File:Venae comitantes.jpg

English: The counter-current flow of blood into a limb in a cold environment. On the left a cartoon of the blood flow into the flipper of a seal in icy water. The arterial blood blood is in close proximity to, indeed surrounded by, veins carrying cold blood back to the torso from the flipper which is almost at the same temperature as the icy water. The arterial blood is cooled by this counter-flow of cold venous blood and arrives in the flipper at only a few degrees above the environmental temperature. As the blood returns in the veins it is warmed by the arterial blood, and arrives back in the torso at almost core temperature. Effectively the body heat has been short-circuited and does not enter the ice-cold flipper. The diagram on the right shows, in slightly more realistic form, the arrangement of the deep veins around the arteries in the limbs of mammals, all of whom can make use of the same mechanism as the seal to prevent heat loss from the body in cold weather.