Think of the body as a heat-sink. We can let off heat by using our fluid exchanges – namely, blood and sweat. When we exercise, sweat cools us off through evaporative cooling. When we’re not exercising or in warm weather, we’re generally not sweating, but we’re still exchanging heat with our surroundings – after all, the metabolism is still active, we’re still “burning energy,” and our skin never reaches air temperature nor does our core cool to the temperature of our skin (so some kind of heat exchange has to be happening). How can that be, when there’s no obvious mechanism to carry away our heat as in the case of sweating?
The answer to this lies at the heart of your question. Human thermoregulation is largely thanks to our blood flow and vasodilation:
Cutaneous sympathetic vasoconstrictor and vasodilator systems also participate in baroreflex control of blood pressure; this is particularly important during heat stress, when such a large percentage of cardiac output is directed to the skin. Local thermal control of cutaneous blood vessels also contributes importantly--local warming of the skin can cause maximal vasodilation in healthy humans and includes roles for both local sensory nerves and nitric oxide. Local cooling of the skin can decrease skin blood flow to minimal levels.