Alternatively, evaporative heat loss for cooling occurs when temperatures above the TNZ, the upper critical zone (UCT), are realized (Speakman and Keijer 2013).
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.
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.
[6][7] Humans produce an obligatory 100 W (0.13 hp) of heat energy at rest as a by-product from basic processes like pumping blood, digesting, breathing, biochemical synthesis and catabolism etc.
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.
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.
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.
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.