Cardiac Cycle

Cardiac Cycle is the recurring sequence of mechanical and pressure events occurring within the heart during a single complete heartbeat, encompassing the coordinated phases of chamber filling, isovolumetric pressure changes, and ejection that together move blood through the atria and ventricles and out into the pulmonary and systemic circulations before the sequence repeats with the next beat.

Major Phases of the Cycle

Atrial Systole

Contraction of the atria actively propels the final portion of blood into the ventricles immediately prior to ventricular contraction, contributing an additional increment of ventricular filling beyond what passive flow alone would provide, a contribution particularly significant at higher heart rates when passive filling time is reduced.

Isovolumetric Ventricular Contraction

Following the onset of ventricular contraction, pressure within the ventricles rises rapidly while both the atrioventricular and semilunar valves remain closed, meaning ventricular volume remains constant during this phase despite the ongoing generation of contractile force, until ventricular pressure exceeds arterial pressure.

Ventricular Ejection

Once ventricular pressure surpasses the pressure within the pulmonary artery and aorta, the semilunar valves open and blood is ejected rapidly from the ventricles into these vessels, with ejection velocity peaking early in this phase before gradually declining as ventricular contraction proceeds toward completion.

Isovolumetric Ventricular Relaxation

After ventricular contraction ends and ventricular pressure falls below arterial pressure, the semilunar valves close, and ventricular pressure continues to fall while both sets of valves remain closed, meaning ventricular volume again remains momentarily constant during this brief relaxation phase.

Ventricular Filling

Once ventricular pressure falls below atrial pressure, the atrioventricular valves open and blood that has accumulated in the atria during ventricular systole flows rapidly into the relaxing ventricles, with this passive filling phase accounting for the majority of total ventricular filling under resting conditions.

Pressure and Volume Relationships

Pressure Gradients Driving Valve Function

Movement of blood between chambers and vessels throughout the cycle is governed entirely by pressure gradients, with heart valves opening and closing passively in response to the relative pressures on either side, ensuring unidirectional blood flow without requiring any active valve control mechanism.

Ventricular Volume Changes Across the Cycle

Ventricular volume rises during the filling phases, remains constant during the two isovolumetric phases, and falls during ejection, with the difference between the maximum volume reached before contraction and the minimum volume reached after ejection representing the stroke volume delivered with each beat.

Timing Relationships Within the Cycle

Relative Duration of Systole and Diastole

Ventricular contraction, encompassing isovolumetric contraction and ejection, occupies a smaller proportion of the total cycle duration than ventricular relaxation and filling, with this proportional relationship shifting as heart rate increases, since the duration of the relaxation and filling phases shortens more substantially than the duration of contraction at faster rates.

Coordination Between Atrial and Ventricular Events

The timing of atrial contraction relative to ventricular events is precisely coordinated by the conduction system, ensuring that atrial contraction consistently occurs immediately before ventricular contraction begins, maximizing the contribution of atrial systole to overall ventricular filling.

Physiological Significance

Efficient Unidirectional Blood Flow

The sequential organization of the cardiac cycle's phases, combined with passive valve function responding to pressure gradients, ensures that blood moves efficiently and unidirectionally through the heart and into the circulation without significant backward flow under normal conditions.

Basis for Clinical Assessment

Because each phase of the cardiac cycle produces characteristic pressure, volume, and sound changes, careful assessment of these parameters, whether through auscultation, pressure measurement, or imaging, allows clinical evaluation of cardiac function and detection of abnormalities affecting specific phases of the cycle.