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Temperature regulation in humans and other mammals I Claus Jessen p. cm. Includes bibliographical references and index.

Using miniature data loggers, we measured the temperatures of carotid blood and brain in four wildebeest (Connochaetes gnou) every 2 min for 3 wk and every 5 min, in two of the animals, for a further 6 wk. The animals ranged freely in their natural habitat, in which there was no shelter. They were subject to intense radiant heat (maximum approximately 1,000 W/m2) during the day. Arterial blood temperature showed a circadian rhythm with low amplitude (< 1 degree C) and peaked in early evening. Brain temperature was usually within 0.2 degrees C of arterial blood temperature. Above a threshold between 38.8 and 39.2 degrees C, brain temperature tended to plateau so that the animals exhibited selective brain cooling. However, selective brain cooling sometimes was absent even when blood temperature was high and present when it was low. During helicopter chases, selective brain cooling was absent, even though brain temperature was near 42 degrees C. We believe that selective brain cooling is controlled by brain temperature but is modulated by sympathetic nervous system status. In particular, selective brain cooling may be abolished by high sympathetic activity even at high brain temperatures.

We used miniature data loggers to record temperature and activity in free-ranging springbok (Antidorcas marsupialis) naturally exposed to severe nocturnal cold and moderate diurnal heat. The animals were active throughout the day and night, with short rests; the intensity of activity increased during daylight. Arterial blood temperature, averaged over many days, exhibited a circadian rhythm with amplitude < 1 degree C, but with a wide range which resulted from sporadic rapid deviations of body temperature. Peak blood temperature occurred after sunset. Environmental thermal loads had no detectable effect on blood temperature, even though globe temperature varied by > 10 degrees from day to day and > 20 degrees C within a day. Brain temperature increased approximately linearly with blood temperature but with a slope < 1, so that selective brain cooling tended to be activated at high body temperature, but without a precise threshold for the onset of brain cooling. Low activity attenuated selective brain cooling and high activity abolished it, even at high brain temperature. Our results support the concept that selective brain cooling serves to modulate thermoregulation rather than to protect the brain against heat injury.

The term selective brain cooling (SBC) refers to the lowering of brain temperature, either locally or as a whole, below arterial blood temperature. SBC has been reported to occur in laboratory experiments on many mammalian and avian species and was often seen as a mechanism serving to protect the brain, which was more or less intuitively supposed to be more susceptible to thermal damage than other organs of the body were. So far, however, no evidence has been found showing that brain tissues are less tolerant of elevated temperatures than other tissues are [1,2], and recent studies have cast great doubt on the validity of the protection concept of SBC (see below). This article reviews the present knowledge of the thermal and nonthermal factors conducive to the development of SBC. It is organized in four major sections. The first deals with species in which the presence of a carotid rete provides a well-defined anatomical structure capable of cooling all arterial blood destined for the brain, and thus cooling the brain as a whole. The second section concerns nonhuman mammals in which the carotid rete is either rudimentarily developed or missing; in these species SBC is very likely to be restricted to certain regions of the brain. The situation in humans deserves a separate section. On the one hand, no specialized heat exchanger like the carotid rete is available to cool the arterial blood before it enters the large brain, but on the other a sweating scalp and face may provide heat sinks of sizable capacity for cooling regions of the brain near its outer Key words: selective brain cooling, brain temperature, carotid rete, mammals, birds.Abstract: Artiodactyls and felids have a carotid rete that can cool the blood destined for the brain and consequently the brain itself if the cavernous sinus receives cool blood returning from the nose. This condition is usually fulfilled in resting and moderately hyperthermic animals. During severe exercise hyperthermia, however, the venous return from the nose bypasses the cavernous sinus so that brain cooling is suppressed. This is irreconcilable with the assumption that the purpose of selective brain cooling (SBC) is to protect the brain from thermal damage. Alternatively, SBC is seen as a mechanism engaging the thermoregulatory system in a water-saving economy mode in which evaporative heat loss is inhibited by the effects of SBC on brain temperature sensors. In nonhuman mammals that do not have a carotid rete, no evidence exists of wholebrain cooling. However, the surface of the cavernous sinus is in close contact with the base of the brain and is the likely source of unregulated regional cooling of the rostral brain stem in some species. In humans, the cortical regions next to the inner surface of the cranium are very likely to receive some regional cooling via the scalp-sinus pathway, and the rostral base of the brain can be cooled by conduction to the nearby respiratory tract; mechanisms capable of cooling the brain as a whole have not been found. Studies using conventiona...