Fig. 5
From: Volume and its relationship to cardiac output and venous return
Limit of the return function. When the pressure inside the great veins is less than the surrounding pressure (which is zero when breathing at atmospheric pressure), the vessels collapse and there is flow limitation. Lowering right atrial pressure (Pra) further does not increase flow. Maximum venous return (VRmax) is then dependent upon MSFP and venous resistance (Rv). The heart cannot create a flow higher than this value