When a healthcare provider listens to the chest (auscultation), the rhythmic sounds heard are not the heart muscle contracting, but rather the acoustic results of heart valves snapping shut. These noises are created by the sudden deceleration of blood and the vibration of the valve leaflets and surrounding structures. The two most prominent noises are described as “lub-dub,” corresponding to the first heart sound (S1) and the second heart sound (S2). They represent the mechanical activity that governs the one-way flow of blood through the heart’s four chambers.
Comparing the Sounds: Volume and Duration
The fundamental difference between the two primary heart sounds lies in their duration, pitch, and relative loudness depending on where they are heard. The first heart sound (S1) is characterized by a longer duration and a lower pitch, giving it a dull quality, and forms the “lub” portion of the heartbeat. The second heart sound (S2) is shorter in duration, sharper, and higher in pitch. S1 is typically louder when listening near the apex of the heart, while S2 is more prominent when listening over the base of the heart, where the great vessels originate. The specific sound characteristics arise from the fact that the two sets of valves close under vastly different pressure gradients. The atrioventricular valves, responsible for S1, are larger and their closure generates a longer, lower-frequency sound. Conversely, the semilunar valves, which produce S2, are smaller and close rapidly under significantly higher pressure, resulting in a quicker, higher-pitched sound.
The First Heart Sound (S1): The “Lub”
The generation of the first heart sound (S1) immediately follows the electrical activation of the ventricles and signals the beginning of systole, the ventricular contraction phase. This sound is primarily caused by the near-simultaneous closure of the two atrioventricular (AV) valves: the mitral valve and the tricuspid valve. As the ventricles contract, pressure rapidly exceeds atrial pressure, forcing the valve leaflets to snap shut. This abrupt closure prevents blood from flowing backward into the atria. The initial vibrations of S1 are sometimes slightly split, as the mitral valve typically closes a few milliseconds before the tricuspid valve. The AV valves are large, and their closure generates significant reverberation within the large ventricular chambers and the contained blood mass. These factors contribute to S1’s characteristic lower frequency and longer duration.
The Second Heart Sound (S2): The “Dub”
The second heart sound (S2) marks the termination of ventricular systole and the start of ventricular relaxation, known as diastole. S2 is generated by the rapid closure of the two semilunar valves: the aortic valve and the pulmonary valve. This closure occurs when the ventricles have completed contraction and the pressure within them drops below the pressure in the great arteries. The semilunar valves are smaller and close against a much higher pressure gradient than the AV valves. This high-tension, rapid closure results in a sound that is short, sharp, and higher in pitch than S1. S2 is composed of two distinct components: the aortic component (A2) and the pulmonary component (P2). A2 usually occurs slightly before P2 because the left side of the heart empties faster. This slight separation, known as a physiologic split, becomes more noticeable during inspiration as increased blood flow into the right side further delays the pulmonary valve closure.
Defining the Cardiac Rhythm
The sequence of the two heart sounds provides the acoustic reference for the entire cardiac cycle. The interval between the first sound (S1) and the second sound (S2) corresponds precisely to systole, the phase of ventricular contraction and blood ejection. Systole is typically shorter than the subsequent phase at normal heart rates. The period spanning from the second sound (S2) to the following first sound (S1) defines diastole, the phase of ventricular relaxation and filling. Diastole is longer than systole, allowing adequate time for the ventricles to refill with blood before the next contraction begins. The heart’s rhythm is established by the regular repetition of this S1-systole-S2-diastole pattern. The sounds also allow for the assessment of heart rate and rhythm regularity, as S1 essentially coincides with the palpable pulse in the major arteries. Identifying S1 and S2 helps distinguish between events occurring during the contraction phase and those occurring during the relaxation phase. This temporal relationship is fundamental to the clinical assessment of the heart’s mechanical performance.