William Harvey
Harvey announced his discoveries about the circulatory system in 1616 during a series of lectures (his lecture notes still exist). He wrote a book (Exercitatio Anatomica de Motu Cordis et sanguinis in Animalibus) about it in 1628. He said that blood was pumped around the body in a closed system. Blood was pumped by the heart through the body before coming back to the heart to repeat the process. This went against what was believed at the time to be true.