Relative importance of venous and arterial resistances in controlling venous return and cardiac output

In dogs with cardiovascular reflexes completely blocked by total spinal anesthesia, the total peripheral resistance was increased five- or more fold in two ways: first, by injecting small plastic microspheres into the arteries, thereby increasing the arterial resistance, and, second, by inflating pneumatic cuffs around the major veins, thereby increasing venous resistance. A small increase in venous resistance decreased cardiac output eight times as much as an increase in arterial resistance of similar magnitude. This difference was caused principally by a) a marked rise in systemic arterial pressure when arterial resistance was increased; this maintained the cardiac output at almost normal levels and b) a fall in systemic arterial pressure when venous resistance was increased; this promoted even more fall in cardiac output than increased total peripheral resistance alone would have caused.