Seals and whales have flippers and flukes that lack blubber and are poorly insulated. These appendages are well supplied with blood vessels an; receive rich blood supply. Therefore, these than structures with their large surface areas can lose substantial amounts of heat and aid in heat dissipation.
Figure: Countercurrent system for conserving heat in the flippers
However, excessive loss of heat is prevented due to the counter- , current heat exchangers in which the blood flows in opposite direction. In the whale flipper, each artery is completely surrounded by veins, and as warm arterial blood flows into the flipper it is cooled by the cold venous blood that surrounds it on all sides. The arterial blood therefore, reaches the periphery precooled and hence loses little heat to the water. The heat has been transferred to the venous blood, which thus is prewarmed before it reenters the body.