How did the valves in veins help William Harvey understand circulation?

William Harvey’s study of venous valves was instrumental in his discovery of the circulatory system. While it was previously believed that these small flaps inside the veins merely prevented blood from pooling due to gravity, Harvey demonstrated that they were cardiocentrically oriented—meaning they allowed blood to flow freely toward the heart while strongly inhibiting flow in the opposite direction.1

By observing that these valves faced toward the heart—such as in the jugular vein—Harvey provided critical evidence that blood was not being consumed by the body, but was instead moving in a continuous, rapid circuit pumped by the heart. This observation, which built upon earlier descriptions by his teacher Hieronymus Fabricius, allowed Harvey to challenge the prevailing medical belief that blood followed an ebb-and-flow movement. His quantitative analysis of the heart's output further confirmed that the body could not possibly produce or consume the volume of blood observed, proving that it must circulate.123