Physiology · Topic 1 of 4
The cardiac cycle
25 min read
The cardiac cycle is the sequence of pressure and volume changes in one heartbeat. At 75 beats per minute it lasts about 0.8 s, with diastole taking roughly two-thirds of that time — which is why tachycardia compromises ventricular filling and coronary perfusion.
The phases
Valve opening and closure define the boundaries between phases.
- Isovolumetric contraction — mitral closed, aortic closed; pressure rises steeply (S1 at the start)
- Ventricular ejection — aortic valve opens once LV pressure exceeds aortic pressure
- Isovolumetric relaxation — aortic valve closes (S2); pressure falls with volume constant
- Rapid filling — mitral valve opens; most filling happens here (S3 if abnormal)
- Atrial systole — the “atrial kick” adds ~20% of filling (S4 if the ventricle is stiff)
Key numbers
End-diastolic volume is about 120 mL and end-systolic volume about 50 mL, giving a stroke volume of ~70 mL and an ejection fraction of ~58%. Normal EF is 55–70%.
The jugular venous pulse
The JVP waveform reflects right atrial pressure: the a wave (atrial contraction), c wave (tricuspid bulging during isovolumetric contraction), x descent (atrial relaxation), v wave (atrial filling against a closed tricuspid) and y descent (tricuspid opening).
High-yield points
- S1 = AV valves closing; S2 = semilunar valves closing.
- Absent a waves occur in atrial fibrillation; cannon a waves in complete heart block.
- EF = SV / EDV.
- Physiological S2 splitting widens on inspiration.