| Popis: |
During hyperthermia, artiodactyls and other orders possessing a carotid rete can lower the temperature of the whole brain below that of the rest of the body core. Recent experiments in free-ranging animals have shown that selective brain cooling (SBC) is active during rest and moderate activity, but not during strenuous exercise when the danger of excessive hyperthermia is greatest. Thus, the previously prevailing concept of SBC as a mechanism protecting the brain from thermal damage is no longer tenable. As an alternative, we suggest that SBC adjusts the activity of heat loss mechanisms to the severity of heat stress by modifying the thermal input that is generated by brain sensors of temperature. In small species devoid of a carotid rete, current evidence indicates that superficial layers of brain near venous plexuses are cooled primarily by conduction, while the mechanisms effecting subtle cooling of the core of the brain remain enigmatic. In humans, some capacity for conductive heat transfer from the brain to the skin of the head, and areas of limited size available for convective heat exchange between arteries and venous plexuses, may permit cooling of superficial brain regions; if so, large thermal gradients between cortical and central regions of the brain must be expected. |
