Systole is the time of heart ventricular contraction that occurs between the first and second heart sounds of the cardiac cycle (the sequence of events in a single heart beat). Blood is ejected into the aorta and pulmonary trunk during systole. Diastole is the time of heart ventricular relaxation after blood is ejected into the aorta and pulmonary trunk. The period from initiation of systole to completion of diastole is called one heartbeat. The ejection fraction is the percentage of blood pumped out of the heart with each heartbeat. Healthy humans have an average ejection fraction of 60-65%. When this number falls below 55%, it can be an indication of heart disease. Diagram showing the phases of the cardiac cycle: systole and diastole. During systole, the walls of the left ventricle contract and move blood into the aorta and pulmonary trunk. During diastole, the walls of the left ventricle relax and allow blood to return to the heart from the aorta and pulmonary trunk. The cardiac cycle consists of three parts: diastole, systole, and diastasis. During diastole, the ventricles are relaxed so that blood can flow back into them.