The carotid rete of the Thomson's gazelle cools its brain via counter-current heat exchange.
The Thomson’s gazelle lives in the East African savannah where is it exposed to high temperatures and predation by big cats, like the cheetah, lion, or leopard. These gazelles have been recorded to run at up to 43-50 miles per hour. Such a burst of speed may raise the metabolic rate, and thus heat production, by as much as 40 fold. Dissipating such heat loads is difficult, especially in arid environments where water is scarce and an animal needs to avoid losing too much.
The brain is a part of the body that is particularly sensitive to high temperature. Hence some ungulates, like the Thomson’s gazelle, have evolved a counter-current heat exchanging structure known as the carotid rete, a configuration of blood vessels in the brain that can keep its temperature lower than body temperature. The blood flowing to the brain moves from the carotid artery into a network of small arteries within a large venous sinus or cavity filled with cooled blood returning from the nasal passages. The warm arterial blood gives up some of its heat to the cool venous blood and this lowers the temperature of the blood on its way to the brain. In the running Thomson’s gazelle, body temperature rises more than brain temperature such that a difference between brain and body temperature has been measured at 2.7° C. A predator like the cheetah must stop running when its body and brain temperature reaches 40.5° C but the gazelle can keep running as its body temperature rises above 43° without its brain temperature exceeding 40.5°. The ability to keep a cool head can thus give the gazelle a survival edge in these predatory pursuits as he can outlast the cheetah who cannot maintain a cooler brain.
Counter-current heat exchangers can be found in many organisms in many configurations. While such mechanisms are well known to engineers, a close look at the design of those used by nature may be useful in designing thermal control systems of human habitations. (Courtesy of The Biomimicry Institute)
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