Human Temperature Control by Eugene H. Wissler

Author:Eugene H. Wissler
Language: eng
Format: epub
ISBN: 9783662573976
Publisher: Springer Berlin Heidelberg

5.2 Empirical Considerations

Given the vital role of sweating in human thermoregulation, it is not surprising that virtually all aspects of sweating have been investigated experimentally. Studies of sweating reveal that the structure of sweat glands and the process of sweat secretion are more complex than they appear to be at first sight. A recent open source paper by Taylor and Machado-Moreira (2013) provides a more detailed description of both structure and process than we will include in this chapter. Another paper by Shibasaki et al. (2006) also discusses aspects of sweating not included in this chapter. Because our principal concern is the effect of sweat secretion and evaporative cooling on human thermoregulation, we will focus attention on quantitative relationships that define sweat rate in terms of physiological variables.

Before we discuss specific studies and construct a quantitative model for sweating, several general considerations are worth mentioning. First, we note that sweating can be measured either for the whole body or for a small area of the skin. Moreover, it is important to differentiate between sweat secretion and evaporation. The rate of evaporation increases as the rate of secretion increases until the maximum rate of evaporation determined by mass transfer limitations is reached. Sweat secreted at a rate higher than the maximum rate of evaporation either accumulates on the skin or drips from the skin. In experimental studies, what happens to sweat that drips from the body and how that affects the “apparent sweat rate” depends on the physical arrangement of the system, and it is important to know what was measured when evaluating experimental data. It is also important to note that the rate of sweat secretion decreases when sweat is secreted more rapidly than it evaporates and accumulates on the skin.

The local mass transfer coefficients for forced convection vary markedly with position and are typically several times larger on the windward side of an arm or leg than on the leeward side. Consequently, the rate of evaporation is often determined by the rate of sweat secretion on the dry windward side of the body and by mass transfer limitations on the wet leeward side.

Two calorimetric methods used to measure the whole-body rate of evaporation in early studies were described in Chap. 2. In direct calorimetry, the rate of evaporation is determined by the rate of absorption in a desiccant such as a sulfuric acid solution, by the rate of condensation on a chilled heat exchanger, or by the difference between mass flow rates of water in air entering and leaving the calorimeter. In partitional calorimetry, continuous or periodic weighing of a subject provides a measure of the whole-body weight loss. During nonsteady-state conditions, the whole-body rate of evaporation is generally not equal to the rate of sweat secretion, because sweat accumulates on the skin when the sweat rate increases and accumulated sweat evaporates from the skin when the sweat rate decreases. Calorimetric and weight-loss methods also measure water loss through the respiratory tract in addition to evaporative water loss from the skin.

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