Final answer:
S1 is the sound caused by the closure of the tricuspid and mitral valves. It is heard loudest at the apex of the heart. It occurs during systole, the contraction phase of the heart, which is part of the cardiac cycle.
Step-by-step explanation:
S1 is the sound created by the closure of the tricuspid and mitral valves, and it is loudest at the apex of the heart.
During the cardiac cycle, when ventricles contract, the tricuspid and mitral valves close to prevent the backflow of blood to the atria, which creates the 'lub' sound, also known as the first heart sound or S1.
This sound is associated with systole, the phase when the heart muscle contracts and pumps blood out of the chambers into the arteries.
On the other hand, the second heart sound, S2, occurs during ventricular diastole when the semilunar valves close after blood is pumped through them, resulting in the 'dub' sound.
The valves' closure reduces the openings in the atrioventricular septum, causing blood flow to become turbulent until the valves are fully closed, and this turbulence generates the audible heart sounds.