What does the heart sound S1 indicate?

The heart sound S1 is primarily associated with the closure of the mitral and tricuspid valves, which are the atrioventricular (AV) valves located between the atria and the ventricles. During the cardiac cycle, S1 marks the beginning of ventricular systole, the phase where the ventricles contract and pump blood to the aorta and pulmonary artery. When the ventricles contract, the pressure increases, causing the mitral and tricuspid valves to close to prevent backflow of blood into the atria. This closure creates the distinctive sound known as S1, often described as a "lub" sound.

The other choices relate to different phases of the cardiac cycle or functions of the heart. The closure of the aortic and pulmonary valves corresponds to heart sound S2, which occurs at the end of ventricular systole. The opening of the aortic valve does not produce a significant sound that can be heard and is associated with the ejection of blood from the ventricles. Lastly, relaxation of the heart pertains to diastole, which occurs after systole but does not correlate with S1. Understanding these phases and sounds is critical for accurate heart assessment and diagnostics.