Is lubb the first heart sound and occurs when the tricuspid and bicuspid valves snap shut?

Yes, "lubb" is the first heart sound, often referred to as S1. It occurs when the atrioventricular valves, specifically the tricuspid and bicuspid (mitral) valves, close at the beginning of ventricular contraction (systole). This sound marks the end of diastole and the start of systole, indicating that blood is being pumped from the heart to the lungs and the rest of the body.