Explain the terms “vasodilation” and “vasoconstriction”. How do these processes contribute in the regulation of the body temperature?

Vasodilation — It is the dilation of blood vessels in the skin that leads to an increase in the blood supply. Vasoconstriction — It is the narrowing of the blood vessels causing reduction in the blood supply to the skin.

Temperature Regulation:
In Cold Weather — When temperature is low, the blood vessels get narrowed. This reduces the blood supply to the skin. As a result, there is less loss of heat by convection, conduction and radiation and also less loss of it through vaporization of sweat because with reduced blood supply, the sweat gland secrete less sweat.
In Hot Weather — When outside temperature is high, there is vasodilation which results in greater loss of heat by radiation etc. and also by vaporization of sweat which is now produced in larger quantities due to the rich system of blood to the skin. Air movements over the body help to speed up evaporation of sweat leading to cooling effect on our body.