The sequence of events that occurs in the heart during one full heartbeat. These events comprise contraction (see systole) and relaxation (see diastole) of the chambers of the heart, associated with opening and closing of the heart valves. When both the atria and the ventricles are relaxed, pressure in the heart is low and blood flows from the vena cava and pulmonary vein into the atria and through to the ventricles. The aortic and pulmonary semilunar valves, at the junction between the left ventricle and aorta and the right ventricle and pulmonary artery, respectively, are closed; therefore, blood can enter but not leave the heart, which increases the pressure in the chambers. As the pressure in the heart increases, the atria begin to contract, forcing the blood into the ventricles and closing the tricuspid valve and the bicuspid valve. A wave of ventricular contraction follows, expelling the blood into the aorta and pulmonary artery to complete the cardiac cycle. At a resting heart rate of, say, 72 beats per minute, the human cardiac cycle lasts approximately 0.85 seconds.

https://library.med.utah.edu/kw/pharm/hyper_heart1.html Animation of the cardiac cycle explaining the changes in blood pressure, heart sounds, and ECG trace