Hey. The first sound is the "lub." This is what you hear once the mitral and tricuspid valves of the heart close, which are the valves between the atria and the ventricles! Super cool. It is your third answer choice.
The closing of the valves between the atria and ventricles.
Explanation:
Heart sounds are defined as noises made by the beating heart and blood flow through heart. The S1 or first heart sound is generated by turbulence due to closing of two components, mitral valve and tricupsid valve.
The mitral valve and tricupsid valve are generally known as atrioventricular valves as they present between the atria and ventricles.
S1 sound is has low frequency, which is described as 'lub' and generates at the beginning of systole
