细脉网或称迷网，是一个牵涉到混合血管和动脉的复杂系统，而大蓝鳍鲔（一种比较发达的鲔鱼）正是其中拥有这系统的，法国的一位名叫George Cuvier的解剖学家称这系统为细脉网。在这个网络里，支动脉和支血管再三地形成精妙的平行小动脉和小血管混合集群，微温的静脉血从肌肉进入这脉管网络后，热量便经由脉管壁渗入邻近较冷、从鳃部迎面而来的动脉血中，当追动脉血进入肌肉时已带有微温。这种热交流作用的效果惊人，留在肌肉中的热量，97%可由流入的血液再收回。这系统的用处在于鱼类之所以游泳时未能加至最高速，是因为热量在血液流经鱼鳃时流出，而大蓝鳍鲔就是正正因为有此系统，牠们也就成为鲔鱼中最发达的一种。即便是人类也很难捕捉牠们。

A rete mirabile (Latin for "wonderful net"; plural retia mirabilia) is a complex of arteries and veins lying very close to each other, found in some vertebrates, mainly warm-blooded ones. The rete mirabile utilizes countercurrent blood flow within the net (blood flowing in opposite directions) to act as a countercurrent exchanger. It exchanges heat, ions, or gases between vessel walls so that the two bloodstreams within the rete maintain a gradient with respect to temperature, or concentration of gases or solutes. This term was coined by Galen.