Final answer:
The first heart sound is caused by the closure of the atrioventricular (tricuspid and mitral) valves and the second by the closure of the semilunar (aortic and pulmonary) valves, marking one cardiac cycle.
Step-by-step explanation:
The first heart sound is a result of the closure of the atrioventricular valves; the tricuspid and mitral valves, while the second heart sound is caused by the closure of the semilunar valves, the aortic and pulmonary valves. When the ventricles contract, the atrioventricular valves close to prevent backflow into the atria, creating the 'lub' sound. Then, when the ventricles relax, the aortic and pulmonary valves close to prevent backflow from the arteries into the ventricles, creating the 'dub' sound. This occurrence marks one complete cardiac cycle and is crucial for effective cardiac function.