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Chapter 12Temperature Regulation

EXERCISE PHYSIOLOGY

Theory and Application to Fitness and Performance, 6th edition

Scott K. Powers & Edward T. Howley

© 2007 McGraw-Hill Higher Education. All rights reserved.

Objectives• Define tem homeotherm• Present an overview of heat balance during

exercise• Discuss the concept of “core temperature”• List the principle means of involuntarily increasing

heat production• Define four processes by which the body can lose

heat during exercise• Discuss the role of hypothalamus as the body’s

thermostat

© 2007 McGraw-Hill Higher Education. All rights reserved.

Objectives

• Explain the thermal events that occur during exercise in both a cool/moderate and hot/humid environment

• List physiological adaptations that occur during acclimatization to heat

• Describe physiological responses to a cold environment

• Discuss physiological changes that occur in response to cold acclimatization

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An Overview of Heat Balance

• In order to maintain a constant core temperature, heat loss must match heat gain

• Thermal gradient from body core to skin surface

Fig 12.1

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Temperature Measurement During Exercise

• Deep-body (core) temperature– Thermocouples or thermistors– Rectum, ear, and esophagus

• Skin temperature– Thermistors at various locations– Calculate mean skin temperature

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Heat Production

• Voluntary– Exercise

• Involuntary– Shivering– Action of hormones

• Thyroxine• Catecholamines

Fig 12.2

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Heat Loss

• Radiation– Transfer of heat via infrared rays– No physical contact between surfaces– 60% heat loss at rest

• Conduction– Heat loss due to contact with another

surface

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Heat Loss• Convection

– Form of conductive heat loss– Heat transferred to air or water

• Evaporation– Heat transferred via water (sweat) on skin surface– Evaporation rate depends on:

• Temperature and relative humidity• Convective currents around the body• Amount of skin surface exposed

– 25% heat loss at rest • Most important means during exercise

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Heat Exchange During Exercise

Fig 12.3

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The Hypothalamus:The Body’s Thermostat

• Increased core temperature– Anterior hypothalamus– Commencement of

sweating– Increased skin blood

flow

• Cold exposure– Posterior

hypothalamus– Increase heat

production• Shivering

– Decrease heat loss• Decreased skin

blood flow

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Responses to Heat Stress

Fig 12.4

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Responses to Cold Stress

Fig 12.5

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Heat Exchange During Exercise

• Metabolic energy (heat) production stimulates heat loss– Evaporative heat loss

• Most important means of heat loss– Convective heat loss

• Small contribution– Radiative heat loss

• Small role in total heat loss

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Heat Exchange During Exercise

Fig 12.6

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Body Temperature Increase During Exercise

• Increase in body temperature with work rate– Linear across wide range of temperatures– Linear for both arm and leg exercise

• Temperature proportional to active muscle mass

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Body Temperature During Arm and Leg Exercise

Fig 12.7

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Heat Exchange During Exercise Effect of Ambient Temperature• As ambient temperature increases,

– Heat production remains constant– Lower convective and radiant heat loss– Higher evaporative heat loss

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Heat Exchange During Exercise Effect of Ambient Temperature

Fig 12.8

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Heat Exchange During Exercise Effect of Exercise Intensity

• With increased exercise intensity– Heat production increases– Higher net heat loss

• Lower convective and radiant heat loss• Higher evaporative heat loss

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Heat Exchange During ExerciseEffect of Exercise Intensity

Fig 12.9

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Exercise in Hot/Humid Environments

• Inability to lose heat– Higher core temperature– Higher sweat rate

• Can result in:– Impaired performance– Hyperthermia

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Core Temperature and Sweat

Rate During Exercise in

Heat/Humidity

Fig 12.10

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Heat Acclimatization

• Increased plasma volume• Earlier onset of sweating• Higher sweat rate• Reduced sodium chloride loss in sweat• Reduced skin blood flow

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Exercise in a Cold Environment

• Enhanced heat loss– Reduces chance of heat injury– May result in hypothermia

• Cold acclimatization– Improved ability to sleep in the cold– Increased nonshivering thermogenesis– Higher intermittent blood flow to hands and feet– Results in ability to maintain core temperature

© 2007 McGraw-Hill Higher Education. All rights reserved.

Chapter 12Temperature Regulation