Background: Although Guyton's concept of venous return (VR) revolutionized circulatory physiology, the pulmonary circulation is invisible in its original framework. Since the pulmonary circulation is critical in left heart failure, we characterized the VR as a surface described by right (P RA) and left atrial (P LA) pressures and demonstrated that the VR surface was capable of representing mechanics of pulmonary as well as systemic circulation. However how baroreflex impacts the VR surface remains unknown. Methods/Results: In 8 dogs, we isolated the carotid sinuses and replaced both ventricles with pumps. We varied cardiac output, shifted blood distribution between the systemic and pulmonary circulation at carotid sinus pressures (CSP) of 100 or 140 mmHg. The coefficient of determination of the VR surface ranged 0.96-0.99 indicating how flat the surface is. Increasing CSP decreased maximum VR (23327 vs. 21633 ml/kg/min, p0.05), whereas did not change the slopes of VR along P RA or P LA axes. Conclusions: Baroreflex parallel shifts the VR surface, thereby stressed volume, without changing its slopes.