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How did Galen’s ideas about blood flow differ from William Harvey’s findings?

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Galen’s physiological theories, which dominated medical thought for over 1,400 years, differed fundamentally from the findings of William Harvey regarding the nature and movement of blood.1

Galenic View

Galen proposed that the body consisted of two separate, non-circulatory blood systems:2
* Venous System: He believed blood was created in the liver from nutrients and then carried by the veins to the rest of the body, where it was consumed as fuel.21
* Arterial System: He posited that a separate, "vital" blood carried life-giving energy from the lungs through the arteries.21
* Heart Function: Galen believed the heart’s active phase was its expansion, which drew blood into itself, and that blood passed between the heart's chambers through invisible pores in the septum.21

William Harvey’s Findings

In his 1628 work, Anatomical Exercise on the Motion of the Heart and Blood in Animals, Harvey demonstrated that the heart acts as a pump, circulating blood in a single, closed system.23

Circulation: Harvey proved that blood is not consumed but rather flows rapidly in a continuous loop. By calculating the volume of blood pumped by the heart, he showed that the body could not possibly produce or consume blood at the rate Galen suggested.23
Heart Mechanics: Harvey observed that the active phase of the heartbeat is the contraction, which expels blood with force.2
Evidence: He utilized experimental evidence, such as the orientation of venous valves, to confirm that blood flow is directed toward the heart, contradicting the idea of blood moving outward to be consumed.2