Circulation of Blood

English physician William Harvey's discovery of what the heart does and how the blood circulates is widely regarded as the single greatest medical achievement of all time: It established the principle of doing experiments in medicine to learn how the body's organs and tissues function. Published in 1628, Harvey's groundbreaking book Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (Anatomical Essay on the Motion of the Heart and Blood in Animals) spurred research into the mechanical functions of many bodily processes, including respiration, digestion, metabolism, and reproduction.
Harvey received his medical degree from the University of Padua, where he learned one very important fact: Veins have valves that permit blood to travel in only one direction. However, the exact role of the valves was unclear.
Realizing that it was still dangerous to contradict Galen, who had claimed that the liver not only makes the body's blood but also pumps it through the body, Harvey decided to study blood flow by operating on live animals. For a period of 12 years Harvey conducted his experiments before members of the influential Royal College of Physicians in London, England. He wanted their support for his book, which praised Galen while challenging many of his ideas.
In the 8th chapter of his 17-chapter book, Harvey carefully introduced the revolutionary idea that blood goes in a circle in the body, traveling from the heart to the arteries to the veins and back to the heart. The next 9 chapters proved, in wonderfully clear English, that he was right.
In a series of brilliant experiments in animals and humans, Harvey demonstrated how blood circulates in the body. When an artery was blocked, the veins draining this artery collapsed. When a vein was blocked, it swelled below the blockage and collapsed above it, but the swelling disappeared when the blockage was removed. He also showed that the valves in veins allow blood to flow only in the direction of the heart. Together, these discoveries proved that blood moves in a circle in the body—that is, there is a “circulation.”
source: encarta encyclopedia