Question
The cardiac cycle has a total duration of 0.8 seconds. During this cycle, which phase lasts the longest, and what is the correct sequence of events? Also, the heart sounds “lub” and “dub” — which valves produce them and when?
This is a straight 4-mark question in NEET 2023, and students lose marks either on the duration values or by mixing up which valves close when.
Solution — Step by Step
The cardiac cycle has exactly three phases, and you must memorise all three durations — NEET asks for any of them.
| Phase | Duration |
|---|---|
| Atrial systole (atria contract) | 0.1 s |
| Ventricular systole (ventricles contract) | 0.3 s |
| Joint diastole (all chambers relax) | 0.4 s |
Total = 0.1 + 0.3 + 0.4 = 0.8 s. The longest phase is joint diastole — 0.4 s. This makes biological sense; the heart rests more than it works.
Atrial systole (0.1 s): SA node fires. Both atria contract simultaneously. Blood is pushed into ventricles through the open AV valves (bicuspid on left, tricuspid on right). Ventricles are relaxed and filling.
Ventricular systole (0.3 s): AV node picks up the signal. Ventricles contract. Pressure inside ventricles shoots up, which forces the AV valves shut (this produces the first heart sound — “lub”). When ventricular pressure exceeds aortic pressure, the semilunar valves (aortic + pulmonary) open and blood is ejected.
Joint diastole (0.4 s): Everything relaxes. Semilunar valves snap shut (this produces the second heart sound — “dub”). AV valves open again as atria fill passively from veins.
Why do the sounds occur at those moments? Sound is produced when valves close suddenly — the pressure change vibrates surrounding tissue.
- “Lub” (S1): AV valves close → marks the start of ventricular systole
- “Dub” (S2): Semilunar valves close → marks the end of ventricular systole
A clean mnemonic: Lub = Lubb = Long phase starting (systole begins). Dub = Done (systole ends).
If one cardiac cycle = 0.8 s, then:
This is the normal resting heart rate. NEET occasionally asks this as a calculation — it’s a 30-second question if you remember 0.8 s.
Why This Works
The whole cycle is orchestrated by the nodal tissue — SA node is the pacemaker (fires at ~72/min), AV node introduces a 0.1 s delay before passing the signal to the bundle of His. That delay is critical: it gives atria time to finish pushing blood into ventricles before ventricles contract. Without it, atria and ventricles would contract simultaneously and cardiac output would drop.
The pressure gradient is the real driver here. Valves don’t have muscles — they open and close purely based on pressure differences. AV valves open when atrial pressure > ventricular pressure, and close when the ventricles contract hard enough to reverse that gradient. This is why understanding pressure-volume relationships in cardiac physiology makes the whole chapter click.
Joint diastole is longer than you might expect (0.4 s) because the heart needs recovery time. During this phase, the coronary arteries also fill — the heart supplies itself with blood during its own relaxation. Interesting design.
Alternative Method
For questions that give you heart rate and ask for cardiac cycle duration, just flip the formula:
At 75 bpm → 60/75 = 0.8 s. At 72 bpm → 60/72 = 0.83 s. Some NEET options use 72 bpm as the “normal” value — don’t get confused, both 72 and 75 bpm appear in NCERT in different contexts. The standard cardiac cycle diagram in NCERT uses 0.8 s at 75 bpm.
If a question gives you atrial systole = 0.1 s and asks for ventricular systole, don’t calculate — just recall. The three values (0.1, 0.3, 0.4) are a fixed set. Ventricular systole is always 0.3 s at normal heart rate.
Common Mistake
The most common error: students write that “lub” is produced by semilunar valves closing and “dub” by AV valves closing — exactly backwards. Remember: Lub = AV valves (L before D, AV valves close first). Dub = semilunar valves. In NEET 2023, one distractor option was worded to exploit exactly this reversal.
A secondary mistake is stating that during atrial systole, the ventricles are empty. They’re not — about 70% of ventricular filling happens passively during diastole, before the atria even contract. Atrial systole contributes only the remaining ~30%. This is why patients with atrial fibrillation (atria not contracting properly) can still survive — the passive filling keeps cardiac output going.