View
220
Download
1
Tags:
Embed Size (px)
Citation preview
Thermal Balance
Body Heat Content
Heat LossHeat Gain
• Radiation
• Conduction
• Convection
• Evaporation
• Muscular activity
• Environmental heat
37±0.5oC
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
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
• The total sweat vaporized from the skin depends on three factors:
1) The body surface exposed to the environment
2) The humidity of the ambient air
3) The convective air currents about the body
• The relative humidity (RH) is the most important factor that determines the effectiveness of evaporative heat loss
amount water in the ambient air total quantity of moisture that can be carried in the air
• RH (%)= 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
Heat Dissipation
Metabolic Heat
Environmental Heat
Circulation (peripheral vasodilation)
Evaporation
Hormonal Adjustment:
-ADH
-Aldosterone
Clothing/Insulation
Time
Bod
y te
mpe
ratu
re בגדי מגן
בגדי ספורט
פעילות גופנית בחום עם בגדי מגן לעומת בגדי
ספורט
Thermoregulation During Exercise
1.Competitive Demands
Muscles that require oxygenMetabolic heat must be transported by the blood from the deep tissues to the periphery
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
Time
Bo
dy
Tem
per
atu
re
Highly Fit (40% of max VO2)
Unfit (60% of max VO2)
הספק כושר גופני אירובי גבוה יהיה קשור בעליה מתונה יותר בחום הנאגר בגוף עבור מוחלט נתון
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 Acclimatization and the Effect of Thermoregulation During Exercise in the
Heat
• 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
Metabolic Heat Endotoxins
DiseaseBacteria
Thermoregulatory set point
Antipyretics )-(
Hydration
התייבשות מרצון• 1קצב הזעה במאמץ בתנאי אקלים חם יכול להיות גבוה מ-•
ליטר/שעה (ואף כפול מכך)הקפדה על שתייה גם כשלא צמאים•עדיף משקה קריר• עד ליטר לשעה0.5במאמץ ממושך בתנאי אקלים חם יש לשתות •תוספת גלוקוז למשקה- במאמצים רצופים הנמשכים למעלה •
משעהתוספת אלקטרוליטים למשקה במאמצים רצופים הנמשכים מעל •
שעות4הופונתרמיה ("הרעלת מים")•