rom "The Hot Brain", MIT Press ...
Thermal Response to Cold
Cold does not exist as a physical entity in and of itself. It is defined as a lack of heat and is not a different form of energy. We are not "pierced" by the cold winds of winter. Cold is simply a sensation that results from heat loss. When humans are exposed to a cold environment, their skin cools and stimulates cutaneous cold receptors, which increase their firing rate as skin temperature decreases . This in turn activates neural pathways to produce superficial vasoconstriction, piloerection or "goose pimples," and possibly shivering. The superficial veins under the skin constrict, and blood returns from the limbs via deep veins, the venae comitantes. This venoconstriction contributes to the lowering of skin temperature, which reduces the temperature difference between the skin and the environment, thereby decreasing heat loss by radiation, conduction, and convection. In very cold weather, vasoconstriction is so effective that tissue insulation can be increased sixfold. This is equivalent to wearing a light wool business suit. Interestingly, the insulation of the head does not change with environmental temperature because cerebral blood flow is kept constant : at -4 C the amount of heat lost from the head is about half of that produced
by the body at rest. It is wise to wear a hat in the winter.