Constriction of blood vessels means that there is less heat radiated to the environment. This is why prolonged cold can lead to frostbite- the fingers and toes don't get enough blood sent to them and the tissues begin to die. Blood vessels dilate to radiate heat to the environment. Due to Homeostasis (maintanance of the internal environment) when your too cold, your blood vessels constrict to preserve heat which makes blood flow slower. when your warm, your blood flows better and mroe heat is released.