Ventricular systole is the phase of the cardiac cycle when the ventricles contract. It follows atrial systole and lasts around 0.3 seconds. As ventricular depolarisation starts (marked by the QRS complex in an ECG), ventricular systole begins. The rising pressure inside the ventricles forces the atrioventricular (AV) valves to shut, and the semilunar (SL) valves open to eject blood from the heart.