Download Exercise and Thermal stress

Exercise and Thermal stress
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Thermoregulation
Heat balance
Heat injuries
Hydration
Thermal Balance
Core temperature is in dynamic equilibrium
Thermal Balance
Body Heat
Content
• Radiation
• Muscular activity
• Conduction
• Environmental heat
• Convection
• Evaporation
Heat Gain
Heat Loss
37±0.5oC
Hypothalamic Regulation of
Temperature
• Thermoregulatory center (thermostat)
• Set point ~ 37oC
Hypothalamic Regulation of
Temperature
Cold stress
Vascular Response:
• Peripheral vascular adjustments – constriction
• Skin temperature
• Insulation by skin and subcutaneous fat
Muscular activity
• Shivering
• Physical activity (can sustain core temperature in air temperature as
low as (-30oC)
• Metabolic rate (VO2) is higher in cold stress
• Hormonal output
• Increased heat production is due partially to the action of
epinephrine, norepinephrine and thyroxin
Heat Stress
• Body heat may be lost by radiation,
conduction, convection and evaporation
Radiation-‫קרינה‬
Conduction- ‫הולכה‬
Convection- ‫הסעה‬
Evaporation-‫נידוף‬
Thermodynamics
30oC
35oC
Heat Loss in High Ambient
Temperature
• When ambient temperature exceeds body temperature,
heat is actually gained by the mechanisms of thermal
transfer (conduction, convection, radiation)
• When ambient temperature exceeds body temperature,
heat dissipation occurs only by sweat evaporation
• The rate of sweating increases directly with the
ambient temperature
• A very small contribution to cooling provided by the
vaporization of water from the respiratory system
Heat Loss in High Humidity
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The total sweat vaporized from the skin depends on three factors:
The body surface exposed to the environment
The humidity of the ambient air
The convective air currents about the body
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The relative humidity (RH) is the most important factor that
determines the effectiveness of evaporative heat loss
• RH (%)=
amount water in the ambient air
total quantity of moisture that can be carried in the air
X 100
• Hot, dry desert climate are more comfortable than cooler but more humid
tropical climates
• Sweat per se does not cool the skin. Only evaporation cool the skin
Physiological Integration of Heat
Dissipation Mechanisms
Circulation (peripheral
vasodilation)
Metabolic Heat
Evaporation
Heat Dissipation
Environmental Heat
Hormonal
Adjustment:
- ADH
- Aldosterone
‫‪Clothing/Insulation‬‬
‫פעילות גופנית בחום עם בגדי‬
‫מגן לעומת בגדי ספורט‬
‫בגדי ספורט‬
‫‪Time‬‬
‫‪Body temperature‬‬
‫בגדי מגן‬
Chimney Effect
Heat
Thermoregulation During Exercise
1.Competitive Demands
Metabolic heat must be
transported by the blood from the
deep tissues to the periphery
Muscles that require oxygen
2. During exercise, evaporation becomes the predominant avenue of
heat
loss.
3. Excessive sweating leads to fluid loss and a reduction in plasma
volume. This may cause circulatory failure and increase the rate of
elevation in body temperature.
Circulatory Adjustments
Constriction and Dilation:
• In the heat adequate cutaneous and muscle blood flow are achieved
at the expense of other tissues (viscera) that can temporarily
compromise their blood supply
• During exercise in the heat skin blood flow may increase to 20% of
cardiac output compared to only 5% of cardiac output during rest in
similar conditions
Maintenance of Blood Pressure
• Vasoconstriction in the viscera serves to increase total vascular
resistance. In this way, arterial blood pressure is maintained during
work in the heat (peripheral vasodilation)
Aerobic vs. Anaerobic Metabolism
• The work in the heat is accomplished with a greater dependence on
anaerobic metabolism than in cooler environment. This may result
in early fatigue during exercise in the heat.
Core Temperature
• Q10 effect
• May increase to as high as 40oC in champion distance
runners with no ill effects
• May point to a heat stroke when having the required
clinical symptoms
Fitness and Exertional Heat Strain
‫כושר גופני אירובי גבוה יהיה קשור בעליה מתונה יותר בחום הנאגר בגוף עבור הספק מוחלט נתון‬
Body Temperature
Unfit (60% of max VO2)
Highly Fit (40% of max VO2)
Time
Body Morphometry
• Surface Area/Body Mass
• Fat- The specific heat of fat is greater than for muscle
tissue. Therefore, excess fat increases the insulatory
quality of the body shell and prevents conduction of heat
to the periphery
Heat Stroke
• Exertional
• Classic
Heat Acclimatization and the Effect of
Thermoregulation During Exercise in the
Heat
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The physiologic adaptive changes that improve heat tolerance
Two weeks of regular daily exercise in hot conditions
Improved cutaneous blood flow
Increased plasma volume
Effective distribution of cardiac output
Lowered sweating threshold
Increased sweat output
More effective distribution of sweat over skin surface
Lowered salt concentration in sweat
Lowered heart rate
Lowered body temperature
Subjective feeling
Hyperthermia vs Fever
Thermoregulatory
set point
Antipyretics
Metabolic
Heat
(-)
Endotoxins
Bacteria
Disease
‫‪Hydration‬‬
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‫התייבשות מרצון‬
‫קצב הזעה במאמץ בתנאי אקלים חם יכול להיות גבוה מ‪ 1-‬ליטר‪/‬שעה‬
‫(ואף כפול מכך)‬
‫הקפדה על שתייה גם כשלא צמאים‬
‫עדיף משקה קריר‬
‫במאמץ ממושך בתנאי אקלים חם יש לשתות ‪ 0.5‬עד ליטר לשעה‬
‫תוספת גלוקוז למשקה‪ -‬במאמצים רצופים הנמשכים למעלה משעה‬
‫תוספת אלקטרוליטים למשקה במאמצים רצופים הנמשכים מעל ‪ 4‬שעות‬
‫הופונתרמיה ("הרעלת מים")‬