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INTRODUCTION

Objectives

By studying this chapter, you should be able to do the following:

  1. Define the term homeotherm.

  2. Present an overview of heat balance during exercise.

  3. Discuss the concept of “core temperature.”

  4. List the principal means of involuntarily increasing heat production.

  5. Define the four processes by which the body can lose heat during exercise.

  6. Discuss the role of the preoptic-anterior hypothalamus as the body’s thermostat.

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

  8. List the physiological adaptations that occur during acclimatization to heat.

  9. Describe the physiological responses to a cold environment.

  10. Discuss the physiological changes that occur in response to cold acclimatization.

Outline

Overview of Heat Balance during Exercise

Temperature Measurement during Exercise

Overview of Heat Production/Heat Loss

  • Heat Production

  • Heat Loss

  • Heat Storage in the Body during Exercise

Body’s Thermostat—Preoptic-Anterior Hypothalamus

  • Shift in the Hypothalamic Thermostat Set Point Due to Fever

Thermal Events during Exercise

Heat Index—A Measure of How Hot It Feels

Exercise in a Hot Environment

  • Sweat Rates during Exercise

  • Exercise Performance Is Impaired in a Hot Environment

  • Gender and Age Differences in Thermoregulation

  • Heat Acclimation

  • Loss of Acclimation

Exercise in a Cold Environment

  • Physiological Responses to Exercise in the Cold

  • Cold Acclimation

Key Terms

acclimation

acclimatization

conduction

convection

evaporation

homeotherms

hyperthermia

hypothermia

preoptic-anterior hypothalamus

radiation

specific heat

thermoreceptors

The regulation of body temperature is critical because cellular structures and metabolic pathways are affected by temperature. For example, enzymes that regulate metabolic pathways are greatly influenced by temperature changes. Indeed, an increase in body temperature above 45°C (normal core temperature is approximately 37°C) can alter the normal structure of enzymes, resulting in the inability to produce cellular energy (i.e., ATP). Ultimately, an inability to produce cellular energy would result in cell death and eventually death to the organism. Further, a decrease in body temperature below 34°C causes a slowed metabolism and abnormal cardiac function (arrhythmias), which can also lead to death. Hence, people and warm-blooded animals live their entire lives only a few degrees from their thermal death point. Therefore, it is clear that body temperature must be carefully regulated.

Humans and other animals that maintain a rather constant body core temperature are called homeotherms. The maintenance of a constant body temperature requires that heat loss must match the rate of heat production. To accomplish thermal regulation, the body is well equipped with both nervous and hormonal mechanisms that regulate metabolic rate as well as the amount of heat loss in response to body temperature changes. The temperature-maintenance strategy of homeotherms uses a “furnace” rather than a “refrigerator” to maintain body temperature at a constant level. That is, the body temperature is set near the high end of the survival range and is held constant by continuous metabolic heat production coupled with a small but continual heat loss. The rationale for this strategy is that temperature regulation by ...

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