Thermal neutral zone

A 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.^[1] 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.

In Humans

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).

Human beings 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. On the flip-side, 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.

The human outer core (skin, subcutaneous fat etc.) acts as an adjustable insulator/radiator with the main mechanism of adjustment being blood flow to this compartment. If the surroundings are warm then heat loss is less, so the body directs more blood to the periphery to maintain the gradient for energy flow. Conversely, if the surroundings are cool, blood flow can be profoundly reduced to the skin, so that heat loss is reduced significantly.

These passive processes determine the TNZ, as negligible work is done to redirect blood to the peripheries or the core.

Physiological mechanisms:

The skin has a huge capacity to accept blood flow resulting in a range of 1ml/100g of skin/min, to 150ml/100g/min. Its metabolic requirements are very low and hence it only requires a very small fraction of the heart's output to maintain its own growth and metabolism. In temperate environments the blood flow to the skin is much higher than required for metabolism, the determining factor is the need for the body to get rid of its heat. In fact, skin can survive for long periods of time (hours) with sub-physiological blood flow and oxygenation, and, as long as this is followed by a period of good perfusion, necrosis will not occur.

One can see that in temperate environments there is room to increase or decrease blood flow to the skin dramatically. This is achieved by way of special arrangements in the vascular beds of the skin. There are significant numbers of extra vessels, especially in the extremities with their large surface areas (hands, ears, toes etc.). These are direct connections between artery and vein which bypass nourishing capillaries, and are controlled by the sympathetic nervous system. These shunts are normally mostly closed, but opening them up allows the skin to become engorged with blood, and because these vessels have low resistance, the blood flow through them is brisk. Conversely, when blood supply to the skin must be reduced these shunts can be closed and furthermore, the normal mechanism of vasoconstriction of arterioles, can dramatically reduce perfusion of the skin.

References