Atrial pressure initially decreases as the atrial base is pulled downward, expanding the atrial chamber. Blood continues to flow into the atria. This causes ventricular active tension to decrease and the rate of ejection and ventricular emptying to fall. Atrial pressures gradually rise due to venous return. As the ventricles continue to relax and intraventricular pressures fall, a point is reached when the total energy of blood within the ventricles is less than the energy of blood in the outflow tracts.
Atrial pressures continue to rise due to venous return.
When the ventricular pressures fall below atrial pressures, the AV valves open and ventricular filling begins. The ventricles continue to relax despite the inflow which causes intraventricular pressure to continue to fall a few mmHg. If the AV valves are healthy, no prominent sounds will be heard during filling. As the ventricles continue to fill with blood and expand, they become less compliant and the intraventricular pressures rise. This reduces the pressure gradient across the AV valves so that the rate of filling falls.