SPECIALIZED BRAIN COOLING IN HUMANS

Humans, compared to other species, have exceptional capability for dis sipation of heat from the entire skin surface. We can secrete more tha n two liters per hour of sweat, indefinitely. The corresponding potent ial for evaporative cooling is near a thousand watts, sufficient to co mpensate for the extreme high levels of heat production during exercis e. Also, the blood vessels of our skin have exceptional capability to dilate and deliver heat to the body surface. These are our special ada ptations for thermal stress. They allow prolonged heavy exercise with modest elevations in the temperature of the fluid that cools all the i nternal organs, not just the brain-arterial blood. The vascular archit ecture of the human head is radically different from that of animals t hat exhibit SBC. These species have special adaptations that reflect t heir dependence on respiratory evaporation, particularly the limitatio n imposed on capability to dispose of heat produced during exercise. T he increase in blood temperature in an intense sprint would heat the w ell-perfused brain rapidly. But the heat exchange over the large surfa ce area of contact between a venous plexus cooled by respiratory evapo ration and the meshwork of arterial vessels in the carotid rete precoo ls blood bound for the brain. Specialized cooling of the brain (SBC) h as not been demonstrated by direct measurements in humans. Changes in tympanic temperature (T(ty)) are taken as evidence for SBC. This conti nues an unfortunate tradition of exaggeration of the significance of T (ty). In the only direct measurements available, brain temperature was unaffected by fanning the face although T(ty) did fall. What may appe ar to be a remnant of the carotid rete heat exchanger in humans is the intimate association between a short segment of the internal carotid artery and the plexus of veins in the cavernous sinus. Fortunately, th e brain need not rely for its cooling on countercurrent heat exchange across this small surface area of contact. In humans, SBC stands for s kin: the body cooler-we use our entire skin surface for heat dissipati on.