How did William Harvey explain the circulation of blood?

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Smallpox vaccination — history of medicine Edward Jenner vaccinating his child against smallpox; colored engraving.

In his 1628 work, Anatomical Exercise on the Motion of the Heart and Blood in Animals, William Harvey revolutionized the history of medicine by demonstrating that blood flows rapidly through a single system of arteries and veins rather than being consumed by the body.1

Harvey’s explanation relied on several key observations:

Quantitative Evidence: By calculating the volume of blood expelled by the heart per beat, he proved that the heart pumps more blood than the body could possibly produce or consume, necessitating a closed, circulating system.1
Venous Valves: He observed that valves in the veins are oriented to allow blood flow only toward the heart, contradicting the belief that they existed merely to prevent pooling due to gravity.1
Heart Function: Harvey corrected the misconception that the heart’s active phase was an expansion; he demonstrated that the heart acts as an organic pump that contracts to expel blood with force.1

While he lacked the microscope to observe capillaries, his work successfully challenged the prevailing theory of two separate, non-circulating blood systems.1