Contraction of the ventricles (ventricular systole)

- Ventricles contract, decreasing their volume, increasing their pressure - Atria relax - Blood moves from high to low pressure into the arteries - Pressure higher in ventricles than atria, forcing AV valves shut to prevent backflow - Pressure is also higher in ventricles than aorta + pulmonary artery - Forces open SL valves + blood forced out into these arteries