA in the Heat
Adequacy of Rehydration
• BW changes show extent of H2O loss from PA and
adequacy of rehydration during and after PA
• Urine and hydration
– Inadequate hydration = Dark yellow urine with
strong odor
– Adequate hydration = Large urine volume, light
color, without strong odor
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Thermoregulation During PA in the Heat
Sodium Facilitates Rehydration
• Add moderate (100 mmol·L−1) Na to beverage for
complete rehydration
• Maintaining a high plasma concentration of Na helps
to:
– Sustain the thirst drive
– Promote retention of ingested fluids
– Rapidly restores lost plasma volume during
rehydration
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Thermoregulation During PA in the Heat
Sodium
Facilitates
Rehydration
FIGURE 10.8 Effect of adding sodium to a
rehydration beverage on retention of ingested
fluid during recovery from PA. Six men
exercised in a warm, humid environment
until sweating produced a 1.9% BW loss.
They then ingested 2045 mL of one of four
test drinks containing sodium in a
concentration of either 2, 26, 52, or 100
mmol·L−1 over a 30-minute period beginning
30 minutes after activity stopped
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Thermoregulation During PA in the Heat
Hyponatremia: Reduced Sodium Conc.
• Low blood sodium level (<135 mEq·L−1)
• Often due to excessive H2O intake
– Sustained low plasma sodium creates osmotic
imbalance across blood-brain barrier—causes rapid
H2O influx into brain
o Resulting brain swelling produces a cascade of
symptoms that range from mild to severe (and
potentially death)
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Thermoregulation During PA in the Heat
Contributing
Factors to
Hyponatremia
FIGURE 10.9 A. Factors contributing
to hyponatremia. B. Physiologic
consequences of hyponatremia. AVP,
arginine vasopressin; CFTR, cystic
fibrosis transmembrane regulatory
gene; CNS, central nervous system
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Thermoregulation During PA in the Heat
Factors That Improve Heat Tolerance
1. Acclimatization
2. Sweating
3. Body fat
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Thermoregulation During PA in the Heat
Factors That Improve Heat Tolerance
•
Acclimatization—“Physiologic adaptations that improve
heat tolerance”
– Acclimatized individual
o Larger quantities of blood shunt to cutaneous vessels
o More effective cardiac output
o Earlier sweating onset
•
Age differences in acclimatization
– Older people have decreased thermoreceptors
sensitivity
– Limited sweat gland output
– Altered skin structure and function
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Thermoregulation During PA in the Heat
Factors That Improve
Heat Tolerance
• Acclimatization
– Acclimatization to
heat stress occurs
during first wk. of
heat exposure (2–
4 h daily) with
complete
acclimatization
after 10 days
FIGURE 10.10 Average rectal temperature, heart
rate, and sweat loss during 100 minutes of daily heat
physical activity exposure for 9 consecutive days. On
day 1, the men walked on a treadmill at an intensity
of 300 kcal · h−1 in a cool climate. Thereafter, they
performed the same daily activity in the heat at
48.9°C (26.7°C wet-bulb)
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Thermoregulation During PA in the Heat
Factors That Improve Heat Tolerance
•
Children
– Prepubescent children have greater number heatactivated sweat glands per unit skin area than
adolescents and adults
o They sweat less and achieve higher core temp during
heat stress
•
Sweating
– Sweating exhibits distinct sex differences in
thermoregulation
o Women possess more heat-activated sweat glands
o Women begin sweating at higher skin/core temp
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Thermoregulation During PA in the Heat
Factors That Improve Heat Tolerance
• Body fat influences thermoregulation
– Excess body fat negatively influences performance
in hot environments
– Excess body fat increases insulatory quality of
shell to retard heat conduction to the periphery
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Thermoregulation During PA in the Heat
Nutrition in Hot Environments
• Cottons and linens readily absorb moisture
• Heavy “sweatshirts” and rubber or plastic garments
produce high RH close to skin
• Dark colors absorb light rays and add to radiant heat
gain
• Light colors reflect heat rays away from body
• Moisture-wicking fabrics provide optimal transfer of
heat and moisture from skin to environment
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Thermoregulation During PA in the Heat
Nutrition in Hot Environments
• Research shows:
– Reduction in voluntary kcal intake per degree
increase from 20 to 100°F (−6.7–37.8°C)
– Reduction in kcal intake cannot be explained by
differences in BMR, body weight, or PA mode
– Individuals eat the same in all environments, but
with fewer kcals consumed in the heat
o Reduced kcal expenditure and intake because of
heat’s blunting effect on PA
• Reduced food intake causes a reduction in food’s
thermic effect
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Thermoregulation During PA in the Heat
Heat Illness
• Heat cramps
– Involuntary muscle spasms after intense PA
• Heat exhaustion
– Most common heat illness
• External heat stroke
– Most serious, requires immediate medical attention
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Thermoregulation During PA in the Heat
Heat-Related Disorders
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Cold Weather PA and Thermoregulation
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Cold Weather PA and Thermoregulation
Acclimatization to Cold
• Humans adapt more successfully to chronic heat
exposure than chronic cold exposure
– Cold adaptation occurs after prolonged exposure
o Increased heat does not accompany heat loss
o Individuals regulate at lower core temp in cold
• Peripheral vasoconstriction in cold exposure causes
declines in extremity skin temp
– Early warning signs of cold injury include fingers
and toes tingling and numbness, or burning
sensation in the nose and ears
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Cold Weather PA and Thermoregulation
Wind Chill Temp Index
FIGURE 10.11 The wind
chill temperature index, the
proper way to evaluate the
“coldness” of an
environment
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