The systole phase is the contraction phase of the heartbeat. One cardiac cycle is the cycle of events where the heart fills with blood and the blood is then pumped back out of the heart. The two phases of the cycle are the diastole phase and the systole phase.
In the cardiac cycle, during the diastole phase, the blood flows into the right and left atria and then the ventricles. During the diastole phase, both atria and ventricles relax. During the systole phase, the right ventricle sends the blood out to the lungs through the pulmonary artery, and the left ventricle pumps the blood into the aorta.