In contrast to the warm muscle and viscera of swimming bluefin tunas, the heart and gills remain at or near ambient water temperature in all tuna species. Tunas achieve regulation of body temperature by employing complex vascular structures called rete mirabile. In bluefin tuna, large lateral cutaneous vessels that branch off into the arteries and veins of rete mirabile supply blood to the red muscle, instead of a centrally located aorta. Rete mirabile function as countercurrent heat exchangers that prevent metabolic heat loss at the gills. Warm-bodied fish, such the southern bluefin tuna, maintain their TB by varying the efficiency of heat exchangers. Some oxygen is typically lost to outgoing venous blood in the process of heat exchange, depending on heat exchanger efficiency, which can be influenced by the rate of blood flow and blood vessel diameter.