The blood vessels of vertebrates regulate temperature by allowing more blood to flow to the skin when cooling is needed and restricting blood flow to the skin when heat must be conserved.
"Blood vessels bring nourishment to the skin, and help to control body temperature. When the body is warm the blood flows more copiously towards the skin, giving off heat to the cooler air outside; it flows even more strongly when the body is being exerted, so someone taking exercise on a warm day will be 'flushed' from the extra blood passing through the skin for cooling. Conversely, in cold weather small valves may close to restrict the blood supply to the skin and hence conserve heat within the body." (Foy and Oxford Scientific Films 1982:77)
Learn more about this functional adaptation.