1.9.1 Cardiac Cycle Definition

Cardiac Cycle Definition is the complete sequence of mechanical and electrical events that occur in the heart from the beginning of one heartbeat to the beginning of the next. It encompasses the coordinated contraction (systole) and relaxation (diastole) of the atria and ventricles, along with the corresponding changes in chamber pressures, blood volumes, and valve states that together drive the unidirectional flow of blood through the pulmonary and systemic circulations. Each cardiac cycle is initiated by an electrical impulse generated at the sinoatrial node and is completed in a fraction of a second, repeating rhythmically to sustain continuous circulation.

Phases of the Cardiac Cycle

The cardiac cycle is classically divided into two principal phases, each subdivided into distinct mechanical events.

Diastole

Diastole is the period during which the heart chambers relax and fill with blood. It is subdivided into:

• Isovolumetric relaxation: the ventricles relax with all valves closed, causing a rapid drop in ventricular pressure without a change in volume.
• Rapid ventricular filling: once ventricular pressure falls below atrial pressure, the atrioventricular (AV) valves open and blood rushes passively into the ventricles.
• Diastasis: a period of slow, additional filling as pressures in the atria and ventricles begin to equalize.
• Atrial systole: atrial contraction delivers the final portion of blood into the ventricles, completing ventricular filling before the onset of ventricular contraction.

Systole

Systole is the period during which the ventricles contract and eject blood. It is subdivided into:

• Isovolumetric contraction: the ventricles begin contracting with all valves closed, causing a sharp rise in pressure without a change in volume.
• Ventricular ejection: once ventricular pressure exceeds the pressure in the aorta and pulmonary artery, the semilunar valves open and blood is ejected into the systemic and pulmonary circulations.

Pressure–Volume Relationships

Throughout the cardiac cycle, ventricular pressure and volume change in a characteristic, reproducible pattern that can be represented graphically as a pressure–volume loop.

End-Diastolic Volume

The end-diastolic volume (EDV) represents the maximum volume of blood in the ventricle just before contraction begins, occurring at the close of diastole.

End-Systolic Volume

The end-systolic volume (ESV) represents the residual volume of blood remaining in the ventricle after ejection, occurring at the close of systole.

Stroke Volume

The difference between EDV and ESV defines the stroke volume, the amount of blood ejected by the ventricle during a single cardiac cycle.

Valvular Events

The opening and closing of the cardiac valves define the boundaries between phases of the cardiac cycle and are responsible for the heart sounds.

Atrioventricular Valves

The mitral and tricuspid valves open during diastole to permit ventricular filling and close at the onset of systole, producing the first heart sound (S1).

Semilunar Valves

The aortic and pulmonary valves open during ventricular ejection and close at the onset of diastole, producing the second heart sound (S2).

Electrical Correlation

Each mechanical phase of the cardiac cycle corresponds to a specific electrical event recorded on the electrocardiogram.

Atrial Depolarization

Atrial depolarization corresponds to the P wave and precedes atrial systole.

Ventricular Depolarization

Ventricular depolarization corresponds to the QRS complex and precedes ventricular systole.

Ventricular Repolarization

Ventricular repolarization corresponds to the T wave and precedes ventricular diastole.

Diagrammatic Summary

Physiological Significance

The cardiac cycle ensures that oxygen-poor blood is continuously routed to the lungs for gas exchange and that oxygen-rich blood is delivered to systemic tissues, all while maintaining the pressure gradients necessary for adequate organ perfusion. Disruptions to the timing, force, or valvular competence of the cardiac cycle underlie many forms of cardiovascular dysfunction, making an understanding of this cycle foundational to cardiovascular physiology.