There are two main phases of a heartbeat: diastole and systole. Each phase possesses additional actions within the phases. Diastole refers to relaxed muscle and systole refers to contracted muscle.
There are many phases to one heartbeat, but they can be summed up in two main phases. The diastole phase is the relaxation of the heart muscles, when blood is being passively moved from the left side of the heart to the right side of the heart and back again. The systole phase refers to when the heart muscle actually contracts and squeezes blood into the aorta and pulmonary artery. The valves that allow blood into the heart are closed during a systole phase and open during a diastole phase.
The actual phases of one heartbeat begin with isovolumetric contraction when the valve to the aorta is opened and the mitral valve is closed. The next phase is the systolic ejection when the heart muscle contracts forcing blood through the aortic valve. The next phase is isovolumetric relaxation when the valve to the aorta closes and the mitral valve opens up. After this are the rapid filling and the slow filling phases, each named after how fast the blood pools into the left ventricle.
When listening to a heartbeat, there are four steps to listen for. The S1 represents the closing of the mitral valve. S2 is the closing of the aortic valve. S3 is the sound of the rapid blood filling the left ventricle. S4 is only heard if there is a stiff ventricle, according to the USMLE Success Academy.