What does the first heart sound (lub) signify?

The first heart sound, commonly referred to as "lub," signifies the closure of the atrioventricular (AV) valves, which occurs at the beginning of ventricular systole. This sound is produced when the ventricles contract, increasing pressure within them, which forces the AV valves (the tricuspid and mitral valves) to close tightly to prevent backflow into the atria. The closure of the AV valves marks the start of ventricular contraction, allowing blood to be ejected from the ventricles into the arteries (the aorta and pulmonary artery) as the semilunar (SL) valves remain closed initially.

This event is crucial because it indicates that the heart is actively pumping blood and transitioning from the filling phase (diastole) to the pumping phase (systole). Understanding the timing and significance of the first heart sound in the cardiac cycle not only aids in the study of heart physiology but also in recognizing how disruptions in normal heart sounds can indicate potential cardiac issues.