During the cardiac cycle, the terms systole and diastole refer to the phases of the heart's activity, specifically in relation to the heart's contraction and relaxation. Systole is the phase of the cardiac cycle during which the heart muscle (myocardium) contracts. This contraction typically begins in the upper chambers of the heart, known as the atria, and then proceeds to the lower chambers, known as the ventricles. During ventricular systole, the ventricles contract and push blood out into the circulation. This is the period when the heart is working hardest and is responsible for the ejection of blood into the aorta and pulmonary trunk, which then carries the blood to the rest of the body and to the lungs for oxygenation, respectively. Diastole, on the other hand, is the phase when the heart is relaxed and filling with blood. This occurs after the ventricles have finished contracting and is the time when the atria and ventricles are at rest, allowing blood to flow into them from the veins. It is the resting phase of the heart and is important for allowing the heart to replenish its oxygen supply and prepare for the next contraction. The transition from systole to diastole is marked by the heart sounds that a doctor can hear with a stethoscope. The first heart sound, known as the "lub," corresponds to the closure of the atrioventricular valves that separate the atria and ventricles, signaling the beginning of ventricular systole. The second heart sound, known as the "dub," is heard when the semilunar valves in the aorta and pulmonary trunk close, marking the end of systole and the beginning of diastole. read more >>

Systole. Systole, period of contraction of the ventricles of the heart that occurs between the first and second heart sounds of the cardiac cycle (the sequence of events in a single heart beat). Systole causes the ejection of blood into the aorta and pulmonary trunk. ... See also blood pressure.read more >>