Question
Describe the cardiac cycle. In your answer, explain the events of systole and diastole, the origin of heart sounds (lub-dub), and how the ECG correlates with these mechanical events.
(NEET 2023 — 5 marks; also a frequent 3-marker in CBSE Class 11 boards)
Solution — Step by Step
One complete heartbeat — one contraction + one relaxation — is the cardiac cycle. At a resting heart rate of 75 beats/min, each cycle lasts 0.8 seconds.
The cycle has three phases: atrial systole (0.1 s), ventricular systole (0.3 s), and joint diastole (0.4 s).
Both atria contract simultaneously, pushing the remaining ~30% of blood into the ventricles. The AV valves (mitral and tricuspid) are open; semilunar valves are closed.
Why does the atrium contract after the ventricle has already partially filled? Because ~70% of ventricular filling happens passively during diastole — the atrial kick just tops it up.
Ventricles contract, pressure rises sharply. The AV valves snap shut the moment ventricular pressure exceeds atrial pressure — this closure produces “lub” (S1), the first heart sound.
Once ventricular pressure exceeds aortic/pulmonary pressure, the semilunar valves open and blood is ejected (stroke volume ≈ 70 mL).
All chambers relax. Ventricular pressure drops below aortic pressure — the semilunar valves slam shut, producing “dub” (S2), the second heart sound.
AV valves open again as atrial pressure now exceeds ventricular pressure, and passive filling begins. The ventricles fill ~70% before the next atrial systole.
The ECG records the electrical events that trigger the mechanical events above:
| ECG Wave | Electrical Event | Mechanical Result |
|---|---|---|
| P wave | Atrial depolarisation | Atrial systole |
| QRS complex | Ventricular depolarisation | Ventricular systole (lub) |
| T wave | Ventricular repolarisation | Ventricular relaxation (dub) |
The PR interval (0.12–0.20 s) is the AV node delay — it gives atria time to finish emptying before ventricles fire.
Why This Works
The key to understanding the cardiac cycle is pressure gradients. Every valve opening and closing is driven purely by pressure differences on either side — there are no muscles controlling the valves directly. Blood always moves from high pressure to low pressure, and the valves simply follow.
This is why the sequence is so precise: atria contract first (low-pressure pump), then ventricles (high-pressure pump). If ventricles fired simultaneously with atria, the AV valves would never close properly and cardiac output would drop.
The ECG-mechanical coupling is a favourite NEET conceptual question. Remember: electrical event always precedes the mechanical event by a tiny interval. The P wave fires → then the atria mechanically contract. QRS fires → then the ventricles contract. T wave → ventricles relax.
Alternative Method — Pressure-Volume (Wiggers Diagram) Approach
For JEE-level or advanced NEET questions, the Wiggers diagram plots aortic pressure, ventricular pressure, and ventricular volume on the same time axis.
The two key crossover points:
- Ventricular pressure > Atrial pressure → AV valve shuts (S1/lub)
- Ventricular pressure < Aortic pressure → Semilunar valve shuts (S2/dub)
Stroke volume = End Diastolic Volume (EDV) − End Systolic Volume (ESV) = 120 − 50 = 70 mL.
Cardiac Output = Stroke Volume × Heart Rate = 70 × 75 = 5250 mL/min ≈ 5 L/min at rest.
NEET loves asking: “Which event corresponds to the QRS complex?” Answer: ventricular depolarisation, which triggers ventricular systole. Many students say “ventricular contraction” — that’s the mechanical consequence, not the electrical event the QRS represents. Write both to be safe in a 3-marker.
Common Mistake
Students often flip S1 and S2. Remember: S1 (lub) = AV valves closing at the start of ventricular systole. S2 (dub) = Semilunar valves closing at the end of ventricular systole. A useful mnemonic: “Lub = Leaving diastole” (ventricular systole starts). Also, atrial repolarisation is hidden inside the QRS complex — it doesn’t appear as a separate wave. This is a direct NEET MCQ trap.