The cardiac cycle is a continuous pattern of events that occur in order for the blood to be pumped around the body and consists of all the stages that transpire in a heartbeat including cardiac systole and diastole. This article will highlight the main stages of the cardiac cycle, the valvular movements of the heart in conjunction to the movement of blood between the cardiac chambers, as well as how the myocardium reacts to the electrical stimulations that it periodically receives. Lastly, a brief overview of some potential pathological conditions will be discussed.
During atrial systole, the atrioventricular valves are open and allow the blood to pass from the atria into the ventricles, meanwhile the semilunar valves remain closed so as to avoid premature ejection into the major vessels. During this phase the atria contract simultaneously with the opening of the mitral valve and tricuspid valve, forcing the blood into the two lower cardiac chambers. The ventricles already have a small amount of blood in them that was left over from the last phase of the previous cycle. This remnant blood volume is increased by another thirty percent so that the final resting volume of ventricular blood is reached at around 130mL.
In the second phase of the cardiac cycle both the atrioventricular and semilunar valves of the heart remain closed. At this point, the ventricles begin to contract and although the ventricular myocardial muscle fibers only shorten a little, the intraventricular pressure rises rapidly. It should be noted however that at this state, despite various cardiac changes, the ventricular volume of blood remains the same.
As opposed to the atrial systole, during ventricular ejaculation the semilunar valves are open and the atrioventricular valves are shut. This occurs because the pressure in the ventricles exceeds the pressure in the arterial trunks and the valves are forced open. The left ventricular pressure overcomes the aortic pressure at 80 mmHg and the right ventricular pressure does the same with the pressure in the pulmonary artery at 10 mmHg. As the systole progresses the ejection of blood volume becomes slower and eventually, once a blood volume of 70-90 mL has been pumped out of the heart at rest, the systole is over and only the blood remnants remain in the ventricles as previously discussed. The volume of the remaining blood is around 50 mL.
During isovolumetric relaxation, which is the second last phase of the cardiac cycle, all the heart valves are closed. This means that there is no blood passing between the cardiac chambers or out of the heart. The pressure drop below 120 mmHg as the ventricles relax causes the semilunar valves to close. As the pressure drops further, the atrial pressure exceeds that of the ventricles and the atrioventricular valves open. Meanwhile, blood is being pumped around the body.
Finally, ventricular filling is the terminal stage of the cardiac cycle. As its name states, this phase is when the ventricles start to fill with blood passed from the atria, meaning that the semilunar valves are closed and both the mitral and tricuspid valves are open. The ventricles are relaxed and are filled to approximately seventy percent of their maximum capacity. The atrioventricular valves slowly start to close as the atrial pressure lessens and the ventricular volume expands. At this time the atria also start to refill themselves.
Potential pathological disturbances to the cardiac cycle are many and stem from a wide range of possible causes. Mechanical failures, such as defective heart valves, hematological disturbances such a hypovolemia and a lack of electrical conduction due to nerve damage are but a few examples.