Until the beginning of the seventeenth century it was generally believed by physicians that blood was produced in the liver and somehow consumed in the tissues after passing through the blood vessels. This idea had been around for 1500 years since the time of Galen. The key figure in replacing this belief by the modern view that blood circulates was William Harvey. I find it interesting that the argumentation which led him to this conclusion and which allowed him to defend it against strong opposition can be seen as an application of mathematics to medicine. Admittedly the mathematics involved is no more than elementary arithmetic but I think it is useful to consider it from this point of view. My main source of information for this story is the book ‘A brief history of medicine‘ by Paul Strathern. I can thoroughly recommend this book as an informative and entertaining account of the way in which advances in medicine have changed the way we live over the centuries.
What was Harvey’s argument? He set out to determine how much blood leaves the heart in an hour. To do this he measured the amount of blood which can be taken up by the heart of a dog and the number of heartbeats of the dog per hour. Multiplying the results and doing some extrapolation from the dog to the case of a human gives the result that the amount of blood which flows out of the heart in one hour has more than three times the weight of the whole human body. That blood could be produced and absorbed again at this rate is so far beyond the limits of plausibility that it served as the strongest argument for Harvey’s theory although it was by no means the only one he presented. Even the fact that capillaries had not yet been discovered and that as a consequence nobody knew how the blood passed from the arteries to the veins could not prevent the triumph of the new theory. This was an important step for the application of scientific arguments in medicine and indeed for the application of the scientific method in general.