The Cardiac Cycle

The cardiac cycle is one complete sequence of cardiac contraction (systole) and relaxation (diastole). At a heart rate of 60 bpm, the total cycle lasts 1 second: diastole occupies approximately 0.66 seconds (two-thirds) and systole approximately 0.34 seconds (one-third). All valve openings and closings are passive, driven entirely by pressure differences.

Diastole has three phases. During isovolumetric ventricular relaxation (~0.05 s), all valves are closed, the ventricles are relaxing, and ventricular blood volume is at its minimum — the end-systolic volume (ESV). Pressure in the atria rises as they fill passively from venous return, until atrial pressure exceeds ventricular pressure and the AV valves open passively. During the ventricular filling phase (~0.5 s), approximately 80% of ventricular filling occurs passively down the pressure gradient in two stages: rapid early filling, then slow mid-diastolic filling. The P wave on the ECG triggers atrial depolarisation → atrial contraction → the final 20% of filling (atrial top-up). The S4 sound, if heard, corresponds to this atrial contraction. S3, heard in youth or left ventricular failure, corresponds to rapid early filling. At the end of diastole, the ventricle is at its maximum volume — end-diastolic volume (EDV, ~130 mL at rest).

Systole has two phases. During isovolumetric ventricular contraction (~0.05 s), the QRS complex triggers ventricular depolarisation. Ventricular pressure rises rapidly, causing the AV valves to close (S1, mitral before tricuspid). All valves are now closed and volume is constant. Pressure continues to rise until it exceeds aortic/pulmonary artery pressure. During ventricular ejection, both semilunar valves open. Approximately two-thirds of the stroke volume is ejected in the first third of this phase (rapid ejection), with the T wave coinciding with ventricular repolarisation. As the ventricles relax in late systole, ventricular pressure falls below aortic/pulmonary pressure and the semilunar valves close (S2, aortic before pulmonary), giving rise to the dicrotic notch from elastic recoil of the aorta.

Stroke volume = EDV − ESV. At rest, EDV is ~130 mL and ESV is ~50 mL, giving SV ~80 mL and ejection fraction ~60%. As heart rate increases, diastole shortens more than systole. The slow-filling mid-diastolic phase and slow ejection phases are preferentially shortened, preserving rapid filling and the majority of ejection. The right heart undergoes the same events but at lower pressures (pulmonary resistance is much less than systemic).