The Cardiac Cycle

The cardiac cycle describes all the activities of the heart through one complete heartbeat—that is, through one contraction and relaxation of both the atria and ventricles. A contraction event (of either the atria or ventricles) is referred to as systole, and a relaxation event is referred to as diastole. The cardiac cycle includes a description of the systolic and diastolic activities of the atria and ventricles, the blood volume and pressure changes within the heart, and the action of the heart valves. A description of each period of the cardiac cycle follows:
  • The isovolumetric ventricular relaxation is the period during which the ventricles are relaxed and both AV and semilunar valves are still closed. The volume of the ventricles remains unchanged (isovolumetric) during this period.

  • Ventricular filling begins as the AV valves open and blood fills the ventricles. The ventricles remain in diastole during this period. The filling of the ventricles can be described as three successive events:

    • Rapid ventricular filling occurs as blood flows into the empty and relaxed ventricles. Volume of the ventricles increases rapidly.

    • Diastasis is a slower filling event than that of the preceding because most of the volume of the ventricle is already occupied by blood.

    • Atrial systole (and the P wave of the ECG) occurs and forces the remaining blood from the atria into the ventricles. The blood volume at the end of this interval is called the end‐diastolic volume (EDV).

  • Ventricular contraction (ventricular systole) begins as the action potential from the AV node enters the ventricles, the ventricles depolarize, and the QRS complex is observed on the ECG. The following intervals during this phase are observed:

    • Isovolumetric contraction occurs when the AV valves are forced shut. During this brief period, while the semilunar valves are still closed, the volume of the ventricles remains unchanged.

    • Ventricular ejection occurs as the continuing contraction of the ventricles increases the pressure in the ventricles and forces the semilunar valves open. At this point, blood is forced out of the ventricles. This interval ends when the ventricles begin to relax, and the semilunar valves close. The closing of the semilunar valves causes a small increase in blood pressure visible as the dicrotic notch on a plot of blood pressure against time. The amount of blood remaining in the ventricles at this time is called the end‐systolic volume (ESV).

The heart sounds associated with the beating of the heart can be heard by auscultating (listening to) the thorax with a stethoscope. The two major heart sounds, described as “lub‐dub,” originate from blood turbulence generated by the closing of the AV valves and the semilunar valves, respectively). Abnormal heart sounds called murmurs are usually caused by improperly functioning valves.