Final answer:
The "Lub" sound of the heartbeat, known as S1, is heard during systole and is caused by the closing of the atrioventricular valves.
Step-by-step explanation:
The sound of the heartbeat heard during systole is the "Lub". This sound, known as the first heart sound or S1, is caused by the closing of the atrioventricular valves during ventricular contraction. The second heart sound, S2, or "Dub," occurs during ventricular diastole and is produced by the closing of the semilunar valves. In a normal cardiac cycle, which includes both diastole and systole, these two sounds can be heard in sequence, creating the characteristic "Lub-Dub" rhythm.