The orderly process of depolarization described in the previous sections triggers a recurring cardiac cycle of atrial and ventricular contractions and relaxations (Figure 14-24). For orientation, we shall first merely name the parts of this cycle and their key events. Then we shall go through the cycle again, this time describing the pressure and volume changes that cause the events.
The cycle is divided into two major phases, both named for events in the ventricles: the period of ventricular contraction and blood ejection, systole, followed by the period of ventricular relaxation and blood filling, diastole. At an average heart rate of 72 beats/min, each cardiac cycle lasts approximately 0.8 s, with 0.3 s in systole and 0.5 s in diastole.
As illustrated in Figure 14-24, both systole and diastole can be subdivided into two discrete periods. During the first part of systole, the ventricles are contracting but all valves in the heart are closed, and so no blood can be ejected. This period is termed isovol-umetric ventricular contraction because the ventricular volume is constant. The ventricular walls are developing tension and squeezing on the blood they enclose, raising the ventricular blood pressure, but because the volume of blood in the ventricles is constant and because blood, like water, is essentially incompressible, the ventricular muscle fibers cannot shorten. Thus, isovolumetric ventricular contraction is analogous to an isometric skeletal-muscle contraction: the muscle develops tension, but does not shorten.
Once the rising pressure in the ventricles exceeds that in the aorta and pulmonary trunk, the aortic and pulmonary valves open, and the ventricular ejection period of systole occurs. Blood is forced into the aorta and pulmonary trunk as the contracting ventricular muscle fibers shorten. The volume of blood ejected from each ventricle during systole is termed the stroke volume (SV).
During the first part of diastole, the ventricles begin to relax, and the aortic and pulmonary valves close. (Physiologists and clinical cardiologists do not all agree on the dividing line between systole and diastole; as presented here, the dividing line is the point at which ventricular contraction stops and the pulmonary and aortic valves close.) At this time the AV valves are also closed. Accordingly, no blood is entering or leaving the ventricles since once again all the valves are closed. Accordingly, ventricular volume is not changing, and this period is termed isovolumetric ventricular relaxation. Note then, that the only times during the cardiac cycle that all valves are closed are the periods of isovol-umetric ventricular contraction and relaxation. The AV valves then open, and ventricular filling occurs as blood flows in from the atria. Atrial contraction occurs at the end of diastole, after most of the ventricular filling has taken place. This is an important point: The ventricle receives blood throughout most of diastole, not just when the atrium contracts. Indeed, in a person at rest, approximately 80 percent of ventricular filling occurs before atrial contraction.
This completes the basic orientation. We can now analyze, using Figure 14-25, the pressure and volume changes that occur in the left atria, left ventricle, and aorta during the cardiac cycle. Events on the right side of the heart are described later. Electrical events (ECG) and heart sounds, the latter described in a subsequent section, are at the top of the figure so that their timing can be correlated with phases of the cycle.
Was this article helpful?