Brengelmann G L
Department of Physiology and Biophysics, University of Washington, Seattle 98195.
FASEB J. 1993 Sep;7(12):1148-52; discussion 1152-3. doi: 10.1096/fasebj.7.12.8375613.
Humans, compared to other species, have exceptional capability for dissipation of heat from the entire skin surface. We can secrete more than two liters per hour of sweat, indefinitely. The corresponding potential for evaporative cooling is near a thousand watts, sufficient to compensate for the extreme high levels of heat production during exercise. Also, the blood vessels of our skin have exceptional capability to dilate and deliver heat to the body surface. These are our special adaptations for thermal stress. They allow prolonged heavy exercise with modest elevations in the temperature of the fluid that cools all the internal organs, not just the brain-arterial blood. The vascular architecture of the human head is radically different from that of animals that exhibit SBC. These species have special adaptations that reflect their dependence on respiratory evaporation, particularly the limitation imposed on capability to dispose of heat produced during exercise. The increase in blood temperature in an intense sprint would heat the well-perfused brain rapidly. But the heat exchange over the large surface area of contact between a venous plexus cooled by respiratory evaporation and the meshwork of arterial vessels in the carotid rete precools blood bound for the brain. Specialized cooling of the brain (SBC) has not been demonstrated by direct measurements in humans. Changes in tympanic temperature (Tty) are taken as evidence for SBC. This continues an unfortunate tradition of exaggeration of the significance of Tty. In the only direct measurements available, brain temperature was unaffected by fanning the face although Tty did fall. What may appear 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, the brain need not rely for its cooling on countercurrent heat exchange across this small surface area of contact. In humans, SBC stands for skin: the body cooler--we use our entire skin surface for heat dissipation.
与其他物种相比，人类具有从整个皮肤表面散发热量的非凡能力。我们每小时能分泌超过两升的汗液，且可持续不断。相应的蒸发冷却潜力接近一千瓦，足以补偿运动期间极高的产热水平。此外，我们皮肤的血管具有非凡的扩张能力，能将热量输送到体表。这些是我们针对热应激的特殊适应机制。它们使我们能够在长时间剧烈运动时，仅让冷却所有内脏器官（不仅仅是大脑——动脉血）的液体温度适度升高。人类头部的血管结构与表现出特殊脑冷却（SBC）的动物截然不同。这些物种具有特殊的适应机制，反映了它们对呼吸蒸发的依赖，特别是对运动期间产热散热能力的限制。在激烈短跑中，血液温度的升高会迅速使灌注良好的大脑升温。但是，通过呼吸蒸发冷却的静脉丛与颈动脉网中的动脉血管网络之间大面积接触所进行的热交换，会预先冷却流向大脑的血液。在人类中，尚未通过直接测量证实存在专门的脑冷却（SBC）。鼓膜温度（Tty）的变化被视为SBC的证据。这延续了一个不幸的传统，即夸大了Tty的重要性。在仅有的直接测量中，尽管Tty确实下降了，但扇脸时大脑温度并未受到影响。在人类中，看似颈动脉网热交换器残余的结构是颈内动脉一小段与海绵窦内静脉丛的紧密关联。幸运的是，大脑的冷却并不依赖于通过这一小面积接触进行的逆流热交换。在人类中，SBC代表皮肤：身体的冷却器——我们利用整个皮肤表面来散热。
