What does S1 heart sound ('Lub') signify?

S1 heart sound, commonly referred to as the "Lub," signifies the beginning of systole. This sound is produced by the closure of the mitral and tricuspid valves at the start of ventricular contraction. When the ventricles contract, they push blood out of the heart into the aorta and pulmonary arteries, and this rapid closure of the valves creates the distinct sound associated with S1. This event marks the transition from diastole, when the heart fills with blood, to systole, when the heart pumps blood out. Understanding that S1 indicates the commencement of systole is crucial for interpreting heart sounds in clinical practice and contributes to assessing a patient's cardiac health.