One pathway conserves metabolic heat at times when their environment is too cold. The other pathway allows them to dissipate heat even when they are well insulated with blubber. The cooling pathway is what we might typically expect to find with deep arteries taking warm blood to the extremity, and veins returning along the under-surface of the skin in such a way that blood cools. The heat conservation pathway is an ingenious counter current heat-exchange system consisting of deep parallel arteries and veins which allows warm outgoing arterial blood to pass its heat over to blood returning in the venous system. The heating and cooling needs of whales change rapidly.
Muscles can flatten veins and shunt blood into one system or the other.
Heat is produced throughout the muscle bulk of the whale when it exercises. With movement, arterial pressure is increased. The enlarged artery pushes on the encircling veins and restricts blood flow in them. The counter-current exchanger is cut off and the blood must return through veins near the skin. Thus, heat is lost at the surface of fins and flippers.