Heart sounds and where they come from !

Mechanism
Exam priority: high

The 'lub-dub' you hear is the sound of heart valves snapping shut; extra sounds can mean the heart is filling against stiffness or overload.

Updated 2026-11-01

Also called: S1, S2, S3, S4, Splitting of S2

See it in 3D: Mitral anterior leaflet · Tricuspid anterior leaflet · Aortic right coronary cusp · Pulmonary anterior cusp · Left ventricle

What it is

Heart sounds are vibrations made when valves close and blood decelerates suddenly. S1 marks mitral and tricuspid closure at the start of systole; S2 marks aortic and pulmonary closure at the end of systole. S3 and S4 are low-pitched diastolic sounds made by blood hitting the ventricular wall during filling.

How it works, step by step

When ventricular pressure rises above atrial pressure, the AV valve leaflets snap shut and the blood column and valve tension vibrate: S1.
At the end of ejection, the falling ventricular pressure lets aortic and pulmonary pressure close the semilunar cusps: S2 (A2 then P2).
On inspiration, lower intrathoracic pressure increases right-heart filling; right ejection lasts longer, so P2 comes later and the split widens.
In early diastole, a large inflow into a dilated ventricle suddenly stops as the wall reaches its limit: S3.
In late diastole, the atrium pushes blood into a ventricle that resists stretch: S4.

Cardiac cycle (Wiggers diagram)

HR 75/minCycle 800 msSystole ≈ 300 msSV 70 mLEF 58%

2. Isovolumetric contraction (≈ 170–220 ms)

The ventricle starts to contract with both valves closed: pressure rises very fast while volume stays at EDV (about 120 mL). The mitral valve bulges into the atrium (c wave).

Valves: mitral and aortic closed
Sound: S1 at the start (mitral and tricuspid closure)
ECG: just after the QRS

Aorta
Left ventricle (LV)
Left atrium (LA)
LV volume
ECG
S1 and S2
Ventricular systole
Ventricular diastole
Valve events (MC, AO, AC, MO)

Tap the sound markers: each one sits at a valve closure or a filling phase.

The four heart sounds

Sound	Timing	Cause	Typical meaning
S1	Start of systole	Mitral and tricuspid closure	Normal
S2	End of systole	Aortic then pulmonary closure	Normal; split varies with breathing
S3	Early diastole	Rapid filling into a compliant, overfilled ventricle	Heart failure with dilation, mitral regurgitation; normal when young
S4	Late diastole, just before S1	Atrial contraction into a stiff ventricle	LV hypertrophy, hypertensive heart disease, aortic stenosis

Clinical notes

A loud P2 suggests pulmonary hypertension; a soft or absent A2 suggests severe aortic stenosis.
In Colombia, rheumatic mitral stenosis still appears: loud S1 and an opening snap after S2, followed by a diastolic rumble.

Test yourself

Think of the answer, then tap the question to see it.

Why does S2 split more on inspiration in a healthy person?

Inspiration lowers intrathoracic pressure and raises venous return to the right heart, so right ventricular ejection lasts longer and pulmonary valve closure (P2) comes later.

A patient with left bundle branch block has S2 splitting that widens on expiration. Why?

Delayed LV activation delays aortic closure (A2) until after P2; on inspiration P2 moves later toward A2 and the split narrows: paradoxical splitting.

Why can S4 not be heard in atrial fibrillation?

S4 is made by an effective atrial contraction into a stiff ventricle; in atrial fibrillation there is no coordinated atrial contraction.

What does a new S3 in a 65-year-old with breathlessness suggest?

Rapid early filling into a dilated, volume-overloaded ventricle with high filling pressure, typical of heart failure with reduced ejection fraction.

Add to my cards (4)

See it in 3D

The structures of Durgical's 3D anatomy where it happens.

← All of physiology

Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.

Sources: 19.3 Cardiac Cycle (OpenStax (Rice University), 2022); Mitral Regurgitation (StatPearls (NCBI Bookshelf), 2025); Rheumatic Heart Disease (StatPearls (NCBI Bookshelf), 2025); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 23). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).

Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake

Representative model, for educational purposes only.