What causes the first heart sound (S1)?

The first heart sound (S1) is primarily caused by the closure of the mitral and tricuspid valves at the onset of ventricular contraction. When the ventricles contract, the pressure within them rises, leading to the closure of these atrioventricular valves to prevent backflow of blood into the atria. This closure creates a distinct sound, which is what constitutes the first heart sound.

Knowing that S1 occurs at the start of ventricular systole helps understand its significance in the cardiac cycle, marking the beginning of pumping blood into the aorta and pulmonary artery. Each of the other options refers to different cardiac events that do not directly contribute to the production of S1. For instance, the closure of aortic and pulmonary valves occurs during S2, while the opening of atrioventricular valves and the contraction of ventricles are events that do not generate heart sounds on their own.