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The cardiac cycle is the time period starting from the atrial contraction and ending with the ventricular relaxation. 

The systole is the period of heart contraction while the blood is pumped into the circulation. The diastole is the period of relaxation occuring as the chambers of heart are filling with blood. The systole and diastole is seen in both atria and ventricles. 

Pressure and flow of blood-

During diastole (when the heart chambers are undergoing relaxation), the atria receives blood from the veins(veins have high pressure than atria). When the blood is filled in atria, it's pressure will increase so the flow of blood from atria to ventricles take place which is passive initially. When there is triggering of the muscles in atria to undergo contraction(atrial systole) by the action potential, the pressure inside the atria increases further, pumping blood into the ventricles. At the time of the ventricular systole, the pressure inside the ventricles rise. This results in pumping blood into the pulmonary trunk from the right ventricle and into the aorta form the left ventricle. 

Cardiac cycle phases- 

Both the atria and ventricles are relaxed(at diastole) at the starting of the cardiac cycle. The blood flows from the inferior vena cavae and coronary sinus to the right atrium. The blood from the four pulmonary veins flow into the left atrium. The blood flows uninterrupted from the atria into the ventricles as the atrioventricular valves, the tricuspid and mitral valves are open. 70-80 percent of the filling of the ventricles take place through this method. The blood is prevented from flowing back into the the left and right ventricles from the pulmonary trunk present on the right and the aorta present on the left as the semilunar valves( pulmonary and aortic valves) are closed. 

Atria systole and diastole- When there is contraction of the muscles of atria (starting from the superior portion of the atria towards the atrioventricular septum), the rise of pressure takes place within the atria and the blood flows into the ventricles through the atrioventricular valves( tricuspid and bicuspid or mitral) valves which are open. When the atrial systole begins, the ventricles of the heart are filled with almost 70-80 percent of their capacity due to the inflow of blood at the time of diastole. The remaining 20-30 percent of the filling of the ventricles is contributed by the atrial contraction. The period of atrial systole is approximately of 100 milliseconds and ends before ventricular systole as the muscles of atria return to diastole. 

Ventricular systole- There is approximately 130 mL of blood in ventricles at the end of atrial systole and before the atrial contraction while standing. It's called as end diastolic volume or preload. 

As there is contraction of the muscles of ventricles, the pressure inside the chamber increases. It's not that high enough that it can cause the opening of semilunar valves( aortic and pulmonary) and can cause ejection of the blood outside the heart. The pressure fastly increases above the atria which are relaxed and in diastole. Now this increase in pressure cause the backflow of blood into the atria, resulting in the closure of the tricuspid and mitral valves. At this early stage, there is no ejection of blood from the ventricles, the blood volume inside the ventricles remain constant. This early phase of ventricular systole is called as isovolumic contraction or isovolumetric contraction. 

During the second phase of ventricular systole, called the ventricular ejection phase, the pressure within the ventricles due to the contraction of the ventricular muscles has reached more then the pressure in the aorta and pulmonary trunk. This causes the flow of blood from the heart by opening the pulmonary and aortic semilunar valves. The pressure created by the left ventricle id more than that by the right ventricle as the pressure in the aorta is very high. But the same amount of blood is pumped from both the ventricles called as stroke volume. This volume is in the range of 70-80 mL. There is still 50-60 mL of blood present in the ventricle after contraction called as end systole volume. 

Ventricular diastole- In the initial phase, there is relaxation of the ventricles causing the pressure of the remaining blood present inside the ventricles to decrease. Blood starts flowing back towards the heart when the pressure of the ventricles is less than the pressure in the aorta and pulmonary trunk. There is closure of the semilunar valves preventing the backflow of blood into the heart. At this point, the atrioventricular valves remain closed so the volume of blood in the ventricles remain the same. Therefore the early phase of ventricular diastole is also known as isovolumic ventricular relaxation phase. 

The late ventricular diastole forms the second phase, there is relaxation of the muscles of ventricles. There is further decrease in pressure on the blood inside the ventricles. Eventually it decreases more than the atrial pressure. Due to this, there is flow of blood from the atria into the ventricles, opening the tricuspid and mitral valves. As the ventricular pressure is decreased, the flow of blood takes place into the relaxed atria from the major veins and then into the ventricles. There is opening of the atrioventricular valves and closing of the semilunar valves as both the chambers are in diastole. This completes the cardiac cycle.