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Heart Sounds: Lub and Dub

Biology · Body Fluids and Circulation · NEET

During each cardiac cycle, two clear sounds are heard through a stethoscope. The FIRST sound, "Lub", comes when the tricuspid and bicuspid (AV) valves CLOSE at the start of ventricular systole. The SECOND sound, "Dub", comes when the semilunar valves CLOSE at the end of ventricular systole. Memory hook: "Lub Locks the AV valves, Dub Divides systole from diastole (semilunar closes)."
Two Heart Sounds in One Cardiac Cyclestart of systoleend of systoleVentricular systoleLUB1st soundAV valves close(tricuspid + bicuspid)DUB2nd soundsemilunar valves closediastole
Lub marks the start of ventricular systole (AV valves close); Dub marks the end of systole (semilunar valves close). Both sounds come from valve closure.

Your doubts, answered

Which valves make the Lub sound and which make the Dub sound?

Lub (first heart sound) is produced by the closure of the tricuspid and bicuspid (atrioventricular) valves. Dub (second heart sound) is produced by the closure of the semilunar valves at the base of the aorta and pulmonary artery. This exact pairing is a favourite NEET one-liner.

Is Lub the first or second heart sound?

Lub is the FIRST heart sound. It comes at the start of ventricular systole when the AV valves shut. Dub is the SECOND heart sound and comes at the end of ventricular systole when the semilunar valves shut. Order matters: systole starts with Lub and ends with Dub.

Are heart sounds caused by valves opening or closing?

Heart sounds are caused by the CLOSING of valves, not by their opening. The sudden closure and the vibration of blood against the shut valve produce the sound. Opening of valves is silent. NEET often traps students by writing 'opening of valves' in an option.

Why is the Dub sound linked to the semilunar valves?

When the ventricles finish contracting, pressure inside them drops below the pressure in the aorta and pulmonary artery. Blood tries to flow back, so the semilunar valves snap shut to stop backflow. This closure produces the second sound, Dub, marking the end of systole.

Why are heart sounds clinically important?

NCERT states these sounds are of clinical diagnostic significance. A doctor listens with a stethoscope; abnormal or extra sounds (murmurs) can indicate a leaking or narrowed valve. For NEET you only need: two normal sounds, Lub then Dub, each from a specific valve closure.

⚠️ The NEET trap
✗ The first heart sound (lub) is produced by the closure of the semilunar valves.
✓ The first heart sound (lub) is produced by the closure of the tricuspid and bicuspid valves; the semilunar valve closure gives the SECOND sound (dub).
🧠 NTA swaps the valves. Fix it in your head: Lub = AV valves (tricuspid + bicuspid), Dub = semilunar valves. Both sounds are from CLOSURE, never opening.

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Frequently asked

What are the two heart sounds called?

They are called Lub (first heart sound) and Dub (second heart sound), heard through a stethoscope during each cardiac cycle.

What produces the Lub sound?

Closure of the tricuspid and bicuspid (atrioventricular) valves at the beginning of ventricular systole produces the Lub sound.

What produces the Dub sound?

Closure of the semilunar valves at the end of ventricular systole produces the Dub sound.

Do heart sounds come from valves opening?

No. Both heart sounds are produced by the CLOSURE of valves. Valve opening does not make a sound.

Why are heart sounds important in NEET and medicine?

NCERT says they have clinical diagnostic significance. Doctors use them to detect valve problems. For NEET, remember the valve linked to each sound and that both come from closure.

Related concepts:
Cardiac Cycle: Systole and DiastoleHeart Valves: Tricuspid, Bicuspid and SemilunarStroke Volume, Cardiac Output and Heart RateElectrocardiogram (ECG): P, QRS and T WavesStructure of the Human Heart
Practise Biology for NEET:
Biology MCQs (free, chapter-wise)Body Fluids and Circulation PYQs