Systemic venous circulation. Waves propagating on a windkessel: relation of arterial and venous windkessels to systemic vascular resistance.

Compared with arterial hemodynamics, there has been relatively little study of venous hemodynamics. We propose that the venous system behaves just like the arterial system: waves propagate on a time-varying reservoir, the windkessel, which functions as the reverse of the arterial windkessel. During later diastole, pressure increases exponentially to approach an asymptotic value as inflow continues in the absence of outflow. Our study in eight open-chest dogs showed that windkessel-related arterial resistance was approximately 62% of total systemic vascular resistance, whereas windkessel-related venous resistance was only approximately 7%. Total venous compliance was found to be 21 times larger than arterial compliance (n = 3). Inferior vena caval compliance (0.32 +/- 0.015 ml x mmHg(-1) x kg(-1); mean +/- SE) was approximately 14 times the aortic compliance (0.023 +/- 0.002 ml x mmHg(-1) x kg(-1); n = 8). Despite greater venous compliance, the variation in venous windkessel volume (i.e., compliance x windkessel pulse pressure; 7.8 +/- 1.1 ml) was only approximately 32% of the variation in aortic windkessel volume (24.3 +/- 2.9 ml) because of the larger arterial pressure variation. In addition, and contrary to previous understanding, waves generated by the right heart propagated upstream as far as the femoral vein, but excellent proportionality between the excess pressure and venous outflow suggests that no reflected waves returned to the right atrium. Thus the venous windkessel model not only successfully accounts for variations in the venous pressure and flow waveforms but also, in combination with the arterial windkessel, provides a coherent view of the systemic circulation.