Cardiac Cycle = Heart Beat
A normal human heart beat would be around 60 beats per minute (bpm).
Which is one beat per second so the cycle is taking one second.
In the diagram it shows the heart in its Diastole (relaxed) state, all three valves are closed here (semi-lunar valve, bicuspid valve and tricuspid valve)
In the diagram below, the bicuspid and tricuspid valves had opened. The reason for this is because the pressure in the atria is larger than the pressure in the ventricles. So the ventricles fill with blood and a final contraction of the atria and forces blood down the ventricles and stretches the walls of the ventricle and fill up the final volume of the ventricles.
In the fourth diagram, the valves have closed again including the semi-lunar valve. We have contraction of the walls of the ventricle which is also called systole. In which the heart is contracting, which increases the pressure of the blood in the ventricles. Since the ventricle pressure is greater than the atria pressure the valves are now closed. This causes what is known as the first heart sound.'lup' The semi-lunar valve are still closed, so the blood there are under increasing pressure since it can't escape.
In the fifth stage, the ventricle pressure is larger than the artery pressure so that the semi-lunar valve have both opened. What happens is we eject the pulse of blood into the artery (pulmonary artery and the aorta).
In the last stage, the heart has returned to the state of diastole. The arterry pressure is greater than the ventricle pressure which means the semi-lunar valve both closes together. This causes the second heart sound. 'dup'