CHAPTER 9

Jiangxi Normal University P.E Department

Main components The purpose of this chapter is to describe the different factors that help regulate body temperature; to explain the effects of climate, physical activity, and acclimatization; and to discuss the limits of tolerance to heat and cold.

contents

SECTION 1 ENVIRONMENT

SECTION 2 HEAT BALANCE

SECTION 3 WATER BALANCE

SECTION 4 TEMPERATURE REGULATION

SECTION 1 ENVIRONMENT

1 ） Physiological alterations in response to prolonged exposure to heat 2 ） State of Training

2 Heat2 Heat

1 Acclimatization1 Acclimatization

1 ） Effect of Physical Training and Diet 2 ） Cold Injury

3 Cold

1 Acclimatization• Continuous or repeated exposure to heat, and possi

bly also to cold, causes a gradual adjustment or acclimatization, resulting in a better tolerance of the temperature stress in question. Many plants prepare for the winter by increasing their carbohydrate content. Certain types of trees can tolerate air temperatures below -40 during the wi℃nter, but during the summer, they fail to survive air temperatures below -3 . Certain insects accumulate the “antifr℃eeze” glycerol in the fall, which enables them to survive cold (Dill 1964).

2 Heat

After a few days of exposure to a hot environment, the individual is able to tolerate the heat much better than when first exposed. This improvement in heat tolerance is associated with increased sweat production, a lowered skin and body temperature, and a reduced heart rate .

3 ColdWhether a clothed human exposed to cold climate really becomes acclimatized to cold, or to what extent, is still an open question. Local acclimatization, however, can occur in the hands of individuals regularly exposed to cold. Such localized cold stress can increase blood flow through the hands, which will improve the ability of the hand and fingers to perform work of a precise nature in the cold. Physically fit subjects seem to have more efficient thermoregulatory abilities against cold stress than unfit subjects, because of an improved sensitivity of the thermoregulatory system.

SECTION 2 HEAT BALANCE

Heat BalanceMaintenance of Thermal BalanceMaintenance of Thermal Balance

Heat BalanceHeat Balance

Methods of Assessing Heat Balance Methods of Assessing Heat Balance

Heat Heat

Cold Cold

Factors Factors

1 Cold The conductance of the tissue is reduced, partly because vasoconstriction of the skin’s blood vessels reduces the blood flow and partly because the blood in the veins of the extremities is redirected from the superficial to the deep veins.

2 Heat When the nude, resting individual is exposed to heat (when the ambient temperature exceeds 28 ), or during muscular activity, the ℃heat content of the body tends to increase. Under such conditions, the blood vessels of the skin dilate, venous return in the extremities takes place through superficial veins, and the conductance of the tissue increases. In the comfort zone, the ski blood flow amounts to about 5% of the cardiac minute volume in extreme heat, it can increase to 20% or more. The increase heat flow to the skin increases the skin temperature.

3 Heat Balance If the heat content of the body is to remain constant, heat production and heat gain must equal heat loss, according to the following equation: M±C-E=0, where M= metabolic heat production; R= radiant heat exchange (positive if the environment is hotter than the skin temperature, but negative if the temperature of the environment is lower than that of the skin), C= convective heat exchange (positive if the air temperature is higher than that of the skin, negative if the reverse), and E= evaporative heat loss. This equation is valid only for conditions when the body temperature is constant. If the body temperature varies, a correction must be introduced, and the following equation is applicable: M±S±R±C-E=0, where S= storage of body heat. This was first derived by Winslow, Gagge, and Herrington (1939). S is positive if the body heat content is decreasing and negative if the heat content increases.

4 Methods of Assessing Heat Balance 1 ） The means skin temperature (Ts) is calculated by assigning certain factors to each measurement in proportion to the fraction of the body’s total surface area represented by each specific area, as originally done by Hardy and Dubois (1938): Head .07

Arms .14 Hands .05 Feet .07 Legs .13 Thighs .19 Trunk .35 Total 1.00

4 Methods of Assessing Heat Balance 2 ） A simple weighting formula for computing the mean skin temperature from observations of four areas of the body was developed by Ramanathan (1964). The formula, which can give values identical to the elaborate Hardy-Dudois formula, is as follows:

3) The following equation can be used to calculate the heat content of the body ：

Heat content =.83 W (.65 Tr +.35 Ts)

Ts=.3Tchest+.3Tarm+.2Tthigh+.2Tleg