Chapter 12 Temperature Regulation EXERCISE PHYSIOLOGY Theory and Application to Fitness and Performance, 6th edition Scott K. Powers & Edward T. Howley

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

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

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

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

Heat Production Voluntary Exercise Involuntary Shivering Action of hormones Thyroxine Catecholamines Fig 12.2

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

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

Heat Exchange During Exercise Fig 12.3

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

Responses to Heat Stress Fig 12.4

Responses to Cold Stress Fig 12.5

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

Heat Exchange During Exercise Fig 12.6

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

Body Temperature During Arm and Leg Exercise Fig 12.7

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

Heat Exchange During Exercise Effect of Ambient Temperature Fig 12.8

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

Heat Exchange During Exercise Effect of Exercise Intensity Fig 12.9

Exercise in Hot/Humid Environments Inability to lose heat Higher core temperature Higher sweat rate Can result in: Impaired performance Hyperthermia

Core Temperature and Sweat Rate During Exercise in Heat/Humidity Fig 12.10

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

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

Chapter 12 Temperature Regulation