The first systematic description of the movement of the blood came to us from Galen, a famous philosopher/physician who lived in the second century A.D. Unfortunately, it was riddled with errors.
According to the Galenic system, blood is created in the liver from ingested food and flows to the right side of the heart. Some of it flows to the lungs where it gives off "sooty vapors" and some flows through invisible pores into the left side of the heart, where it gains "vital spirits" when mixed with pneuma brought in by the trachea. Several arteries flow into a fictional rete mirabile at the base of the brain, where vital spirits are changed to animal spirits before being distributed to the rest of the body through hollow tubes called nerves, where the blood is consumed by the tissues.
Such an authority was Galen that the practice of medicine was built around his defective understanding of physiology for the next 1500 years.
Galen's findings were first challenged in the 1200s by an Arab physician, Ibn-al-Nafiz, who insisted there were no invisible passages from the right side to the left side of the heart and he also correctly traced the pulmonary circulation. But Nafiz's writings were ignored until the twentieth century, even in the Arab world.
It was left to William Harvey to bring an accurate idea of the circulatory system to a skeptical world in the seventeenth century. His first clue that something was amiss with Galenic theory was a consideration of the volume of blood that is pumped with each heartbeat. He realized that the liver would have to create several times the body weight in blood every day if the blood was actually being consumed. In Harvey's experiments, he also found out what every butcher already knew -- that an animal could be completely exsanguinated in a matter of minutes by cutting an artery. The blood supply was extremely limited; hence it had to be recirculated. This Harvey established through careful experimentation, sometimes using cold blooded animals, as their circulation could be slowed down.
In 1628, Harvey published his De Moto Cordis. "...since this only book does affirm the blood to pass forth and back through unwonted tracts," wrote Harvey at the time, "contrary to the received way, through so many ages of years, and evidenced by innumerable, and those famous and learned men, I was greatly afraid to let this little book ... either to come abroad or across the sea, lest it might seem an action too full of arrogancy..."
Harvey was right to be concerned; his medical practice suffered greatly from all the criticism he received. A whole practice of medicine involving purging and bleeding rested on Galen's system, and it would not pass away for generations to come. In later life, Harvey became something of a recluse. "Much better is it oftentimes to grow wise at home and in private," said Harvey," than by publishing what you have amassed with infinite labour, to stir up tempests that may rob you of peace and quiet for the rest of your days."