This marks the beginning of diastole, the phase where the heart relaxes and fills with blood again. After the ventricles have pumped blood out to the lungs and the body, they begin to relax. The pressure inside them drops rapidly.
The human heart is a marvel of biological engineering, working tirelessly to pump blood through miles of vascular pathways. At the center of its function is a rhythmic cadence often described as "lub-dub." While these sounds might seem like simple thumps, they are actually precise acoustic signatures of the heart’s internal mechanical actions. The "lub," scientifically known as the first heart sound or S1, is the foundational beat that signals the start of ventricular contraction.
The heart is divided into four chambers: two upper atria and two lower ventricles. Separating these chambers are the atrioventricular (AV) valves—the mitral valve on the left and the tricuspid valve on the right. When the ventricles have filled with blood, they begin to contract. This sudden increase in pressure forces the mitral and tricuspid valves to snap shut, preventing blood from flowing backward into the atria. lub heart sound
This sound occurs at the beginning of systole, the phase where the heart contracts to pump blood out. Imagine the ventricles squeezing tight. As the pressure inside the ventricles rises, the blood inside tries to rush backward into the atria where the pressure is lower.
| Condition | S1 Intensity | Mechanism | |-----------|--------------|------------| | | Loud | Increased ventricular filling, valves float open | | Mitral stenosis | Loud (if valve pliable) | Elevated left atrial pressure, valves close forcefully | | Tachycardia | Loud | Short diastole increases closing velocity | | Long PR interval | Soft | Reduced ventricular filling, valves nearly closed | | Mitral regurgitation | Soft | Incomplete valve closure | | Heart failure | Soft | Reduced contractility | | First-degree AV block | Soft | Prolonged PR interval | | Atrial fibrillation | Variable | Irregular cycle length changes filling time | This marks the beginning of diastole, the phase
There is even a fourth sound, , which occurs just before the "Lub" (late diastole). This is almost always pathological, indicating a stiff ventricle that is resisting the inflow of blood from the atria. It creates a "Tenn-es-see" rhythm.
Contrary to popular belief, the sound isn't the valve leaflets clapping together like hands. Instead, it is caused by the sudden tensing of the valve leaflets and the surrounding structures (the chordae tendineae and papillary muscles), followed by the sudden deceleration of blood against the closed valves. It creates a vibrational thud. The human heart is a marvel of biological
S1 is caused primarily by the :
