The rhythmic “lub-dub” sound heard with every heartbeat represents the mechanical action of the heart pumping blood. These sounds are not the muscle contracting, but the vibrations generated by the rapid, forceful closing of the heart’s four valves. Understanding the source of the “lub” and the “dub” requires understanding the precise timing of pressure changes and blood flow within the heart’s chambers. This process ensures blood moves forward efficiently and never flows backward.
The Cardiac Cycle: The Pumping Action
The heart’s rhythm is defined by the cardiac cycle, which consists of two main phases: systole and diastole. Systole is the period of contraction, when the chambers squeeze to eject blood to the lungs and the rest of the body. This pumping phase generates the higher pressure measured in a blood pressure reading. Diastole is the period of relaxation and filling. During diastole, the chambers expand, allowing blood to flow back in from the body and lungs to prepare for the next contraction.
The Source of the “Lub” (S1)
The first heart sound, or S1, is the longer “lub” sound, marking the beginning of ventricular contraction (systole). It is caused by the near-simultaneous closure of the two atrioventricular (AV) valves: the mitral and tricuspid valves. These valves separate the upper chambers (atria) from the lower chambers (ventricles). As the ventricles begin to contract, the internal pressure rapidly exceeds the atrial pressure, forcing the AV valves to snap shut. This closure prevents backward blood flow and creates the resulting vibration heard as the “lub” sound.
The Source of the “Dub” (S2)
The second heart sound, or S2, is the shorter “dub” sound. It marks the end of ventricular contraction (systole) and the start of relaxation (diastole). The “dub” is produced by the closure of the two semilunar valves: the aortic valve and the pulmonary valve. These valves control the flow of blood from the ventricles into the main arteries. After the ventricles eject blood, the pressure inside them drops quickly, causing the high pressure in the major arteries to push back. This back-pressure forces the semilunar valves to slam shut, creating the sharp “dub” sound.
Listening Closer: When Sounds Change
While S1 and S2 are the characteristic normal sounds, a doctor listening with a stethoscope may hear additional sounds that indicate changes in the heart’s function. Heart sounds are generated by turbulent blood flow, and any deviation from smooth flow can create an audible change.
Extra Heart Sounds (S3 and S4)
Sometimes, two faint extra sounds, S3 and S4, can be detected. An S3 sound occurs early in the relaxation phase. While it may be normal in children or young adults, in older adults it can suggest an issue like ventricular dilation or fluid overload. An S4 sound occurs just before the “lub” and is often associated with a stiffened ventricular wall, which can occur with conditions like chronic high blood pressure.
Heart Murmurs
The most common deviation is a heart murmur, which is a swishing or whooshing sound that lasts longer than the brief “lub” or “dub.” Murmurs are caused by blood flowing abnormally across the valves or through a hole in the heart wall, creating turbulence. While some murmurs are considered “innocent” and harmless, others can signal a valve problem, such as a valve that is stiff and unable to open fully, or one that is leaky and allows blood to flow backward.