The circulation of the blood might sound like something we all accept but, in fact, it wasn't discovered until 1628.
Before that it was believed that blood came from food in your liver, then entered the heart where it was heated before it shot out into the veins, not the arteries. This is why Shakespeare and people like that talk about the blood “coursing through their veins” instead of their arteries.
William Harvey was the physician to James I. Through a meticulous study of what you might call the plumbing of the chest he came to the conclusion that the heart didn’t heat the blood, it pumped it into the arteries. He knew from Fabricius that the veins had stepladder valves in them, which Harvey realised helped the blood get back to the heart, completing the circuit. Harvey was working before the microscope and didn’t know how the blood got from the arteries to the veins but he made a very bold guess that this was done by tiny vessels so small he couldn’t see them. He was perfectly right of course and we call them capillaries.
It was a discovery of colossal importance. There have been numerous advances since but I’d suggest that circulation was so crucial because without it the others wouldn’t have emerged. You couldn’t undertake modern surgery or give an injection without circulation and can you imagine any modern medical discovery without the knowledge of the blood pumping from the heart?
Harvey’s theory was published in 1628 in a book called On the Motion of the Heart and Blood and you might think that he would have been inundated with patients afterwards. Yet it almost ruined his career as a doctor. In those days doctors were very conservative and wouldn’t make innovations – this was associated with quacks. Good doctors, it was thought, dispensed medicine and diagnosed purely in accordance with the way the ancients had taught.
So, curiously enough, the greatest medical discovery of all time caused a considerable amount of financial distress to its discoverer!
Allan Chapman is the author of England's Leonardo: Robert Hooke and the Seventeenth Century Scientific Revolution (Taylor & Francis, 2004)
This extract is taken from 12 giant leaps for mankind, an article first published in the July 2009 issue of BBC History Magazine