Exercise Thermoregulation, Fluid
Balance, and Rehydration
Chapter 10
Part 1
Temperature Regulation
• Body core temperature regulation
– Critical for:
• Cellular structures
• Metabolic pathways
– Too high - hyperthermia(5oC)
• Protein structure of cells destroyed
– Too low – hypothermia (10oC)
• Slowed metabolism
• Cardiac arrhythmias
Temperature Regulation
• Thermal gradients
– Determine rate and direction of heat transfer
– Heat transfer from higher to lower
temperature
– Core to shell (skin)
• Typical gradient when ambient temp is less than
body temp
– Skin to core
• If ambient temperature is Greater than body temp
Temperature Regulation
• Net gain of body heat
– Heat loss is less than heat production
– Body temperature rises
– Positive thermal balance
• Net loss of body heat
– Heat loss exceeds heat production
– Body temperature decreases
– Negative thermal balance
Temperature Regulation
• Homeotherms (mammals)
– Internal body temperature is kept fairly constant
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•
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Humans are unable to tolerate wide changes
Hour to hour, day to day variations < 1o C
Normal temperature: 37o C
Normal range 36.1 – 37.8o C (97 – 100o F)
– Conversion: Tf = (Tc * 1.8) + 32
– So, if it is 37o C, Tf = (37 * 1.8) + 32
– 66.6 + 32 = 98.6o F
– Heavy exercise, illness, extreme conditions of heat
and cold – can push an individual outside this range
Heat Balance
• Greater extremes
– Early morning (<36oC)
– Heavy exercise >40oC
• Muscles >42oC or 107.6oF
• Body temperature (core temperature) reflects
the balance between heat production and heat
loss
Body Temperature
 Core body temperature: deep tissues
 Skeletal muscles and organs
 Shell body temperature: peripheral
 Skin
 Initiate response to external temperature
 Thermoreceptors relay information to
 Hypothalamus
 Cerebral cortex
Hypothalamic Regulation
 Hypothalamus (37  1oC)
 Central coordinating center
 Acts as a thermostat
 Heat gain
 Heat loss
Thermal Balance
Heat-Regulating Mechanisms
 Activated by:
• Temperature changes in blood
• Skin heats up/cools down changes temperature of
blood
• Directly stimulate hypothalamus
• Thermal receptors in the skin
• Provide input to modulate hypothalamic activity.
 Receptors (temperature regulation)
 Skin
 Subcutaneous tissue.
Heat Transfer
Thermal Regulation
Radiation
 Objects “radiate” heat
 Body temperature is warmer than the
environment.
 Radiant heat energy transfers to solid cooler
objects around us.
 Body temperature is cooler than the
environment.
 Radiant heat energy transfers to us.
Conduction
 Transfers heat directly through a liquid, solid, or
gas from one molecule to another
 We conduct heat to cooler air, water or objects
 Warmer air, water or objects conduct heat to us
Convection
 Air, water movement
 So, if cool/hot air is travelling over the
body (e.g. wind) or cool/hot water, we will
be cooled/heated
Evaporation
 Major physiologic defense against overheating
 Respiratory water loss
 Sweat
 Heat stress
 2-4 million sweat (eccrine) glands
 Secrete large quantities of hypotonic saline solution.
 *Cooling occurs when sweat evaporates from
the skin surface.
 Has to evaporate
Environmental Temperature
 Increased ambient temperature
 Reduces heat loss by conduction, convection,
and radiation.
 This heat loss stops when ambient temp
exceeds body temp
 At that point, the only way to dissipate heat is
 Sweat evaporation from the skin
 depends upon:
• Surface exposed to the environment
• relative humidity of ambient air
• Convective air currents around the body
High Humidity
Heat-Dissipating Mechanisms
 Circulation
• Cardiac output (HR x SV)
• Rest – 15-20% to skeletal muscles
• Heavy exercise – 80-85% to skeletal muscles
• “Workhorse” to maintain thermal balance
• 15-25% of cardiac output – periphery
• If cardiac output falls, skin blood flow falls and
ability to dissipate heat is reduced
• So, when exercising in the heat
• Cardiac output is directed more to periphery
• Less to muscle
• Cause of fatigue
Cardiac Drift
Heat-Dissipating Mechanisms
 Sweating: rate dependent upon:
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Climate conditions
Exercise intensity
Clothing
Conditioning
 Evaporation
 Major cooling mechanism
Fluid conserving Mechanisms
 Hormones
• Antidiuretic hormone - ADH (Vasopressin)
• Posterior Pituitary
• Increases water reabsorption from kidneys
• Conserves water and maintains BP
• Aldosterone
• Adrenal cortex
• Increases reabsorption of sodium
Heat-related Deaths
• Since 1936 (National Safety Council)
– 30,000 people have died from heat related illnesses.
•
On the average, 384 people die each year from heat
stroke.
– Associated with normal hot temperatures.
– In the event of a heat wave, the number of deaths increase
tremendously.
•
Occurrence of heat related injuries
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–
–
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Often with the elderly;
People who are not in good physical condition;
or not acclimatized to the heat
Or heat waves
Exercise in the Heat
• Increased heat load
• Increased sweating
• Increased Fluid loss
– Decreased plasma volume
– Decreased cardiac output
– Decreased cutaneous circulation
Distribution of Cardiac Output
• Thermoneutral
environment vs hot
environment
Water Loss
 Dehydration
 Sweat loss 0.5 – 1.5 L/h – moderate exercise
– Maximal daily sweat rates-10-15 L
– Highest sweat rates 2 to 3 L/hr
• Can be sustained for only a few hours
 Considerable water loss
 Both intracellular and extracellular compartments
 The risk of heat illness
 Increases in a dehydrated state.
Sweat Rates
• 1 to 3% loss in body weight through
dehydration can adversely affect
performance and body’s physiologic
function
• Marathon runners
– Loss of 5 L
– 6-10% of body weight
Core
Temperature
and Sweat
Rate During
Exercise in
Heat/Humidity
Fig 12.10
Exercise in the Heat
• Compromised BF to
muscles
• Increased use of
glycogen
• Increased production
of LA
• Hastens fatigue
Dehydration and Exercise Performance
Dehydration and Exercise Performance
Dehydration and Exercise Performance
 Any degree of dehydration impairs
 Capacity of circulatory system
 Temperature-regulating mechanisms
 Dehydration - 2% body mass
 Impairs physical work capacity and physiologic
function
 Predisposes to heat injury when exercising in a hot
environment.
Dehydration and Exercise
Performance
 The risk for dehydration increases during
vigorous cold-weather exercise.
 Less moisture in cold
 Respiratory fluid loss is increased
 Excessive clothing
 Creates a heated microclimate under the clothing