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The first heart sound, or S 1, forms the "lub" of "lub-dub" and is composed of components M 1 (mitral valve closure) and T 1 (tricuspid valve closure). Normally M 1 precedes T 1 slightly. It is caused by the closure of the atrioventricular valves , i.e. tricuspid and mitral (bicuspid), at the beginning of ventricular contraction, or systole .
The closure of the semilunar valves causes the second heart sound. The aortic valve , which has three cusps, lies between the left ventricle and the aorta . During ventricular systole , pressure rises in the left ventricle and when it is greater than the pressure in the aorta, the aortic valve opens, allowing blood to exit the left ventricle ...
As diastole ends, the ventricles begin depolarizing and, while ventricular pressure starts to rise owing to contraction, the atrioventricular valves close in order to prevent backflow to the atria. At this stage, which corresponds to the R peak or the QRS complex seen on an ECG, the semilunar valves (aortic and pulmonary valves) are also closed.
The second heart sound (S2) is caused by the closure of the aortic and pulmonic valves, which causes vibration of the valve leaflets and the adjacent structures. The aortic valve closes slightly before the pulmonic, and this difference is accentuated during inspiration when S2 splits into two distinct components (physiological splitting).
Semilunar valves: Atrioventricular valves: A: Atrial systole: P: S4* closed: open B: ... all the heart valves are closed; at no time are all the heart valves open. [1] *
The second heart sound, S2, is the sound of the semilunar valves closing during ventricular diastole and is described as "dub". [1] Each sound consists of two components, reflecting the slight difference in time as the two valves close. [9]
Ejection causes pressure within the ventricles to fall, and, simultaneously, the atria begin to refill (atrial diastole). Finally, pressures within the ventricles fall below the back pressures in the aorta and the pulmonary arteries, and the semilunar valves close. Closure of these valves give the second heart sound (S2). The ventricles then ...
Heart sounds result from vibrations created by the closure of the heart valves. There are at least two; the first (S1) is produced when the atrioventricular valves (tricuspid and mitral) close at the beginning of systole and the second (S2) when the aortic valve and pulmonary valve (semilunar valves) close at the end of systole. [4]