Chilling out

 作者:邓甄     |      日期:2019-03-08 07:10:02
By Michael Day THE blood vessels and airways in our heads may be arranged to act as a heat exchanger that cools blood before it enters the brain. What’s more, the failure of this system to prevent the brain overheating might be a cause of sudden infant death syndrome, a team of researchers believes. The brain is particularly sensitive to the dangerous effects of overheating. Physiologists know that some mammals, such as cats and sheep, have a complex network of arteries—the rete caroticum—extending from the left and right internal carotid arteries that feed the brain. The network weaves through the large cavernous sinus in their heads, so as blood is pumped through the arterial network, it loses heat to the cooler venous blood in the cavernous sinus. This ensures the brain does not get too hot. Humans and other primates don’t have this complex structure. But George du Boulay and his colleagues at the Institute of Neurology in London suspected that the internal carotid arteries that run from the neck to the brain in primates might still play a similar role. The blood passing through them might lose enough heat to the nearby airways and jugular veins to help cool the brain. As a test, the team threaded a tiny thermometer called a thermocouple up through the internal carotid artery of a macaque monkey. They compared the temperature of blood in this artery with other tissues. To their surprise, they found that blood in the artery was consistently about 1.5 °C below rectal or brain tissue temperatures before it even reached the cavernous sinus. “We were very surprised to find such a large temperature difference,” says du Boulay, who plans to submit the team’s results for publication. “It’s very, very likely that the arterial blood is being cooled by the airway and that further cooling comes from the cavernous sinus.” He hopes his work will encourage other scientists to study this area. Louis Sokoloff, head of research into brain metabolism at the National Institutes of Health near Washington DC, says he thinks the result is intriguing. “This backs up earlier research suggesting there is heat exchange between vessels and airways in the head,” he says. If this cooling system fails to lower brain temperature in an infant, it might even explain some cot deaths, du Boulay suggests. He speculates that cool air may not reach the nose or face of a baby lying face down, which might be enough to make the brain’s heat exchanger fail and trigger cot death. However, Bob Schroter, a specialist in circulatory physiology at Imperial College in London, says he will remain unconvinced until the result is confirmed by further research. He does not believes that the internal carotid arteries’ proximity to the airways and other vessels in the head could account for so much cooling. “It’s far from proven,