Cardiac output of the hypothermic rat

As the colonic temperature of the rat was lowered the heart rate and cardiac output fell linearly with the temperature. The arterial pressure did not fall linearly indicating an increase of total peripheral resistance. The increase of hematocrit ratio and the effect of cold on blood per se combined to increase the in vitro viscosity threefold as the colonic temperature approached 15°C. It appears from these data that the increase in viscosity of the blood is the important factor in the increase in total resistance to flow and that little change in total or average vascular geometry took place. However, comparison of the local clearances of 1131 from specific extravascular areas shows that individual vascular geometries may be changing but in such a fashion as to balance out each other.