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1. The effects of isoprenaline on the pressure-flow relationships in the peripheral circulation were studied in five anaesthetized dogs during right heart bypass procedures. 2. Isoprenaline (1 mug/kg per min) produced a significant drop in arterial pressure and arterial resistance. 3. There was a significant decrease in the resistance to venous return which resulted in a significant increase in venous return. 4. The findings suggest that the increase in cardiac output produced by isoprenaline results in large part from dilatation of venous resistance vessels.

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We developed and tested a new two-compartment serial model of the arterial vasculature which unifies the capacitance (downstream arterial compliance) and waterfall (constant downstream pressure load) theories of blood flow through the arteries. In this model, blood drains from an upstream compliance through a resistance into a downstream compliance which empties into the veins through a downstream resistance which terminates in a constant pressure load. Using transient arterial pressure data obtained from an isolated canine hindlimb preparation, we tested this model, using a stop-flow technique. Numerical parameter estimation techniques were used to estimate the physiologic parameters of the model. The downstream compliance was found to be more than ten times larger than the upstream compliance and the constant pressure load was significantly above venous pressures but decreased in response to vasodilation. Our results support the applicability of both the capacitance and waterfall theories.

1. The influence of practolol (a proposed beta1-adrenoreceptor antagonist) upon the pressure-flow relationships in the peripheral circulation was studied in eight anaesthetized dogs during right heart by-pass procedures. 2. Practool (1 mg/kg) produced a significant increase in the resistance to venous return which resulted in a significant fall in venous return. 3. There was no significant change in arterial resistance. 4. This study suggests that practolol should not be classified as an exclusive cardioselective beta-adrenoreceptor antagonist.