What is the cardiac cycle?

The cardiac cycle is the continual sequence of contraction and relaxation of the atria and ventricles to keep blood continually circulating around the body.

First the atria contract decreasing their chamber volumes increasing the pressures inside them forcing blood into the ventricles through the open atrioventricular valves. Next the ventricles contract and the atria relax decreasing their volumes and thus their pressure increases to higher than the atria and aorta and pulmonary artery forcing the closure of the AV valves to prevent the back-flow of blood as well as the opening of the semi-lunar valves pushing blood out of the heart and into these arteries. During diastole, when both the atria and ventricles relax, the SL valves shut to prevent back-flow of blood back into the heart from the arteries. Blood begins to return to the heart increasing atrial pressure to above ventricular pressure opening the AV valves allowing passive flow into the ventricles before the atria contract beginning the whole cycle again.