Baleen whales such as the gray whale move huge quantities of cold, ocean water through their very large mouths and across the filtering surface of the baleen. The tongue of a whale can represent as much as 5% of its total body surface area. The whale’s body is well insulated with blubber but not the tongue. Thus, to avoid losing too much of its body heat to the cold water passing through its mouth, the gray whale’s tongue has the largest counter-current heat exchanger yet described. This lingual rete (blood vessel network, inside each side of the tongue) is comprised of more than 50 sets of very long and small diameter arteries each surrounded by many small veins. This structure insures a slowed blood flow and a large surface area for exchange of heat between the cool blood in the veins leaving the tongue and the warm blood in the arteries coming into the tongue. This way blood is pre-cooled very effectively before approaching the surface of the tongue and thus does not lose much heat to the cold water in the mouth. The surface temperature of the tongue of a young gray whale has been measured to be only 0.5° C higher than the water.
The presence of very long, small diameter arterial and venous vessels in close proximity with low flow is key to the efficient recapture of heat and maintenance of a cool tongue surface.
“Vascular structures for heat conservation in the tongue of the gray whale (Eschrichtius robustus) are reported here. Numerous individual countercurrent heat exchangers are found throughout the massive tongue. These converge at the base of the tongue to form a bilateral pair of retia. Temperature measurements from the oral cavity of a live gray whale indicate that more heat may be lost through the blubber layer over the body than through the tongue, despite the fact that the tongue is far more vascularized and has much less insulation. These heat exchangers substantially reduce heat loss when these whales feed in cold waters.” (Heyning and Mead 1997:1138)