Cardiac Cycle

The coordinated and rhythmic series of muscular contractions associated with the heart comprise the cardiac cycle.

In humans, the cardiac cycle can be subdivided into two major phases, the systolic phase and the diastolic phase. Systole occurs when the ventricles of the heart contract. Accordingly, systole results in the highest pressures within the systemic and pulmonary circulatory systems. Diastole is the period between ventricular contractions when the right and left ventricles relax and fill.

The cardiac cycle cannot be described as a linear series of events associated with the flow of blood through the four chambers. One can not accurately describe the cardiac cycle by simply tracing the path of blood from the right atrium, into the right ventricle, into the pulmonary circulation, the venous pulmonary return to the left atrium, and finally the ejection into the aorta and systemic circulation by the contraction of the left ventricle. In reality, the cardiac cycle is a coordinated series of events that take place simultaneously on both the right pulmonary circuit and left systemic circuit of the heart.

The cardiac cycle begins with a period of rapid ventricular filling. The right atrium fills with deoxygenated blood from the superior vena cava, the inferior vena cava, and the coronary venous return (e.g., the coronary sinus and smaller coronary veins). At the same time, the pulmonary veins return oxygenated blood from the lungs to the left atrium. During the early diastolic phase of the cardiac cycle, both ventricles relax and fill from their respective atrial sources. The atrio-ventricular valves (the tricuspid valve is located between the right atrium and right ventricle; the mitral valve is between the left atrium and left ventricle) open and allow blood to flow from the atria into the ventricles.

The flow of blood through the atrio-ventricular valves is unidirectional and as volume related pressure increases within the ventricles, the atrioventricular valves close to prevent backflow from the ventricles into the atria.

At the onset of the systolic phase, specialized cardiac muscle fibers within the sino-atrial node (S-A node) contract and send an electrical signal propagated throughout the heart. In a sweeping fashion, the right atrium contracts and forces the final volume of blood into the right ventricle. The left atrium contracts and contributes the final 20% of volume to the left ventricle.

The S-A node signal is delayed by the atrioventricular node to allow the full contraction of the atria that allows the ventricles to reach their maximum volume. A sweeping right to left wave of ventricular contraction then pumps blood into the pulmonary and systemic circulatory systems. The semilunar valves that separate the right ventricle from the pulmonary artery and the left ventricle from the aorta open shortly after the ventricles begin to contract. The opening of the semilunar valves ends a brief period of isometric (constant volume) ventricular contraction and initiates a period of rapid ventricular ejection.

As muscle fibers contract, they lose their ability to contract forcefully (i.e., the greatest force of muscular contraction in the ventricle occurs earlier in the contraction phase and decreases as contraction proceeds). When ventricular pressures fall below their respective attached arterial pressures, the semilunar pulmonary and aortic valves close. At the end of systole, the semilunar valves shut to prevent the backflow of blood into the ventricles.

After emptying, both ventricles collapse to undergo a period of repolarization and refilling. The receptivity of the ventricles to filling corresponding lowers atrial pressures and allows them to fill from their respective venous sources. At the outset, ventricular pressures remain greater than atrial pressures and the atrioventricular valves remain closed. Because the volume of blood in the ventricle is once again static—closed off by both the atrio-ventricular and semilunar valves—this period is described as isometric (same volume) relaxation.

The cardiac cycle is complete with the onset of another period of rapid ventricular filling that takes place when atrial pressures exceed ventricular pressures and the atrio-ventricular valves open to allow rapid filling.

It is the opening and snapping shut of the atrio-ventricular and semilunar pulmonary and aortic valves that creates the familiar pattern of sound associated with the cardiac cycle. Because the right to left contractions of the atria and ventricles are sweeping, the opening and closings of the right side and left side valves are separated by a short interval. The first heart sound results from the closure of the atrio-ventricular valves. The second heart sound results from the closure of the semilunar valves.

The electrical events associated with cardiac cycle are measured with the electrocardiogram (EKG).