During the S1 heart sound, which cardiac structures are closed?

The S1 heart sound, commonly referred to as the "lub" sound, is predominantly associated with the closure of the atrioventricular (AV) valves—specifically, the mitral and tricuspid valves. This occurs at the beginning of ventricular contraction (systole) when blood is pumped from the ventricles into the aorta and pulmonary artery.

AV nodes are not structures that open or close as part of heart valve mechanics; rather, they are part of the electrical conduction system of the heart that receives impulses from the atria and transmits them to the ventricles. Since the S1 sound is related to the mechanical action of the heart, the closure of the mitral and tricuspid valves during this phase is critical. The perception of the S1 sound signifies that these valves have closed tightly to prevent backflow of blood into the atria.

The aortic and pulmonary (semilunar) valves close during S2 ("dub"), marking the end of ventricular systole. Thus, the positioning of the sound (S1 vs S2) clarifies which valves are relevant during the respective heart sounds, making a solid understanding of cardiac cycles essential for interpreting heart sounds effectively.