Chapter 10
Exercise Thermoregulation,
Fluid Balance, and
Rehydration
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Simple Facts
• A reduction in body water results in a decrease in
performance
• Hydration status is determined by water intake and loss
• Lack of water consumption impairs the body and after
several days, can result in death
• Over consumption of water can also result in problems
• 50-60% of the body’s mass is water
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Simple facts (cont.)
• Lean body mass (LBM) is comprised of 75% water
• Fat contains 5% water
• 154lb male=42L
• 154lb female=35L
• Sweating: uses water to cool body and prevent
hyperthermia
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Body Temperature
 Hyperthermia is an increase in body temperature by
5°C or more.
 Core body temperature: deep tissues
 Shell body temperature: peripheral
 Core temperature rises quickly when heat gain exceeds
heat loss during vigorous exercise in a warm
environment.
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Hypothalamic Regulation
 Hypothalamus is the central coordinating center for
temperature regulation.
 Acts as a thermostat
 Initiates responses to protect the body from heat gain or
heat loss
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Exercise Generates Heat
• Core temperature normally increases during exercise.
• The relative stress of exercise determines the magnitude
of the increase.
• A well-regulated temperature increase creates a more
favorable environment for physiologic and metabolic
functions.
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• Cutaneous and muscle blood flow increase during
exercise in the heat, while other tissues temporarily
compromise their blood supply.
• For every liter of oxygen (O2), 4 calories of heat are
produced, with only 1 calorie used for energy
• An athlete using 4L O2/minute, heat production is about
917 calories for hour
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Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Core temperature
• Humans tolerate relatively small variations in core
temperature.
• Exposure to heat or cold initiates thermoregulatory
mechanisms that generate and conserve heat at low
ambient temperatures and dissipate heat at high
temperatures.
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Heat-Regulating Mechanisms
 Become activated in two ways:
•
Temperature changes in blood perfusing the
hypothalamus directly stimulate this
thermoregulatory control center.
•
Thermal receptors in the skin provide input to
modulate hypothalamic activity.
 Structures in the skin and subcutaneous tissue help to
regulate temperature.
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Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Thermoregulation
• Thermoregulation is the body’s mechanism to regulate body
temperature and prevent overheating
• The hypothalamus serves as the “thermostat” for temperature
regulation.
• The hypothalamus initiates adjustments from:
– Thermal receptors in the skin.
– Changes in hypothalamic blood temperature.
– Nervous signals from osmoreceptors and pressure
receptors of blood volume
– Hormones
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Heat Storage
• Heat production is directly proportional to exercise
intensity, so both hot and cool environments can
increase body temperatures
• If body temp> 103°F, it will cause the CNS (brain) to
begin shutting down and fatiguing
• Normal body temp=96.8-100°F
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Heat Loss
 Can occur due to:
•
Radiation
•
Conduction
•
Convection
•
Evaporation
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Heat Loss
• Increased body core temperature is sensed by the
thermoregulatory center in the hypothalamus and
increases blood flow to the skin to induce sweating
• Warm blood diverts from the body’s core to the skin in
response to heat stress.
• Heat loss occurs by radiation, conduction, convection,
and evaporation.
• Evaporation provides the major physiologic defense
against overheating at high ambient temperatures and
during exercise.
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Radiation
 Objects emit electromagnetic heat waves.
 Body temperature is warmer than the environment.
 Radiant heat energy leaves the body through air to solid
cooler objects around us.
 The body absorbs radiant heat energy when the
temperature of objects in the environment exceeds skin
temperature.
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Conduction
 Transfers heat directly through a liquid, solid, or gas
from one molecule to another
 The circulation transports most of the body heat to the
shell.
 A small amount continually moves by conduction directly
through the deep tissues to the cooler surface.
 Conductive heat loss then involves the warming of air
molecules and cooler surfaces in contact with the skin.
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Convection
 Air movement
 Warm air next to the skin acts as a zone of insulation.
 If cool air continuously replaces the warmer air
surrounding the body, heat loss increases.
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Evaporation
 Major physiologic defense against overheating
 Water vaporization from the respiratory passages and
skin surface continually transfers heat to the
environment.
 In response to heat stress, 2-4 million sweat (eccrine)
glands secrete large quantities of hypotonic saline
solution.
 Cooling occurs when sweat evaporates from the skin
surface.
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Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Environmental Temperature
 Increased ambient temperature reduces the effectiveness
of heat loss by conduction, convection, and radiation.
 Sweat evaporation from the skin depends on:
•
Surface exposed to the environment
•
Temperature and relative humidity of ambient air
•
Convective air currents around the body
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Environmental Heat Stress and Heat Loss
Via Sweat Evaporation
• Determined by:
• ambient temperature
• Humidity
• Wind velocity
• Solar radiation
• During exercise, heat production and loss from the skin occurs via
evaporation (convection) and conduction
• If the environment is hotter, heat is gained
• If the environment is humid, decreased evaporation of heat occurs
• One L of water takes 573 calories to evaporate
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Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Heat-Dissipating Mechanisms
 Circulation
•
“Workhorse” to maintain thermal balance
 Evaporation
 Hormones
•
Antidiuretic hormone
•
Aldosterone
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Exercise Clothing
 Cottons and linens readily absorb moisture.
 Heavy “sweatshirts” and rubber or plastic garments
produce high relative humidity close to the skin.
 Dark colors absorb light rays and add to radiant heat
gain.
 Light colors reflect heat rays away from the body.
 Moisture-wicking fabrics provide optimal transfer of heat
and moisture from the skin to the environment.
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Water Loss
 Dehydration
 Considerable water loss occurs during several hours of
intense exercise in a hot environment.
 Both intracellular and extracellular compartments
contribute to fluid deficit.
 The risk of heat illness greatly increases when a person
begins exercising in a dehydrated state.
 Sweat is hypotonic with other body fluids.
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Sweating and Exercise
• Increased sweating strains fluid reserves, creating a
relative state of dehydration.
• Excessive sweating without fluid replacement decreases
plasma volume and causes core temperature to rise
precipitously.
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Dehydration and Exercise
 Just about any degree of dehydration impairs the
capacity of circulatory and temperature-regulating
mechanisms to adjust to exercise demands.
 Dehydration of as little as 2% body mass impairs
physical work capacity and physiologic function and
predisposes to heat injury when exercising in a hot
environment.
 The risk for dehydration increases during vigorous coldweather exercise.
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Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Dangerous Conditions:
Physiological Effects of Dehydration
• Decreased blood volume
• Decreased blood flow to skin
• Decreased sweat rate (due to increased blood osmolarity)
• Decreased heat dissipation
• Increased core temperature
• Increased muscle glycogen use
• Decreased cardiac output
• Decreased gastric emptying 20-25% (@5% dehydration)
• Heat stress from the environment alone can decrease VO2max
by 7%
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Dangerous Conditions:
Effects of Dehydration on Exercise
Performance
• 2% dehydration impairs performance
• 5% dehydration decreases work capacity by about 30%
(impedes heat dissipation, compromises cardiovascular
function, and diminishes exercise capacity)
• 2.5% dehydration decreases sprinting capacity (can
occur with acute dehydration)
• 7% dehydration will increase fatigue due to heat strain
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Prevention
• Factors to be aware of:
– Hydration status
– Environmental temperature
• Consume fluids prior to exercise (hyperhydrate)
• Consume fluid during early stages (minimize dehydration
and maximize availability)
• Acclimate slowly
• Be aware of heat-related illness signs
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Effects of Fluid Intake on Performance
• Pre-exercise hyperhydration
• Helps to decrease thermal and cardiovascular strain
• May help to decrease HR, core temp., and increase
sweating
• Consume large volume of water or water+electrolytes
1-3 hours prior to exercise
• Increased fluid retention with glycerol added to fluids
• May help to decrease heart and core temperatures and
increase performance
• Should consume 500 ml 2 hours pre-exercise and
500ml 15’ prior to event
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Hydration during exercise
• One can avoid dehydration if intake=output
• Difficulties:
1) Sweat rates =2-3L/hr, volume consumed > 1L/hr
can be uncomfortable
2) Sweat rates vary
3) Thirst is not a good indicator (occurs after2%
loss/mild dehydration)
4) Limited ability to intake water during competition
(soccer, tennis)
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• During exercise
• Every 15-20’ consume 120-180ml (~1/4 cup)
• Only need water if activity is 30-60’
• Glucose and electrolyte beverages if longer (offer at 30’
mark of exercise to prevent an insulin response)
• Concentration: glucose=20-60g/L, sodium=2060mmol/L, and osmolality<290
• Most sports drinks: glucose=60-80g/L, Na+=20-25
• Have athletes practice drinking during training (estimate how
much they will/can consume)
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Rehydration
 Properly scheduling fluid replacement maintains plasma
volume, so circulation and sweating progress optimally
 A well-hydrated individual always functions at a higher
physiologic and performance level than a dehydrated person.
 Achieving hyperhydration before exercising in a hot
environment protects against heat stress because it:
•
Delays dehydration
•
Increases sweating during exercise
•
Diminishes the rise in core temperature
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Adequacy of Rehydration
 Body weight changes indicate the extent of water loss
from exercise and adequacy of rehydration during and
after exercise or athletic competition.
 Urine and hydration:
•
Dark yellow urine with a strong odor = inadequate
hydration
•
Large volume, light color, without a strong odor =
adequate hydration
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Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Sodium and Rehydration
 A moderate amount of sodium added to a rehydration
beverage provides more complete rehydration.
 Maintaining a relatively high plasma concentration of
sodium helps:
•
Sustain the thirst drive
•
Promote retention of ingested fluids
•
More rapidly restore lost plasma volume during
rehydration
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• 1-1.5 L fluid for every kg lost
• Should consume beverages over 6 hours to rehydrate
• Avoid caffeine and alcoholic beverages
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Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Hyponatremia
 Low blood level of sodium (<135 mEq/L)
 Can occur due to excessive water intake
 A sustained low plasma sodium concentration creates an
osmotic imbalance across the blood–brain barrier that
causes rapid water influx into the brain.
 The resulting swelling of brain tissue produces a cascade
of symptoms that range from mild to severe.
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Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Acclimatization
 Heat acclimatization refers to the physiologic adaptations
that improve heat tolerance.
 The acclimatized individual:
•
Has larger quantities of blood shunt to cutaneous
vessels
•
Has more effective cardiac output
•
Has an earlier onset of sweating
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• Repeated heat stress initiates thermoregulatory
adjustments that improve exercise capacity and reduce
discomfort on subsequent heat exposure.
• Acclimatization triggers favorable redistribution of cardiac
output and increases sweating capacity.
• Exercise improves temperature regulation, but exercise
intensity must be increased (70-100%) in order to
improve a response
• Full acclimatization generally occurs in about 10 days of
heat exposure.
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• Increased size of sweat glands
• Increased total blood volume and cardiac output
• Marathoners have a lower body temperature at rest and
a lower threshold to sweating (lower BMR and skin temp)
• Aging affects thermoregulatory function, yet
acclimatization to moderate heat stress does not
appreciably deteriorate with age.
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• When controlling for fitness and acclimatization levels,
women and men show equal thermoregulatory efficiency
during exercise.
• Women produce less sweat than men do at the same
core temperature.
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Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Age Differences in Acclimatization
 Older individuals have:
•
A decreased sensitivity of thermoreceptors
•
Limited sweat gland output
•
Dehydration-limited sweat output with insufficient
fluid replacement
•
Altered structure and function of the skin and its
vasculature
•
A decreased recovery from dehydration
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Other Factors Affecting Acclimatization
 Children
 Gender
•
Men sweat more.
•
Women show heat tolerance similar to men.
 Body fat
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Wet Bulb-Globe Temperature
 Used to evaluate the environment for its potential
thermal challenge
 Index of environmental heat stress
 Incorporates ambient temperature, relative humidity,
and radiant heat
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Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins
Heat Illnesses
 Heat cramps
•
Involuntary muscle spasms that occur after intense
physical activity
 Heat exhaustion
•
Most common heat illness
 Heat stroke
•
Most serious and requires immediate medical
attention
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Heat Illnesses
• Increases if athlete is dehydrated, less trained, and less
acclimatized
• Initial stages:
– Exercise increase energy production & increases heat
which will increase core temperature
– Increased blood flow to the skin, decreased blood to
organs
– Increased toxins occur, causing blood poisoning,
decreased BP, and fainting
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Copyright © 2009 Wolters Kluwer Health | Lippincott Williams & Wilkins