The relevance of right atrial pressure (RAP) as the backpressure for venous return (Q_VR) and mean systemic filling pressure as upstream pressure is controversial during dynamic changes of circulation. To examine the immediate response of Q_VR (sum of caval vein flows) to changes in RAP and pump function, we used a closed-chest, central cannulation, heart bypass porcine preparation (n = 10) with venoarterial extracorporeal membrane oxygenation. Mean systemic filling pressure was determined by clamping extracorporeal membrane oxygenation tubing with open or closed arteriovenous shunt at euvolemia, volume expansion (9.75 ml/kg hydroxyethyl starch), and hypovolemia (bleeding 19.5 ml/kg after volume expansion). The responses of RAP and Q_VR were studied using variable pump speed at constant airway pressure (P_AW) and constant pump speed at variable P_AW. Within each volume state, the immediate changes in Q_VR and RAP could be described with a single linear regression, regardless of whether RAP was altered by pump speed or P_AW (r² = 0.586–0.984). RAP was inversely proportional to pump speed from zero to maximum flow (r² = 0.859–0.999). Changing P_AW caused immediate, transient, directionally opposite changes in RAP and Q_VR (RAP: P ≤ 0.002 and Q_VR: P ≤ 0.001), where the initial response was proportional to the change in Q_VR driving pressure. Changes in P_AW generated volume shifts into and out of the right atrium, but their effect on upstream pressure was negligible. Our findings support the concept that RAP acts as backpressure to Q_VR and that Guyton’s model of circulatory equilibrium qualitatively predicts the dynamic response from changing RAP.