Cardiac cycle
Also known as: cardiac cycle phases
The cardiac cycle is the sequence of electrical and mechanical events that occurs with every heartbeat. It divides into systole, when the ventricles contract and eject blood, and diastole, when they relax and fill.
A single cycle passes through several phases. Atrial contraction adds the final increment of filling, the "atrial kick," bringing the ventricle to its end-diastolic volume. Isovolumetric contraction follows: the mitral and tricuspid valves have closed, the aortic and pulmonic valves have not yet opened, and pressure climbs with no change in volume. When ventricular pressure exceeds aortic pressure the semilunar valves open and ejection begins, first rapidly and then more slowly. Isovolumetric relaxation begins when the semilunar valves close, and filling resumes once the atrioventricular valves open — rapidly at first, then slowly during diastasis.
Heart sounds mark the valve transitions. S1 is closure of the mitral and tricuspid valves at the start of systole; S2 is closure of the aortic and pulmonic valves at the start of diastole, and it splits physiologically on inspiration as increased venous return delays pulmonic closure. S3 occurs during rapid ventricular filling and suggests a dilated, volume-overloaded ventricle; S4 occurs with atrial contraction against a stiff, noncompliant ventricle.
Volumes make the cycle quantitative. A typical ventricle holds roughly 120 mL at end-diastole and about 50 mL at end-systole, so stroke volume is near 70 mL and ejection fraction is stroke volume divided by end-diastolic volume. Multiplying stroke volume by heart rate gives cardiac output. Plotting ventricular pressure against volume produces the pressure-volume loop, whose four sides correspond to filling, isovolumetric contraction, ejection, and isovolumetric relaxation. Diastole shortens far more than systole as heart rate rises, which is why tachycardia limits filling and coronary perfusion.
The cardiac cycle is tested widely. USMLE Step 1 expects you to place heart sounds, pressure tracings, and valve events on the correct phase; the NCLEX applies the same physiology to hemodynamic monitoring and critical care; and the CCMA exam connects the electrical events of the cycle to the ECG waveform and rhythm interpretation.
Key takeaways
- The cardiac cycle alternates systole, when ventricles contract and eject, with diastole, when they relax and fill.
- Isovolumetric contraction and relaxation are the phases when all four valves are closed and volume does not change.
- S1 marks mitral and tricuspid closure; S2 marks aortic and pulmonic closure.
- Stroke volume equals end-diastolic volume minus end-systolic volume, and stroke volume times heart rate equals cardiac output.