Harvey Describes Blood Circulation
In 1628, the English physician William Harvey published Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus, a short Latin treatise arguing that the heart pumps blood in a continuous circuit around the body. The idea overturned more than a thousand years of medical teaching and is often seen as a founding moment of modern physiology, because it combined careful observation, measurement and experiment to explain how a living organ works.
For centuries, European medicine had relied on the ideas of the Greek physician Galen, who worked in the second century AD. Galen taught that the liver made blood from food and that the body steadily consumed it. He held that blood ebbed and flowed through the veins and seeped through invisible pores in the wall between the heart's ventricles. Some correction had already begun. The thirteenth-century physician Ibn al-Nafis had described blood passing through the lungs, and in the sixteenth century Andreas Vesalius, Realdo Colombo and others challenged parts of Galen's anatomy. Harvey, who studied at Padua, also knew that his teacher Fabricius had described valves in the veins, although Fabricius did not grasp their purpose.
Harvey, later physician to James I and Charles I, drew on dissections and on experiments with living animals, which was common practice at the time. He observed that the heart's contraction drives blood out into the arteries. He also reasoned that the valves in the veins allow blood to travel only toward the heart. Simple demonstrations on the human arm, using a tight bandage and pressure on the veins, showed the one-way flow. His most famous argument was quantitative: even on cautious estimates, the heart expels far more blood in an hour than the body could plausibly manufacture from food, so the same blood must be circulating repeatedly.
Harvey's work had a real gap. He could not explain how blood passed from the smallest arteries into the veins, because the microscope had not yet revealed capillaries. Marcello Malpighi observed them in the 1660s, supplying the missing link. Harvey's theory also met resistance. Some physicians defended Galen, and others doubted that the theory improved treatment. Harvey reportedly saw his practice suffer for a time, though the evidence for this is mostly anecdotal. By the later seventeenth century, however, circulation was widely accepted in European medical circles.
The legacy of the book goes beyond the discovery itself. It showed that the body could be studied as a mechanism, with quantities that could be measured and claims that could be tested. That approach influenced the emerging scientific method and the physiology that followed. Harvey's treatise remains a model of how evidence and argument can displace long-standing authority, even when a theory is still incomplete.