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11
Exercise in Hot
and Cold
Environments:
Thermoregulation
Learning Objectives
• Find out how the body gets rid of excess body heat to
maintain homeostasis at rest and during exercise
• Discover how the body adapts to exercise in a hot
environment
• Learn why humidity is important when exercising in the
heat
(continued)
Learning Objectives (continued)
• Differentiate between heat cramps, heat exhaustion,
and heatstroke
• Learn how the body minimizes excessive heat loss
during exposure to cold
• Find out the dangers of exercising in the cold
Temperature Conversions
In physiology, temperatures are expressed in degrees
Centigrade
– To go from °F to °C: subtract 32 °, then divide by 1.8
– To go from °C to °F: multiply by 1.8, then add 32 °
To Maintain a Steady-State Core
Temperature, the Body Must Balance
Heat Gain With Heat Loss
Modes of Heat Transfer
Conduction: the transfer of heat from one solid
material to another though direct molecular contact
Convection: moving heat from one place to another by
the motion of gas or liquid across a heated surface
Radiation: heat is given off in the form of infrared rays
Evaporation: heat is lost when fluid evaporates
Removal of Heat From the Skin
Thermograms of the Body Showing
Variations in Radiant Heat
From Department of Health and Human Performance, Auburn University, Alabama. Courtesy of John Eric Smith, Joe
Molloy, and David D. Pascoe. By permission of David Pascoe.
Evaporation
• Primary avenue for heat dissipation during exercise
• Evaporation accounts for 80% of heat lost during
exercise vs. 10-20% at rest
• As body core temperature increases, sweat production
increases
• Sweat must evaporate to provide cooling
Heat Balance Equation
M–W±R±C±K–E=S
M = Metabolism
W = Work
R = Radiation
C = Convection
K = Conduction
E = Evaporation
S = Storage
Humidity
• Water vapor pressure of the air plays a major role in
evaporative heat loss
• High humidity (regardless of temperature) limits
evaporation of sweat
• Low humidity is ideal for sweat evaporation
The Complex Interaction Between the
Body’s Mechanisms for Heat Balance and
Environmental Conditions
Adapted, by permission, from C.V. Gisolfi and C.B. Wenger, 1984, “Temperature regulation during exercise: Old
concepts, new ideas,” Exercise and Sport Sciences Reviews 12: 339-372.
Heat Balance
Key Points
• Humans are homeothermic; they regulate an internal
temperature between 36.1 to 37.8 °C (97.0 to
100.0 °F)
• Body heat is transferred by conduction, convection,
radiation, and evaporation
• At rest most heat is lost via radiation
• During exercise, evaporation is the most important
avenue of heat loss
• Higher humidity decreases the capacity to lose heat by
evaporation
Internal Body Temperature
• Can exceed 40 °C (104 °F) during exercise
• May be 42 °C (107.6 °F) in active muscles
• Small increases can make muscles’ energy systems
more efficient
• Temperatures above 40 °C can affect the nervous
system and reduce the ability to unload excess heat
A Simplified Overview of the Role
of the Hypothalamus in Controlling
Body Temperature During Hyperthemia
A Simplified Overview of the Role
of the Hypothalamus in Controlling
Body Temperature During Hypothermia
Thermoregulatory Control
of Heat Exchange
• Preoptic-anterior hypothalamus
• Central and peripheral thermoreceptors
• Effectors that alter body temperature
– Sweat glands
– Smooth muscle around arterioles
– Skeletal muscles
– Endocrine glands
Thermoregulatory Control
of Heat Exchange
Key Points
• The preoptic-anterior hypothalamus houses the
thermoregulatory center, acting as a thermostat,
monitoring temperature and accelerating heat loss or
heat production as needed
• Peripheral thermoreceptors in the skin and central
thermoreceptors in the brain transmit information about
the body’s internal temperature
• Central thermoreceptors are far more sensitive to
temperature change than peripheral thermoreceptors
• Effectors stimulated by the hypothalamus can alter the
body temperature
Thermoregulatory Effector Organs
Key Points
• Smooth muscle in the skin arterioles can cause these
vessels to dilate to direct blood to the skin for heat
transfer or to constrict to retain heat deep in the body
• Increased skeletal muscle activity increases
temperature by increasing metabolic heat production
• Metabolic heat production can also be increased by
actions of hormones such as thyroxine and the
catecholamines
• Increased sweat gland activity decreases the
temperature by increasing evaporative heat loss
Cardiovascular Function in the Heat
The circulatory system has to continue to transport
blood to the working muscle and increase blood
flow to the skin for thermoregulation
– Increased cardiac output
– Redistribution of blood flow from inactive organs
(gut, liver, and kidneys)
What Limits Exercise in the Heat?
• At some point the cardiovascular system can no longer
compensate for the increased demands of endurance
activity and thermoregulation
– Heart rate approaches maximum
– Muscle blood flow is maintained
• Critical temperature theory: the brain will send signals
to stop exercise when some critical temperature is
reached, usually between 40 and 41 °C
Rectal Temperature and Cardiovascular
Responses in Thermoneutral
and Hot Environments
Adapted, by permission, from L.B. Rowell, 1974, “Human cardiovascular adjustments to heat stress,” Physiological
Reviews 54: 75-159.
Body Fluid Balance: Sweating
Anatomy of an eccrine sweat gland that is innervated by a
sympathetic cholinergic nerve
Exercise in the Heat
Key Points
• Skin competes with the active muscles for more of the
limited cardiac output
• Muscle blood flow is well maintained unless severe
dehydration occurs
• Blood flow is redistributed from nonessential regions to
the skin to aid in heat dissipation
• Although cardiac output may remain constant or
decrease slightly, stroke volume may decline, resulting
in a gradual upward drift in heart rate
(continued)
Exercise in the Heat (continued)
Key Points
• Sweating increases during exercise in the heat, which
can quickly lead to dehydration and electrolyte loss
• Aldosterone and ADH release are increased, causing
sodium and water retention, which can expand the
plasma volume
Variables of Environmental
Heat Load
•
•
•
•
Air temperature
Humidity
Air velocity
Amount of thermal radiation
Wet-Bulb Globe Temperature
• Simultaneously accounts for conduction, convection,
evaporation, and radiation
• Dry bulb measures air temperature (Tdb)
• Wet bulb measures temperature as water evaporates
from it (Twb)
• Black globe absorbs radiated heat (Tg)
• WBGT = 0.1Tdb + 0.7Twb + 0.2Tg
Wet-Bulb Globe Temperature
Heat-Related Disorders
• Heat cramps
• Heat exhaustion
• Heatstroke
Warning Signs and Symptoms of Heat
Cramps, Heat Exhaustion, and Heatstroke
Adapted by permission of All Sport, Inc.
Heat Cramps
Symptoms: Severe and painful cramping of the large
muscle groups
Causes: Sodium losses and dehydration
Prevention: Proper hydration and liberally salt food
Treatment: Move to cooler location and administer
fluids or saline solution
Heat Exhaustion
Symptoms: Extreme fatigue, dizziness, nausea,
vomiting, fainting, rapid pulse
Causes: Cardiovascular system’s inability to
adequately meet the body’s needs as it becomes
dehydrated, blood volume decreases, and/or rarely
sodium depletion
Treatment: Move to cooler environment, elevate feet;
give oral saline if conscious or intravenous saline if
unconscious
Heat Stroke
Symptoms: Increase in internal body temperature
(>40 °C), cessation of active sweating, rapid pulse and
respiration, and confusion, disorientation, or
unconsciousness
Cause: Failure of the body’s thermoregulatory system
Treatment: Rapidly cool body in cold water or ice bath
or with wet towels; seek medical attention
Preventing Hyperthermia
• Recognize the symptoms of heat illness
• Avoid exercising in humid conditions above a WBGT of
28 °C (82.4 °F)
• Schedule practices or events in early morning or at
night
• Wear lightweight, light-colored, loosely woven clothing
• Make fluids readily availably to encourage fluid
consumption to match sweat losses
Effects of Fluid Intake
on Core Body Temperature
Archives of Environmental Health, “Fluid ingestion during distance running,” D.L. Costill, 1: 520-525, 1970. Adapted
with permission of the Helen Dwight Reid Educational Foundation. Published by Heldref Publications, 1319 18th
Street NW, Washington, DC 20036-1802. www.heldref.org. Copyright © 1970.
Heat Stress
Key Points
• WBGT measures air temperature and accounts for heat
exchange via conduction, convection, evaporation, and
radiation
• Heat cramps are caused by loss of fluids and sodium
due to excessive sweating
• Heat exhaustion results from the cardiovascular
system’s failure to meet the needs of the active muscles
and skin due to dehydration and low blood volume
• Heat exhaustion can deteriorate into heat stroke
• Heatstroke is caused by failure of the body’s
thermoregulatory system, which can be fatal
Heat Acclimation
• How can athletes prepare for prolonged activity in the
heat?
• Does repeated exercise in the heat make us better able
to tolerate thermal stress?
Effects of Heat Acclimation
• Improvement in the ability to get rid of excess body
heat
• Decreased body core temperature for a given
submaximal work rate
• Sweating increases and becomes more dilute
• Blood flow to skin is reduced; more blood is available to
muscles
• Blood volume increases
• Attenuated heart rate
• Stroke volume is maintained
Effects of Heat Acclimation on Body Core
Temperature and Heart Rate
During a 90 Minute Run
Adapted, by permission, from D.S. King et al., 1984, “Muscle metabolism during exercise in the heat in unacclimatized
and acclimatized humans,” Journal of Applied Physiology 59: 1350-1354.
Changes in Rectal Temperature, Heart Rate
and Sweat Loss During Heat Acclimation
Achieving Heat Acclimation
You can achieve heat acclimation by exercising in the
heat for 1 hour or more each day for 9 to 14 days.
Cardiovascular changes generally occur first, starting
with—and supported by—plasma volume expansion
during the first 1-3 days. Changes in the sweating
mechanisms generally take longer, up to 10 days or
more.
Heat Acclimation
Key Points
• Repeated exposure to heat stress during exercise
improves your ability to get rid of excess heat
• Sweating starts earlier and sweat rate increases
especially in well-exposed areas
• Skin temperature decreases facilitating heat transfer
by increasing the thermal gradient
• Core temperature and heart rate during exercise are
reduced, whereas stroke volume increases
• Requires exercise in a hot environment
• Amount and rate of heat acclimation depends on
training status, duration of exposure, and rate of
internal heat production
Exercise in the Cold:
Body Heat Conservation
Peripheral vasoconstriction reduces blood flow to
skin
Nonshivering thermogenesis is stimulation of
metabolism by the sympathetic nervous system
Shivering involves rapid involuntary cycle of contraction
and relaxation of muscles to increase heat production
Summary of Human
Thermoregulatory Mechanisms
Factors That Affect Body Heat Loss
•
•
•
•
Body size and composition
Air temperature
Windchill
Water immersion
Windchill Equivalent
Temperature Chart
Heat Loss in Cold Water
• Water has a thermal conductivity about 26 times
greater than air
• Heat loss is 4 times faster in water
• Heat transfer is accelerated if the water is moving
around the individual
Exercise in the Cold
Key Points
• Peripheral vasoconstriction decreases the transfer of
core heat to the skin and the environment
• Nonshivering thermogenesis increases metabolic heat
production through the actions of the sympathetic
nervous system
• Shivering increases metabolic heat production
• Increased surface area, decreased muscle mass, and
decreased subcutaneous fat facilitate the loss of body
heat to the environment
• Wind increases heat loss by convection
• Immersion in cold water tremendously increases heat
loss through convection
Physiological Responses
to Exercise in the Cold
• When muscle is cooled, it is less able to produce force,
and fatigue can occur more rapidly
• During prolonged exercise in the cold people may
become more susceptible to hypothermia
– Energy supplies diminish
– Metabolic heat production decreases
• Exercise triggers the release of catecholamines, which
increase the mobilization and use of FFA for fuel. But in
the cold, vasoconstriction impairs circulation to
subcutaneous fat tissue
Hypothermia
• Ability of the hypothalamus to regulate body
temperature is lost if body core temperature drops
below 34.5 °C (94.1 °F)
• Hypothermia causes heart rate to drop, from impaired
cardiac conduction through the SA node
• Decreased respiratory rate and volume
The Warming of Inspired Air as it
Moves Through the Respiratory Tract
Frostbite
• Exposed skin can freeze quickly
• This can lead to gangrene and loss of tissue
Health Risks During Exercise
in the Cold
Key Points
• The hypothalamus begins to lose its ability to regulate
body temperature below 34.5 °C (94.1 °F)
• Hypothermia critically affects the heart’s SA node,
decreasing heart rate and cardiac output
• Breathing cold air does not freeze the respiratory
passages or lungs when ventilation is low
• Exposure to extreme cold decreases respiratory rate
and volume
• Frostbite occurs as a consequence of the body’s
attempts to prevent heat loss and can lead to tissue
necrosis