Figure 40-2 Schematic illustrating thermoregulatory control mechanisms. Mean body temperature is the integrated thermal input from a variety of tissues, including the brain, skin surface, spinal cord, and deep core structures. This input is shown entering the hypothalamus from the left. However, thresholds are usually expressed in terms of core temperature. A core temperature below the thresholds for response to cold provokes vasoconstriction, nonshivering thermogenesis, and shivering. Core temperature exceeding the hyperthermic thresholds produces active vasodilation and sweating. No thermoregulatory responses are initiated when the core temperature is between these thresholds; these temperatures identify the interthreshold range, which in humans is usually only about 0.2°C. (Threshold data from Lopez M, Sessler DI, Walter K, et al: Rate and gender dependence of the sweating, vasoconstriction, and shivering thresholds in humans. Anesthesiology 80:780–788, 1994. Figure redrawn from Sessler DI: Perioperative hypothermia. N Engl J Med 336:1730–1737, 1997.)