Human Thermoregulation at a Glance
- Thermoregulatory reflexes involve changes in skin blood flow and sweating that act to preserve thermal balance with an internal temperature of approximately 37°C (98.6°F).
- Thermal balance is determined by metabolic heat production; evaporative heat loss; heat gain or loss through radiant, convective, and conductive mechanisms; and useful mechanical work done.
- Dermal papillary loops, arteriovenous anastomoses, and sweat glands are the major skin effectors of thermoregulation.
- Heat stress evokes large increases in skin blood flow and sweating through cholinergic cotransmitter and nitric oxide-dependent mechanisms to facilitate heat dissipation.
- Local skin heating causes a local vasodilation through antidromic neurotransmitter release from afferent skin nerves and increased nitric oxide generation.
- Cold stress evokes reduced skin blood flow through noradrenergic cotransmitter mechanisms to facilitate heat conservation.
- Local skin cooling causes a local vasoconstriction through noradrenergic and afferent neural mechanisms as well as nonneural mechanisms.
Human beings are homeotherms: we maintain our internal, or core temperature of the body within a narrow range despite thermal stresses. Thermal stress can arise from variations in environmental temperature or from the human body itself, as with heat generation by skeletal muscle during dynamic exercise. When thermal stresses arise from the environment, changes in skin temperature occur prior to any change in internal temperature. When thermal stresses arise from the body itself as with exercise, changes in core temperature occur prior to any change in skin temperature. In either case, thermal gradients are established between the skin and the body core. If skin temperature is lower than core temperature, heat will be lost from the body unless skin blood vessels constrict. If skin temperature is greater than core temperature, the body will gain heat, unless skin vessels dilate and sweat glands produce perspiration. The skin is thus a crucial component of human thermoregulation.
Human thermoregulation is achieved through an integration of several physiological processes. These integrated processes make up thermoregulatory reflexes that maintain a stable internal temperature at a “set point” of 37°C (98.6°F) despite thermal stresses. The set point is not invariant and may fluctuate as much as 0.5°C–1.0°C (0.9°F–1.8°F) according to circadian rhythms and during the menstrual cycle in females. The thermoregulatory reflexes designed to preserve internal temperature at the set point are coordinated by thermally sensitive neurons in the anterior hypothalamic–preoptic area and spinal cord, which respond to the changes in internal and skin temperatures. For example, cold-sensitive neurons in the anterior hypothalamic–preoptic area and spinal cord integrate afferent sensory inputs and activate heat-conserving mechanisms that include cutaneous vasoconstriction and increased metabolic heat production (shivering). Conversely, the stimulation of heat-sensitive neurons in the anterior hypothalamic–preoptic area and spinal cord integrate afferent sensory inputs and activate heat-dissipating mechanisms that include cutaneous vasodilation and sweat production.
To maintain thermal balance, heat gained or lost by the body must equal heat dissipated from, or produced by the body. This concept can be mathematically expressed as: