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[Physiology](https://durgical.com/en/physiology/) · [Cardiovascular](https://durgical.com/en/physiology/#s-cardio)

# 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](https://durgical.com/en/anatomy/anterior-leaflet-of-left-atrioventricular-valve/) · [Tricuspid anterior leaflet](https://durgical.com/en/anatomy/superior-leaflet-of-right-atrioventricular-valve/) · [Aortic right coronary cusp](https://durgical.com/en/anatomy/right-coronary-leaflet/) · [Pulmonary anterior cusp](https://durgical.com/en/anatomy/anterior-semilunar-leaflet-of-pulmonary-valve/) · [Left ventricle](https://durgical.com/en/anatomy/left-ventricle/)

## Key points

- S1 = mitral + tricuspid closure (start of systole), loudest at the apex; S2 = aortic + pulmonary closure (end of systole), loudest at the base.
- Physiological splitting of S2 widens on inspiration: more venous return to the right heart delays P2, and pooled blood in the lungs brings A2 a bit earlier.
- Paradoxical splitting (splits on expiration) = delayed A2: left bundle branch block or severe aortic stenosis. Fixed splitting = atrial septal defect.
- S3 (early diastole, rapid filling) = volume overload or a dilated failing ventricle; it can be normal in children, athletes and pregnancy.
- S4 (late diastole, atrial kick into a stiff ventricle) = hypertrophy, hypertension, ischemia; never present in atrial fibrillation.

## 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/min **Cycle** 800 ms **Systole** ≈ 300 ms **SV** 70 mL **EF** 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](https://durgical.com/en/conditions/heart-failure/) | Early diastole | Rapid filling into a compliant, overfilled ventricle | Heart failure with dilation, mitral regurgitation; normal when young |
| [S4](https://durgical.com/en/conditions/hypertensive-heart-disease/) | 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.

## Memory aid

S3 sounds like 'Ken-tuc-ky' (the slosh comes after S2); S4 sounds like 'Ten-nes-see' (the push comes before S1).

## 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)](https://durgical.app/estudiar/?add=/en/physiology/heart-sounds-and-where-they-come-from/)

## See it in 3D

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

- [Mitral anterior leaflet](https://durgical.com/en/anatomy/anterior-leaflet-of-left-atrioventricular-valve/)
- [Tricuspid anterior leaflet](https://durgical.com/en/anatomy/superior-leaflet-of-right-atrioventricular-valve/)
- [Aortic right coronary cusp](https://durgical.com/en/anatomy/right-coronary-leaflet/)
- [Pulmonary anterior cusp](https://durgical.com/en/anatomy/anterior-semilunar-leaflet-of-pulmonary-valve/)
- [Left ventricle](https://durgical.com/en/anatomy/left-ventricle/)

## Related topics

- [The cardiac cycle](https://durgical.com/en/physiology/the-cardiac-cycle/)
- [Murmurs and the hemodynamics of valve disease](https://durgical.com/en/physiology/murmurs-and-the-hemodynamics-of-valve-disease/)
- [Heart failure physiology](https://durgical.com/en/physiology/heart-failure-physiology/)
- [Physiology of congenital shunts](https://durgical.com/en/physiology/physiology-of-congenital-shunts/)

## Related conditions

- [Heart failure](https://durgical.com/en/conditions/heart-failure/)
- [Aortic Valve Stenosis](https://durgical.com/en/conditions/aortic-valve-stenosis/)
- [Mitral regurgitation](https://durgical.com/en/conditions/mitral-regurgitation/)
- [Atrial septal defect](https://durgical.com/en/conditions/atrial-septal-defect/)
- [Rheumatic heart disease](https://durgical.com/en/conditions/rheumatic-heart-disease/)

[← All of physiology](https://durgical.com/en/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](https://openstax.org/books/anatomy-and-physiology-2e/pages/19-3-cardiac-cycle) (OpenStax (Rice University), 2022); [Mitral Regurgitation](https://www.ncbi.nlm.nih.gov/books/NBK553135/) (StatPearls (NCBI Bookshelf), 2025); [Rheumatic Heart Disease](https://www.ncbi.nlm.nih.gov/books/NBK538286/) (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).

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Representative model, for educational purposes only.

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License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/heart-sounds-and-where-they-come-from/