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What Are the Four Heart Sounds?

What are heart sounds?

Using a stethoscope to assess different sounds the heart makes is an important diagnostic tool.

Using a stethoscope to assess different sounds the heart makes is an important diagnostic tool.

Heart sounds are generated by blood flowing in and out of the heart’s chambers through the valves as they open and close. Listening to the heart sounds through a stethoscope (auscultation) is one of the first steps a physician takes in evaluating a patient’s medical condition.

Heart sounds provide the doctor valuable information about heart function. Auscultation is used to detect abnormal heart sounds and decide on further course of action.

How does the heart function?

The heart is a muscular organ and has four chambers that receive and pump blood:

  • Right atrium
  • Right ventricle
  • Left atrium
  • Left ventricle
  • The left atrium receives oxygenated blood from the lungs and pumps it into the left ventricle.
  • The left ventricle pumps the oxygen-rich blood to the rest of the body through a network of arteries.
  • The right atrium receives the oxygen-depleted blood from the body through veins and pumps it into the right ventricle.
  • The right ventricle pumps the blood to the lungs for oxygenation.

The left ventricle’s contractions while pumping out blood create the systolic blood pressure in the arteries (the higher number in a blood pressure reading). A web of nerve tissue runs through the heart to send electric signals to the heart muscle to initiate the heart’s contraction. 

Heart valves ensure that the flow of the blood is in only one direction, by opening and closing as the heart pumps blood. The four heart valves are

  • Tricuspid valve separating right atrium and right ventricle
  • Mitral valve separating left atrium and ventricle
  • Pulmonic valve between right ventricle and pulmonary artery
  • Aortic valve between the left ventricle and aorta

What creates the heart sounds?

Blood flow creates vibrations in the heart chambers and valves which produce audible sounds that can be heard through a stethoscope. Smooth, low-resistance blood flow is called a laminar flow. When the flow is rough with high resistance it is known as a turbulent flow.

Vibrations increase along with blood flow turbulence and depend on the diameter of the blood vessel as well as the blood’s

  • viscosity
  • density
  • velocity

Some of the functions of the heart that generate heart sounds are

  • Opening or closing of the heart valves
  • Flow of blood through the valve opening
  • Flow of blood into the heart’s ventricles
  • Rubbing of cardiac surfaces

What are the four heart sounds?

The cardiac cycle is made of two phases: 

  • Systole while the ventricles contract to pump out blood 
  • Diastole when the ventricles relax and fill with blood. 

These two phases constitute the heartbeat. 

In a healthy adult, the heart makes two sounds, commonly described as ‘lub’ and ‘dub.’ 

The third and fourth sounds may be heard in some healthy people, but can indicate impairment of the heart function. S1 and S2 are high-pitched and S3 and S4 are low-pitched sounds.

First sound

When the two ventricles contract and pump out blood into the aorta and pulmonary artery the mitral and tricuspid valves close to prevent the blood flowing back into the atria. The first sound S1 is generated by vibrations created by the closing of these two valves.

Normally the mitral valve closes just before the tricuspid valve, and when the two different sounds are detectable, it is called a “split S1.” A split S1 may be indicative of certain conditions affecting the heart. 

Second sound

After pumping the blood, the ventricles relax to receive blood from the atria, and the diastole phase starts. The aortic and pulmonic valves close and cause vibrations, giving rise to the second heart sound, S2. The increase in intensity of this sound may indicate certain conditions.

When the aortic valve closes just before the pulmonic valve, it may generate a split S2. This may indicate impairment in the heart function.

Third sound

The third heart sound is a low-pitched sound audible with the rapid rush of blood from the atrium into the ventricle as it starts relaxing. This may be a normal sound in some people but in people with heart conditions, S3 may indicate heart failure.

Fourth sound

The fourth is a low-intensity sound heard just before S1 in the cardiac cycle. The sudden slowing of blood flow by the ventricle as the atrium contracts causes this sound, which may be a sign of heart disease.


In the U.S., 1 in every 4 deaths is caused by heart disease.
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Other heart sounds

Opening snap

Opening snap (OS) is a high-pitched sound that is caused by rapid opening of the mitral or tricuspid valve following the aortic valve closing sound (S2). This may indicate narrowing (stenosis) of the mitral or tricuspid valve; the closer in time the OS is to S2, the more severe the stenosis.

Ejection systolic sounds

These sounds are heard during the early part of the ventricular contraction, which may be

  • Valvular ejection sounds due to defects in the aortic or pulmonic valves 
  • Vascular ejection sounds due to defects of the aortic or pulmonary artery
  • Nonejection systolic click due to mitral or tricuspid valve prolapse

Heart murmurs

In some people, heart murmurs are just the sound of blood flow characteristic to that person. Doctors call this an “innocent heart murmur.” Heart murmur may also be caused by turbulent flow of blood across the heart valves, however, which may indicate heart disease.


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