The atrial systole is the phase of the cardiac cycle where the atria contract while the ventricles relax. It lasts approximately 0.1 seconds. During this phase, the depolarisation of the sinoatrial (SA) node leads to atrial depolarization (marked by the P wave in an ECG). The contracting atria exert pressure on the blood within them, forcing blood through the open atrioventricular (AV) valves into the ventricles.