A heartbeat is the sound caused by the heart valves opening and closing while they pump blood. A properly working heart only has blood flowing in one direction. This is possible by the valves opening and closing in exact coordination with the heart’s pumping action. Assuming you meant lub-dub, the lub sound is the tricuspid and mitral valves closing. The dub sound is the aortic and pulmonary valves closing. Combined, they make the sound of a heartbeat/the sound of someone's heart working properly.
