Harvey is most famous for observing and putting forth the working of the circulation system. He outlined how the heart pumps blood and also traversed the path it takes. While the valves in veins were already hinted at, by earlier physiologists, Harvey took it up as a challenge to find out the real purpose of valves and its functioning in the circulatory system of the body.
He also discovered the pulmonary circulation and the systemic circulation. He discovered that while the heart pumps blood through the arteries, that flow into the body, while the veins carried impure blood back to the heart. He experimented upon this by pushing in blood in the opposite direction, which certainly did not happen.
En route to discovering and letting t he world know about his great discovery, Harvey did stumble upon great many stones, mainly critics! Harvey's ideas were eventually accepted during his lifetime. His work was attacked, notably by Jean Riolan in Opuscula anatomica (1649) which forced Harvey to defend himself in Exercitatio anatomica de circulations sanguinis (also 1649) where he argued that Riolan's position was contrary to all observational evidence. [www.wikipedia.com]