The rhythmic “lub-dub” sound of the heart provides important clues about its function. These distinct sounds are generated by the closing of the heart’s valves, reflecting the intricate mechanics of blood flow within the body. The “lub” is known as the first heart sound (S1), while the “dub” is the second heart sound (S2). This article explores the origin and timing of the S2 sound during each heartbeat.
Understanding the “Dub”: What S2 Represents
The “dub,” or S2 heart sound, signals the closure of two crucial valves in the heart: the aortic valve and the pulmonic valve. These valves are known as the semilunar valves, and they are positioned at the exits of the heart’s pumping chambers, the ventricles. The sound arises from the vibrations created by the snapping shut of these valve leaflets.
The aortic valve controls blood flow from the left ventricle into the aorta, the body’s main artery, while the pulmonic valve regulates blood flow from the right ventricle into the pulmonary artery, which leads to the lungs. Their closure occurs after the ventricles have forcefully ejected blood into these large arteries. In contrast, the S1 sound, the “lub,” is produced by the closure of the mitral and tricuspid valves, which are the heart’s atrioventricular valves. S2 therefore marks the completion of blood ejection from the ventricles and the end of the heart’s contraction phase.
Timing S2 in the Cardiac Cycle
The S2 heart sound is heard at a specific point within the cardiac cycle. It occurs at the end of ventricular systole, the phase when the heart’s ventricles contract and pump blood. As the ventricles finish contracting and begin to relax, their pressure drops. When this pressure falls below the pressure in the aorta and pulmonary artery, the semilunar valves close to prevent blood from flowing backward into the heart.
The closure of these valves generates the S2 sound, signifying the onset of ventricular diastole. Diastole is the relaxation and filling phase, during which the ventricles prepare for the next contraction by refilling with blood. The sequence of a heartbeat involves S1 initiating systole, followed by blood ejection, and S2 marking the transition to diastole.
Hearing S2: Auscultation and Normal Splitting
Healthcare professionals listen to heart sounds using a stethoscope, a process called auscultation. S2 is best heard at the “base” of the heart, over the second right intercostal space (aortic area) and the second left intercostal space (pulmonic area) along the sternum. When listening to S2, it usually presents as a single, sharp sound during exhalation.
S2 is composed of two distinct components: the sound of the aortic valve closing (A2) and the sound of the pulmonic valve closing (P2). Normally, A2 occurs slightly before P2. During inspiration, S2 “splits,” sounding like two separate events, sometimes described as “lub-da-dub.”
The reason for this normal splitting is that inspiration increases the amount of blood returning to the right side of the heart. This greater volume of blood in the right ventricle causes the pulmonic valve to stay open a fraction of a second longer, delaying its closure (P2). Meanwhile, the aortic valve closure (A2) remains relatively unaffected.
This slight delay in P2, when compared to A2, becomes noticeable as a split sound. During exhalation, the blood flow dynamics return to their previous state, and A2 and P2 once again close almost simultaneously, making the S2 sound single. This physiological splitting is a normal finding, best appreciated over the pulmonic area.