Guttural pouches, brain temperature and exercise in horses

Abstract

Selective brain cooling (SBC) is defined as the lowering of brain temperature below arterial blood temperature. Artiodactyls employ a carotid rete, an anatomical heat exchanger, to cool arterial blood shortly before it enters the brain. The survival advantage of this anatomy traditionally is believed to be a protection of brain tissue from heat injury, especially during exercise. Perissodactyls such as horses do not possess a carotid rete, and it has been proposed that their guttural pouches serve the heat-exchange function of the carotid rete by cooling the blood that traverses them, thus protecting the brain from heat injury. We have tested this proposal by measuring brain and carotid artery temperature simultaneously in free-living horses. We found that despite evidence of cranial cooling, brain temperature increased by about 2.5 degrees C during exercise, and consistently exceeded carotid temperature by 0.2-0.5 degrees C. We conclude that cerebral blood flow removes heat from the brain by convection, but since SBC does not occur in horses, the guttural pouches are not surrogate carotid retes.

Figure 1

(a) Sites on horses at which temperatures were measured with implanted miniature thermometric loggers. (b) Blood and brain temperatures recorded at 5 min intervals in a free-living horse, at 32 °C globe temperature. Except for a short transient at the beginning of exercise, brain temperature exceeded carotid blood temperature before, during and after exercise. The difference between jugular venous blood temperature and carotid arterial blood temperature widened during exercise. (c), (d) Plots of mean, maximum and minimum hypothalamic temperature recorded at each 0.1 °C class of carotid blood temperature, and the line of identity, showing all data collected in horses at rest (lower end) and exercise (higher end), over 11 days ((c) 3168 data points and (d) excluding the 315 data points for which carotid blood temperature changed by 0.1 °C or more in the preceding 5 min). Except during the occasional transients, hypothalamic temperature always exceeded carotid blood temperature.