Orderly recruitment of thermoeffectors in resting humans

The recruitment of thermoeffectors, including thermoregulatory behavior, relative to changes in body temperature has not been quantified in humans. We tested the hypothesis that changes in skin blood flow, behavior, and sweating or metabolic rate are initiated with increasing changes in mean skin temperature (T_skin) in resting humans. While wearing a water-perfused suit, 12 healthy young adults underwent heat (Heat) and cold stress (Cold) that induced gradual changes in T_skin. Subjects controlled the temperature of their dorsal neck to their perceived thermal comfort. Thus neck skin temperature provided an index of thermoregulatory behavior. Neck skin temperature (T_skin), core temperature (T_core), metabolic rate, sweat rate, and nonglabrous skin blood flow were measured continually. Data were analyzed using segmental regression analysis, providing an index of thermoeffector activation relative to changes in T_skin. In Heat, increases in skin blood flow were observed with the smallest elevations in T_skin ( P < 0.01). Thermal behavior was initiated with an increase in T_skin of 2.4 ± 1.3°C (mean ± SD, P = 0.04), while sweating was observed with further elevations in T_skin (3.4 ± 0.5°C, P = 0.04), which coincided with increases in T_core ( P = 0.98). In Cold, reductions in skin blood flow occurred with the smallest decrease in T_skin ( P < 0.01). Thermal behavior was initiated with a T_skin decrease of 1.5 ± 1.3°C, while metabolic rate ( P = 0.10) and T_core ( P = 0.76) did not change throughout. These data indicate that autonomic and behavioral thermoeffectors are recruited in coordination with one another and likely in an orderly manner relative to the comparative physiological cost.