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2007, Piran, Slovenia

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Environmental Ergonomics XII<br />

Igor B. Mekjavic, Stelios N. Kounalakis & Nigel A.S. Taylor (Eds.), © BIOMED, Ljubljana <strong>2007</strong><br />

464<br />

PERFORMANCE OF THE TEMPERATURE AND HEAT<br />

REGULATION MODELS IN REPETITIVE WATER IMMERSION<br />

Andreas D. Flouris and Stephen S. Cheung<br />

Dalhousie University, Halifax, Nova Scotia, Canada<br />

Contact person: Andreas Flouris (aflouris@dal.ca)<br />

INTRODUCTION<br />

The most prominent theory proposed to date describing human thermoregulation recognizes a<br />

thermal ‘set-point’ that the body’s control systems attempt to maintain, but considerable<br />

dissension still exists as to whether the identification, monitoring, and maintenance of this setpoint<br />

are based on temperature or heat. The model of temperature regulation implies the<br />

existence of temperature-sensitive cells that respond appropriately to provide the body with a<br />

fixed temperature level, while its control is driven in proportion to the difference between the<br />

current temperature and the normal fixed resting temperature. Contrary, the model of heat<br />

regulation holds that the human body maintains a heat content equilibrium over a wide range<br />

of heat loads by sensing heat flow to/from the body and subsequently defending the body heat<br />

content through specific physiological responses. Each of the two thermoregulatory models<br />

exhibits inherent advantages and limitations in explaining the various phenomena of human<br />

nature and, after almost two centuries of research, the precise mechanisms of human<br />

thermoregulation remain uncertain.<br />

Application of the set-point theory suggests that the human body regulates its thermal status<br />

to a neutral and comfortable temperature level. The temperature regulation model suggests<br />

variations in skin surface temperatures across the different body regions to maintain core<br />

temperature within a prescribed range, but this does not necessarily extend to a stable body<br />

heat content which can vary significantly. In contrast, temperature in the heat regulation<br />

model is viewed as an end-result of the equilibrium in heat production and loss that serves to<br />

minimize changes in body heat content, with varying temperatures across the surface and the<br />

core of the body.<br />

The main challenge in examining these fundamental mechanisms of human thermoregulation<br />

stems from the complexity in the body’s responses to different stimuli, as well as the<br />

difficulty to explicitly identify the system of interest and isolate it from other systems within<br />

the body, as well as external dynamics. Nevertheless, the human being remains an automaton,<br />

constantly sensing the surrounding environment and making adept responses to preserve its<br />

homeostasis. Our purpose in this experiment was to disrupt the body’s thermal balance and to<br />

monitor its responses aiming at preserving its homeostasis in relation to the temperature and<br />

heat regulation models.<br />

METHODS<br />

Volunteers were 8 healthy adult males (23.8±2.0 years; 1.8±0.1 m; 79.6±11.4 kg]. Written<br />

informed consent was obtained from all participants after full explanation of the procedures<br />

involved. The experimental protocol conformed to the standards set by the Declaration of<br />

Helsinki and was approved by the appropriate ethical review board at Dalhousie University.<br />

Following an initial period of 15 minutes to obtain baseline values, participants entered a<br />

water tank maintained at 42ºC water temperature and passively rested until their core<br />

temperature was raised by 0.5ºC above baseline. Thereafter, they exited the warm bath and<br />

entered a different water tank maintained at 13ºC water temperature until their core<br />

temperature was decreased by 0.5 ºC below baseline. This procedure was repeated twice, as

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