Figure 2.

Illustration of the impact of evaporative cooling on heat transfer. A. Scheme of potential modes of heat loss. Loss of heat from body temperature (Tbody) to the outside (Tamb) is governed by at least three mechanisms that relate to the resistance of the skin biofabric (described by a factor, Rf) and the biophysical properties of the boundary layer (described by another factor, Rc). Heat can be lost by conduction (Qcon), radiation (Qrad) or evaporation (Qevap); together, these determine the surface temperature (Ts). The rate of radiation is controlled by the gradient of temperature from body to environment (Tamb); conduction by contact surfaces, and evaporative cooling by ambient humidity (ɸamb). B. Dry block assay. Skins were excised from mice, transferred to a heat block (37^oC), and promptly imaged using infrared thermography (FLIR). Skin thickness was measured by histological analysis of sections. Impeded heat flow was visualized as a lower surface temperature (color scale shown at foot of image), and quantified as [T0-T1]dry, where higher values show higher insulation. C. Wet block assay. Skins were transferred to a water-saturated tissue substrate (emissivity >0.98, homogeneous background; 37 °C), and imaged by FLIR. The surface temperature recorded is lower for skins with high rates of transpiration and also lower for skins with more insulation (opposing metabolic effects). The surface temperature is quantified as [T0-T3]wet, where values combine these two distinct aspects of skin physiology. D. Trans-epidermal water loss assay reveals basis for discrepant dry and wet block images. TEWL was measured using a closed-chamber instrument; results for these skins varied over 3-fold, from skins ranging in thickness by nearly 6-fold (75 and 522 μM). Note that SKO skins appear black due to trapped hair follicles (see detailed descriptions in legend to Figure 4). Thermographic images were analyzed using the FLIR camera software as per Materials and Methods section; shown are mean values ± SEM, analyzed by unpaired 2-tailed t tests (****P < 0.0001; ***P < 0.001; *P < 0.05).