ADAPTATION Thermoregulation

Cetaceans can maintain or lose body heat by adjusting their circulatory system accordingly. The arteries in the limbs are surrounded by veins. The heat in the arterial blood can be moved to the venous blood returning to the body core to maintain body heat. This is called a countercurrent heat exchange system and is advantageous when the water is cold or the cetacean is not active. During diving, the blood is transported only in the body core, away from the surface to preserve heat.(Ridgway, 1972)

When a cetacean needs to lose body heat either because it is very active or the water is warm, more blood is transported to the veins near the surface of the limbs and the heat is lost to the environment. This decreases the blood in the body core, therefore decreasing body heat.(Ridgway, 1972).
Diagram shows how arteries are surrounded by veins to conduct heat.(Seaworld website)