The cardiac cycle

1. Atrial systole. Atria contract and pump blood. Ventricles, already partially filled, receive last ~30% of blood, for a final resting volume of approximately 130mL.

2. Isovolumic ventricular contraction. Ventricles begin to contract.  Initial contraction increases ventricular pressure, closing the AV valves. Blood is pressurized. Ventricular volume unchanged. This phase is 0,1 s.

3. Ventricular ejection. The semilunar valves open when ventricular pressures exceed pressures in the aorta and pulmonary artery. Ventricular ejection (systole) of blood follows. This phase is 0,25 s.

4. Isovolumic relaxation. Ventricles relaxed. The semilunar valves close when the ventricles relax and pressure in the ventricles decreases. Atria fill with blood throughout ventricular systole, allowing rapid ventricular filling at the start of the next diastolic period. Duration of this phase is 0,1 s.

5. Ventricular diastole. Ventricles relaxed. The AV valves open when pressure in the ventricles decreases below atrial pressure, and the ventricles fill.  Duration of this phase is 0,35 s.

The first heart sound, or S1, caused by the closure of the atrioventricular valves, i.e. tricuspid and mitral (bicuspid), at the beginning of ventricular contraction, or systole. 

The second heart sound, or S2, caused by the closure of the semilunar valves (the aortic valve and pulmonary valve) at the end of ventricular systole and the beginning of ventricular diastole.

The third heart sound, or S3, is benign in youth, some trained athletes, and sometimes in pregnancy but if it re-emerges later in life it may signal cardiac problems. S3 is thought to be caused by the oscillation of blood back and forth between the walls of the ventricles initiated by blood rushing in from the atria.

The fourth sound, or S4, is a sign of a pathologic state. Atrial contraction must be present for production of an S4.