🏴󠁧󠁢󠁥󠁮󠁧󠁿William
Harvey

For fourteen hundred years, the circulation of the blood had been explained by Galen of Pergamon, the second-century Greek physician whose authority over medicine was nearly as absolute as Aristotle's over natural philosophy. According to Galen, blood was continuously manufactured in the liver from food, distributed through the veins to nourish the body, consumed by the tissues, and periodically replenished — a one-way, open-ended system, like water flowing from a spring into dry earth. William Harvey, working in London in the early decades of the seventeenth century, dismantled this picture through a combination of anatomical dissection, vivisection, and a calculation so simple that it should have been made centuries earlier: if the heart pumps approximately two ounces of blood per beat, and beats seventy-two times per minute, then in a single hour it expels more blood than the entire body contains. The liver could not possibly manufacture blood at that rate. The same blood must be going round and round. The heart was not a furnace. It was a pump.

William Harvey was born on April 1, 1578, in Folkestone, Kent, the eldest of seven sons of a prosperous farmer and merchant who would later serve as mayor. He was educated at the King's School, Canterbury, and then at Gonville and Caius College, Cambridge, where he studied medicine. He then traveled to Padua — the finest medical school in Europe — to study under Hieronymus Fabricius, who had discovered the valves in the veins. Fabricius believed these valves prevented blood from pooling in the lower limbs, an essentially static explanation. Harvey would eventually understand that they were directional valves ensuring blood flowed only toward the heart — a central piece of evidence for circulation. He returned to England in 1602 with his doctorate, married the daughter of the court physician, and began a distinguished medical career. He was appointed physician to St. Bartholomew's Hospital in 1609 and physician to King James I in 1618.

Harvey's research on circulation proceeded for at least twelve years before he was confident enough to publish. He conducted hundreds of dissections of living and dead animals — deer, eels, shrimp, pigeons, dogs, fish — observing the heart in action across dozens of species. He applied ligatures to arteries and veins and demonstrated that blood in veins flows toward the heart, not away from it. He showed that the valves in the heart are unidirectional. He measured the capacity of the heart's chambers. And he performed the simple volumetric calculation that made Galen's model arithmetically impossible. In 1628 he published De Motu Cordis et Sanguinis in Animalibus — On the Motion of the Heart and Blood in Animals — a slim, densely argued Latin treatise that established the complete circulation of the blood as a measurable physical fact.

The reception was mixed. Many physicians refused to believe it; Galen's authority was deeply entrenched, and Harvey was criticizing not just a theory but the entire educational tradition of European medicine. Some called him a charlatan. His practice dropped off. But the argument was too tight, the evidence too systematic, to be permanently dismissed. By the 1650s, the circulation of the blood was accepted throughout Europe, and Harvey was recognized as one of the great figures of European science. He served as physician to Charles I during the English Civil War and was present at the Battle of Edgehill, where the King reportedly put his sons in Harvey's care during the fighting. He died in 1657, aged seventy-nine, having lived to see his discovery universally accepted.

One component of the circulatory system Harvey could not identify was the capillaries — the microscopic vessels connecting the arteries to the veins. He knew they must exist, reasoned their existence explicitly, but lacked a microscope powerful enough to see them. They were discovered by Marcello Malpighi in 1661, four years after Harvey's death, using the first practical compound microscope. Harvey had predicted them correctly. The loop he described in 1628 was closed at last.