Blood enters the atria - the walls of the atria contract, raising the pressure of blood and forcing the valves open. the blood then enters the ventricles. Once the ventricles become full, they contract and increase the blood pressure-closing the valves and opening the semi-lunar valves where blood is ejected into the pulmonary artery and aorta. As the ventricles empty, higher pressure in the aorta and pulmonary artery closes the valves and the cycle restarts.