Selective brain cooling in the horse during exercise and environmental heat stress

Five horses were exercised on a treadmill [to central blood temperature (Tcore) approximately 42.5 degrees C]. Three of those horses were heated at rest in a climate room (53 degrees C, 90% relative humidity) (to Tcore approximately 41.5 degrees C). Temperatures were measured in the rectum, hypothalamus (Thyp), cerebrum, and cavernous sinus (Tsinus), on the skin of the head and midside, and Tcore. When Tcore increased above 38.5 degrees C, Thyp remained 0.6 +/- 0.1 degree C (SE) lower during heat exposure and 1 +/- 0.2 degrees C lower during exercise. During heat exposure, Tsinus was 2.2 +/- 0.4 degrees C below Tcore, and during exercise, Tsinus was 5 +/- 0.9 degrees C below Tcore. Upper respiratory tract bypass during exercise in one horse resulted in substantial reductions in Tcore-Thyp to 0.4 +/- 0.3 degrees C and Tcore-Tsinus to 0.9 +/- 0.2 degrees C. Thus the horse, a species without a carotid rete, can selectively cool the brain during exercise or heat exposure; this occurs, at least in part, via cool blood within the cavernous sinus, presumably resulting principally from cooling of venous blood within the upper respiratory tract.