How do blood vessels control body temperature?

Blood vessels control body temperature both, when the temperature gets too high as well as when it gets too low, since heat is carried in the blood stream. When the core body temperature gets too high, the blood vessels that supply the skin will widen, if the core body temperature gets too low, then the blood vessels will narrow. This means that more or less blood will flow through the blood vessels near the surface of the skin. This all results in more or less heat being lost by radiation when the body is trying to heat up or cool down.