Exercise and Thermal stress

• Thermoregulation

• Heat balance

• Heat injuries

• Hydration

Thermal Balance

Core temperature is in dynamic equilibrium

Thermal Balance

Body Heat Content

Heat LossHeat Gain

• Radiation

• Conduction

• Convection

• Evaporation

• Muscular activity

• Environmental heat

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

• 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

Chimney Effect

Heat

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 Stroke

• Exertional

• Classic

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")•