Human Skin: Sweating, Thermoregulation, and Water Balance
Humans are tropical animals, and given access to shade and adequate water, healthy persons can tolerate extended exposure to any naturally occurring climatic heat stress. This talk examines the role of skin in human thermoregulation as a potentially important evolutionary factor to modern man. Two strong selective factors for survival in early hominins were the ability to forage during peak daily heat when their predators were not a threat, and the capability for persistence hunting (track and pursue prey to cause hyperthermia-induced exhaustion of prey). Unlike most animals, humans can thermoregulate while performing physical exertion so that body core temperature elevates primarily as a function of metabolic rate (exercise intensity) and is only slightly influenced by the climate, if sufficient biophysical heat exchange is possible (e.g., convection, evaporation). Human phenotypes that contribute importantly to thermoregulation include: upright posture, large skin surface area, active skin vasodilation, eccrine sweating and sweat sodium reabsorption, and sensory information. Although these characteristics enhance thermoregulation, they often impose cardiovascular challenges during physical exertion. However, several skin sensory mechanisms help optimize the balance between thermoregulation and cardiovascular strain.