Explain the stages of the cardiac cycle

The cardiac cycle consists of series of chamber systole and diastole. Systole is the name given to a period when cardiac muscle contract to cause blood ejection from the respective chamber to another. Diastole is the period when the chamber gets filled with blood and the muscle is relaxed.  It's easier to begin the cycle with atrial diastole. In atrial diastole the atria of the heart become filled with blood from their respective vessels (for left it's the pulmonary veins, whilst for the right it is the vena cava). This occurs as the pressure in the atria is lower than in those veins. Once sufficiently filled, the pressure in the atria will exceed the pressure in the ventricles so the atrioventricular valves will open and ventricles will start filling with blood. This will begin a period known as ventricular diastole. Eventually the SAN will stimulate the contraction of the atria to cause atrial systole and ejection of a small volume of blood into the ventricles.  As the ventricles fill, the pressure inside them will rise and eventually will exceed the pressure in the atria, this will lead to closure of the AV valves. Semilunar valves stay closed until the pressure in the ventricles exceeds that of the pulmonary artery and aorta. Ventricular contraction in ventricular systole causes the necessary pressure rise to open the semilunar valves and eject the blood into the great vessels. After this the whole cycle repeats again as the ventricular muscle relaxes.