Figure 3. Idealized "venous return curve". Basis of interpretation of venous return (Qr) as driven by (Pmc – Pcv). The linear sloped segment can be described mathematically as Qr = K(Pmc – Pcv) where K is the the slope of the segment and Pmc is the value of Pcv at zero flow. This would describe a hydraulic system in which venous return was driven through a resistance by the pressure difference between a fixed source at Pmc and the varied pressure, Pcv, at the outflow. The value of the resistance is obtained as the negative of the reciprocal of the (negative) slope, i.e. (-1)/K. However, this resistance does not correspond to the physical resistance in the model in Figure 1. In electrical terms, it is a composite whose value depends upon the values of all the resistances and capacitances in the circuit