Limitations on arteriovenous cooling of the blood supply to the human brain.

Arteriovenous heat transfer (AVHT) is a thermoregulatory phenomenon which enhances tolerance to thermal stress in a variety of animals. Several authors have speculated that human responses to thermal stress reflect AVHT in the head and neck, even though primates lack the specialized vascular arrangements which characterize AVHT in other animals. We modeled heat transfer based on the anatomical relationships and blood flows for the carotid artery and associated venous channels in the human neck and cavernous sinus. Heat transfer rate was predicted using the "effectiveness-number of transfer units" method for heat exchanger analysis. Modeling showed that AVHT is critically dependent upon (1) heat exchanger effectiveness and (2) arteriovenous inlet temperature difference. Predicted heat exchanger effectiveness is less than 5.5% for the neck and 0.3% for the cavernous sinus. These very low values reflect both the small arteriovenous interface for heat exchange and the high flow rate in the carotid artery. In addition, humans lack the strong venous temperature depression required to drive heat exchange; both the cavernous sinus and the internal jugular vein carry a large proportion of venous blood warmed by its passage through the brain as well as a small contribution from the face and scalp, whose temperature varies with environmental conditions. Under the most optimistic set of assumptions, carotid artery temperature would be lowered by less than 0.1 degrees C during its passage from the aorta to the base of the brain. Physiologically significant cooling of the blood supply to the brain cannot occur in the absence of a suitably scaled site specialized for heat exchange.