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 through evaporative cooling.
The brain is a part of the body that is particularly sensitive to high temperature. Hence some ungulates, like the Thomson’s gazelle, use a counter-current heat exchanging structure known as the carotid rete to keep the brain cooler than the body. The rete is a configuration of arteries and veins in a sinus at the base of the brain. Warm blood flowing to the brain travels from the carotid artery into a network of small arteries within the sinus, where it transfers some of its heat to cooler venous blood flowing the opposite direction as it returns from the nasal passages. The cooled arterial blood then continues toward 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)
“In artiodactyls, the arterial blood destined for the brain passes through the carotid rete at the base of the brain…It consists of hundreds of small arteries, arising from branches of the carotids and after 10–15 mm joining again to enter the circle of Willis. The rete is embedded in the cavernous sinus, which carries cool blood returning via the angularis oculi vein from the evaporating surfaces of the upper respiratory tract. This is a heat exchanger of considerable capacity: in one species of antelope, Thomson’s gazelle, a 2.7°C gradient between Tbrain and Ttrunk was observed.” (Jessen 1998:281)