Effect of blood flow on thermal equilibration and venous rewarming

In this study we have explored the feasibility of using an isolated rat limb as an animal model for studying countercurrent arterial thermal equilibration and venous rewarming in muscle tissue. Unlike in vivo experiments in which animal models have been used for studying thermoregulation or temperature response in tissue under various physiological conditions, isolated organ or tissue provides for better control and more accurate measurement of the blood perfusion rate. It has been shown that the induced perfusion rate in the rat limb can vary from 3 ml/(min x 100 g) at normal physiological conditions to 25 ml/(min x 100 g) during hyperemic conditions. Temperature distributions along the countercurrent arteries and veins have been measured using fine thermocouple wires. We observed a 25%-78% thermal equilibration along the femoral artery and its branches in intermediate size vessels between 700 and 300 microm diameter. This equilibration depends strongly on the local perfusion rate. In comparison, local perfusion rate plays a minor role in determining the overall venous rewarming in the rat hind limb. Approximately 70%-80% of the heat leaving the artery is recaptured by the countercurrent vein. This agrees well with our previous theoretical and experimental results, which show a dramatic shift in thermal equilibration between the supply artery and vein tissue cylinder and the secondary vessel tissue cylinder as the flow rate changes.