A heartbeat is a two–part pumping action that takes about a second.
One complete sequence of filling and pumping blood is called a cardiac cycle, or heartbeat. Heartbeat is followed by systole (contraction) and diastole (relaxation). The cardiac cycle is a term referring to all or any of the events related to the flow or blood pressure that occurs from the beginning of one heartbeat to the beginning of the next. Simply put, the cardiac cycle is the sequence of events that occurs when the heart beats.
The chambers of the heart alternately contract and relax in a rhythmic cycle. During the period of contraction (systole), the heart pumps blood out through the arteries; during the period of relaxation (diastole), the heart fills with blood. One complete sequence of filling and pumping blood is called a cardiac cycle or heartbeat.
The contraction phase (systole) is followed by a relaxation phase (diastole). At the end of ventricular systole, the ventricles start relaxing (ventricular diastole). Meanwhile the atria (auricles) have also been relaxing (atrial diastole) and for a short period, both the atria and the ventricles are in a relaxed state (joint diastole). The whole sequence of events in the heartbeat is called cardiac cycle.
The frequency of the cardiac cycle is described by the heart rate. Each beat of the heart involves five major stages. The first two stages, often considered together as the "ventricular filling" stage, involve the movement of blood from atria into ventricles. The next three stages involve the movement of blood from the ventricles to the pulmonary artery (in the case of the right ventricle) and the aorta (in the case of the left ventricle.)