Thermoneutral zone

The thermal neutral zone (TNZ) is essentially an endotherm's temperature tolerance range. Within the TNZ the basal rate of heat production is in equilibrium with the rate of heat loss to the external environment. The endotherm does not have to use large amounts of energy to control its temperature within the thermal neutral zone, the organism adjusts to the temperatures within the zone through different responses requiring little energy. It can show postural changes where it changes its body shape or moves and exposes different areas to the sun/shade, and through radiation, convection and conduction, heat exchange occurs. Vasomotor responses allow control of the flow of blood between the periphery and the core to control heat loss from the surface of the body. Lastly, the organism can show insulation adjustments; a common example being “goosebumps” in humans where hair follicles are raised by pilomotor muscles, also shown in animals’ pelage and plumage. This allows heat to be trapped between hairs. Below the thermal neutral zone there is the zone of LCT (lower critical temperature) and above there is the zone of UCT (upper critical temperature). The organism reaches the LCT when the Ta (ambient temp.) decreases. When an organism reaches this stage the metabolic rate increases significantly and thermogenesis increases the Tb (body temp.) If the Ta continues to decrease far below the LCT hypothermia occurs. When the Ta reaches too far out of the UCT the rate heat gain and heat production become higher than the rate of heat dissipation (heat loss through evaporative cooling), resulting in hyperthermia.

The thermoneutral zone describes a range of temperatures of the immediate environment in which a standard healthy adult can maintain normal body temperature without needing to use energy above and beyond normal basal metabolic rate. Its value is 25 - 30 degrees Celsius (77 - 86 degrees Fahrenheit) for a naked man, standing upright, in still air. Note this is for a resting human and does not allow for shivering, sweating or exercising. Even with light clothing, radiation and convection losses are dramatically reduced, effectively reducing the TNZ. Hence a comfortable temperature within a controlled environment like an air-conditioned building will be more like 18 - 22 degrees Celsius (64.4 - 71.6 degrees Fahrenheit).

Humans produce an obligatory ~100 watts of heat energy as a by-product from basic processes like pumping blood, digesting, breathing, biochemical synthesis and catabolism etc. This is comparable to a common incandescent light-bulb. Hence, if the body were perfectly insulated, core temperature would continue to increase until lethal core temperatures were achieved. Conversely, we are normally in surroundings that are considerably colder than body core temperature (37 degrees Celsius or 98.6 degrees Fahrenheit) and hence there is a large gradient for thermal energy flow from the core to the surroundings. Therefore, the body must ensure it can also minimize the loss of heat to around 100 watts, if it is to maintain core temperature. In short, the skin must be able to get rid of 100 watts of heat in relatively warm environments, but also ensure that it does not lose too much more than this in relatively cold environments.

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Wikipedia