When William Harvey published his revolutionary book on the circulation of the blood in 1628, his reward was vicious criticism and abuse from fellow physicians. The reason was simple. In his Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (An Anatomical Exercise Concerning the Motion of the Heart and Blood in Animals), Harvey challenged beliefs that had been accepted by the medical establishment since they were put forward by the Greek physician, Galen, 1400 years earlier, in spite of growing evidence against them.
Galen's explanation for one of the age-old mysteries - the role blood played in the human body - suggested that the body continuously made and then consumed large amounts of blood and thus the nutrition gained from food. He believed that blood from the arteries was used to “top-up" the blood carried in the veins, passing from one to the other through tiny holes in the heart, from the right side to the left.
Solving the mystery of the human heart
Harvey first heard this theory questioned as a medical student in around 1600. Having qualified as a doctor in 1602 and returned to England, he started a medical practice in London, later becoming a consultant physician at St Bartholomew's
Hospital. His skepticism about Galen's theory led to 14 years of experiments and theorizing before he arrived at a workable idea of how blood circulated.
In 1616 he announced the results. Blood, he said, travels from the left of the heart through the arteries to the tissues, returns via the veins and enters the right side of the heart, travels on to the lungs and finally arrives back at the heart's left side.
Heartbeats provide the breakthrough
Harvey went on to conduct further experiments on live snakes, sheep and human corpses in an attempt to prove his hypothesis. Then he noticed when dissecting some of the animals that their hearts continued to beat after they had been removed from their bodies. This convinced him that the heart was a muscle which contracted to pump blood through the veins. "The movement of blood occurs constantly in circular manner," he noted, "and is the result of the beating of the heart."
He also established that the left ventricle of the human heart contains 2 oz. (60 g.) of blood and that it beats between 60 and 200 times a minute, pumping out 4 lb. (1.8 kg.) of blood. This processed a colossal amount of blood - far more than the body could produce from eating food and drinking liquids. From this, he deduced that the flow of blood was contained in a closed system and was constantly being recycled rather than replaced. In an experiment on a live snake, Harvey showed that when a vein was tied with thread, the piece of vein between the thread and the heart emptied. But if an artery was tied, the section between the heart and the thread remained full of blood, while the section beyond it was empty. This proved that the blood flows in one direction; out from the heart through the arteries and back to the heart via the veins. It certainly did not flow from one side of the heart directly to the other.
Tiny vessels provide the proof
By 1628, Harvey had accurately established the workings of the heart and the movement of the blood. The only remaining puzzle was the apparent absence of any connection between the arteries and veins. Capillaries, minute blood vessels which supplied the missing link, were discovered by Marcello Malpighi four years after Harvey's death.
By the end of the 17th century Harvey's explanation of how blood circulated was an accepted medical fact. Galen's theory was seen as just another wrong turning in the progress of science.