exercise thermoregulation, fluid balance, and rehydration
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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(5 o C) Protein structure of cells destroyed Too low – hypothermia (10 o C) - PowerPoint PPT PresentationTRANSCRIPT
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Exercise Thermoregulation, Fluid Balance, and Rehydration
Chapter 10Part 1
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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
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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
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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
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Temperature Regulation• Homeotherms (mammals)
– Internal body temperature is kept fairly constant• 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
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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
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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
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Hypothalamic Regulation
Hypothalamus (37 1oC) Central coordinating center
Acts as a thermostat Heat gain Heat loss
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Thermal Balance
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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.
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Heat Transfer
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Thermal Regulation
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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.
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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
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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
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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
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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
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High Humidity
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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
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Cardiac Drift
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Heat-Dissipating Mechanisms
Sweating: rate dependent upon: Climate conditions Exercise intensity Clothing Conditioning
Evaporation Major cooling mechanism
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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
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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
– Often with the elderly; – People who are not in good physical condition; – or not acclimatized to the heat– Or heat waves
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Exercise in the Heat
• Increased heat load• Increased sweating• Increased Fluid loss
– Decreased plasma volume– Decreased cardiac output– Decreased cutaneous circulation
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Distribution of Cardiac Output• Thermoneutral
environment vs hot environment
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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.
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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
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Core Temperature and Sweat
Rate During Exercise in
Heat/Humidity
Fig 12.10
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Exercise in the Heat• Compromised BF to
muscles• Increased use of
glycogen• Increased production
of LA• Hastens fatigue
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Dehydration and Exercise Performance
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Dehydration and Exercise Performance
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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.
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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