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Heart Sounds

*Toni cordis*

For medical students2 min readUpdated 2026-10-10

Heart sounds (toni cordis) are regular acoustic phenomena that accompany cardiac activity during various phases of the cardiac cycle. Graphic recording of these sounds using specialized equipment is called a phonocardiogram, which sequentially displays four main heart sounds.

Recording MethodA phonocardiogram registers acoustic vibrations of the cardiac cycle

Systolic SoundThe 1st sound occurs during isometric contraction

Diastolic SoundThe 2nd sound is associated with ventricular relaxation

Muscular FactorThe 3rd and 4th sounds are caused by ventricular wall vibration

First Heart Sound (S1)

The first heart sound is one of the loudest and longest. It occurs during the transition from ventricular relaxation to active ventricular contraction.

Phase of the Cardiac Cycle: The sound is formed during the phase of isometric ventricular contraction and captures the very beginning of the subsequent rapid ejection phase.

Primary Mechanism of Formation: The fundamental cause of the S1 sound is the closure of the atrioventricular (AV) valves (mitral and tricuspid valves). As ventricular pressure rapidly rises, the valves between the atria and ventricles snap shut.

S1 is a complex acoustic phenomenon consisting of several components:

Vibrations of tightly tensed AV valve cusps.
Vibration of chordae tendineae (tendinous cords) attached to the cusp edges, preventing them from everting into the atrial cavities.
Contributing component: vibrations of the semilunar valves (aortic and pulmonary valves) at the moment they open to eject blood are mixed with the AV valve closure sound.

Second Heart Sound (S2)

The second heart sound marks the end of systole and the transition of the heart into diastole.

Phase of the Cardiac Cycle: This acoustic phenomenon is strictly tied to the phase of isometric ventricular relaxation. At this moment, ventricular cavity pressure drops.

Primary Mechanism of Formation: The key event here is the sharp slamming shut of the semilunar valves—the aortic and pulmonary valves. Blood from the great vessels attempts to return to the relaxing ventricles, fills the pockets of the semilunar valves, and forces them to snap tightly shut.

The source of the sound is the intense vibration of these valve cusps following their collision.

Third and Fourth Heart Sounds (S3 and S4)

Unlike S1 and S2, which are predominantly valvular in origin, S3 and S4 are associated with hemodynamic blood impacts against the muscular walls of the heart.

Third Heart Sound (S3)

Cycle Phase: Rapid ventricular filling phase.
Mechanism: Following the opening of the AV valves, blood passively and very rapidly rushes from the atria into the ventricles. The impact of this blood mass against the muscular ventricular walls causes low-frequency vibrations, recorded as the third heart sound.

Fourth Heart Sound (S4)

Cycle Phase: Atrial systole.
Mechanism: At the very end of ventricular diastole, the atria actively contract, expelling the remaining portion of blood. This supplemental ventricular filling once again triggers ventricular wall vibration, forming the fourth heart sound.

Mnemonic

Remember the phases by major events: S1 = AV valves close (tension before ejection); S2 = semilunar valves snap shut (relaxation after ejection).

Frequently asked questions

Where are the normal auscultation points for the first heart sound?

Auscultation points for S1 correspond to the best listening locations for the valves involved in its formation (mitral and tricuspid).

Apex of the heart (mitral valve) — typically the 5th intercostal space, 1–1.5 cm medial to the left midclavicular line (linea medioclavicularis sinistra).
Lower third of the sternum body (tricuspid valve) — base of the xiphoid process.

Where is the second heart sound best auscultated (auscultation points)?

S2 is best auscultated over the projections of the valves whose closure forms it (aortic and pulmonary trunks).

Aortic valve — 2nd right intercostal space at the sternal border.
Pulmonary valve — 2nd left intercostal space at the sternal border.

Additionally, Erb's point (3rd left intercostal space at the sternal border) is used to evaluate the aortic valve.

What is the main source of sound during the formation of S2?

The primary sound source for the second heart sound is the vibration of the closing cusps of the semilunar valves (aortic and pulmonary valves).

What causes the appearance of the third and fourth heart sounds (S3 and S4)?

Both sounds are linked to ventricular wall vibrations during filling: S3 during rapid passive filling, and S4 during supplemental filling from atrial systole.

Go deeper

Detailed analysis of the isometric contraction phase
Physiology of the rapid ventricular ejection phase
Hemodynamics of the isometric ventricular relaxation phase
Mechanics of atrioventricular valves and chordae tendineae
Atrial systole and its contribution to ventricular filling