The cardiac cycle is the term given to the processes that takes place during a single contraction and relaxation of the heart muscles. The term can also be explained based on the path taken by oxygen depleted blood to become oxygen saturated blood and thereafter pumped back in to the great arteries. Thus, for explanation purpose, the cardiac cycle will be described here from the time where oxygen depleted blood enters the heart through superior and inferior vena cava.
The blood once used up will return to the heart through the superior and inferior vena cava and will enter the right atrium of the heart. During the diastolic phase, the ventricles will be relaxed and the atrio-ventricular valves will open and thus allows the atrial blood to flow into the right ventricle.
When the diastole ends and the systolic contraction begins, the atrio-ventricular valves will close and prevent the flow from the ventricle entering back in to the right atrium. But, the opened pulmonary valves will allow the blood to flow towards the lungs during this contraction and this will be the time in which the oxygen depleted blood becomes saturated with oxygen.
Once the blood becomes saturated or during the diastolic phase of the cardiac cycle, similar to the way in which blood returns from the great vena cava, the pulmonary veins will allow the oxygenated blood from the lungs to fill up the left atrium, which is in the relaxed mode and thus enter the left ventricle through the atrio-ventricular valves of the left side.
As mentioned earlier, during the systole, the left ventricle will also contract and pump the oxygenated blood through the aortic valves in to the great vessels. This blood will traverse through to the body tissues and will provide them with the necessary oxygen. At the same time, the closing up of the atrio-ventricular valve will make sure the oxygenated blood would not be flowing back towards the left atrium.
It should be made clear that, the contraction of both atria and the ventricles takes place at the same time and therefore, the things which happens during diastole and the things which happens in the systole should occur together and not in a sequential manner.
The cardiac cycle is a well timed sequence of events and its disruption either functionally or structurally will be detrimental for the life to sustain. The sequences of events are largely triggered by the two nodes that are present in the cardiac muscle wall which are the Sinus node and the SA node. The two nodes which gives off the impulse for contractions and the bundle of nerves that traverses through the cardiac muscles in order to take the message are thus considered highly important for an effective cardiac cycle.