Cardiac Cycle
Direct answer
The cardiac cycle is the sequence of electrical and mechanical events during one heartbeat, from the start of one systole to the start of the next. At 75 beats per minute it lasts 0.8 seconds: atrial systole 0.11 s, ventricular systole 0.3 s and ventricular diastole 0.5 s, the final portion being joint diastole. Its phases — atrial contraction, isovolumetric contraction, ejection, isovolumetric relaxation and filling — are coordinated by valve movements that generate the heart sounds.
What you must remember
- Ventricular systole (0.3 s): isovolumetric contraction (about 0.05 s, all valves closed, first heart sound at its onset), then ejection, rapid followed by reduced; the aortic valve opens when ventricular pressure exceeds aortic diastolic pressure (about 80 mmHg) and peaks near 120 mmHg.
- Ventricular diastole (0.5 s): isovolumetric relaxation (second heart sound at its onset), rapid filling (0.11 s, may give a third heart sound), diastasis (about 0.19 s) and atrial systole (0.11 s), which contributes roughly a fifth to a quarter of filling.
- Volumes: end-diastolic about 120–130 ml, end-systolic about 50 ml, stroke volume about 70 ml — ejection fraction about two-thirds.
- Pressures: left ventricle up to about 120 mmHg; right ventricle about 25 mmHg (pulmonary artery 25/10 mmHg); atrial pressures stay in low single digits.
- Heart sounds: S1 (mitral and tricuspid closure) begins systole; S2 (aortic then pulmonary closure) begins diastole, splitting wider on inspiration; S3 in rapid filling is normal in children and young adults; S4 with atrial contraction is absent in atrial fibrillation and usually abnormal in adults.
- ECG correlation: P wave precedes atrial systole, QRS precedes ventricular systole, T wave is ventricular repolarisation.
- JVP: a wave (atrial contraction), c wave (onset of ventricular systole), v wave (atrial filling against the closed tricuspid valve), with x and y descents.
Common confusion
Errors cluster around the isovolumetric phases: during contraction and relaxation all valves are closed, and the atrioventricular valves open only when ventricular pressure falls below atrial pressure early in diastole. Students also overstate atrial systole — most filling (70–80 per cent) is passive — and forget that at higher heart rates diastole shortens more than systole, cutting filling and coronary perfusion time.
Exam-focused takeaway
In theory, reproduce the Wiggers diagram — pressures, volume, ECG and phonocardiogram on one time axis — marking phases and valve events. In viva, give the durations, the cause of each heart sound and why S2 splits on inspiration. In practicals, identify S1 and S2 on auscultation, record and label a normal ECG, and name the JVP waves.
Practise MCQs and previous-year questions on the cardiac cycle and heart sounds in the PrepElephant app. Free notes continue on this website.
Frequently asked questions
How long does the cardiac cycle last?
At 75 beats per minute, 0.8 seconds: atrial systole 0.11 s, ventricular systole 0.3 s and ventricular diastole 0.5 s, including joint diastole of about 0.4 s.
What causes the first and second heart sounds?
S1 — mitral and tricuspid closure at the onset of ventricular systole; S2 — aortic and pulmonary closure at the onset of ventricular diastole.
What is the ejection fraction?
Stroke volume divided by end-diastolic volume — about 70 ml out of 120 ml, or roughly 65 per cent; a key index of ventricular function.
When are S3 and S4 heard, and are they normal?
S3 occurs in rapid filling and can be normal in children, young adults and pregnancy; S4 follows atrial contraction, cannot occur in atrial fibrillation, and in adults usually suggests a stiff ventricle.
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