What is primarily responsible for the first and second heart sounds?

The first and second heart sounds, commonly referred to as "lub" and "dub," are primarily caused by the mechanical actions of the heart during the cardiac cycle. The first heart sound occurs with the closure of the atrioventricular valves (the mitral and tricuspid valves) at the beginning of ventricular contraction, or systole. This is when the ventricles contract and push blood into the aorta and pulmonary arteries. The rapid closure of these valves generates vibrations that produce the distinct "lub" sound.

Similarly, the second heart sound is produced when the semilunar valves (the aortic and pulmonary valves) close at the end of ventricular contraction and the beginning of ventricular filling or diastole. This closure prevents blood from flowing back into the ventricles and contributes to the characteristic "dub" sound.

In summary, the primary responsibility for both heart sounds lies with the contraction of the ventricles, as these actions lead to the closing of the valves within the heart, which generates the audible sounds associated with the heartbeat.