The continuous movement of blood throughout the body, known as blood circulation, is a foundational concept in modern biology and medicine. Before this process was fully understood, the inner workings of the human body were shrouded in mystery, leading to incorrect medical practices. The person credited with establishing the true nature of this systemic flow is the English physician William Harvey, whose work fundamentally changed the understanding of human physiology.
The Prevailing Theory Before Harvey
Before Harvey’s work, medical understanding was dominated by the theories of the ancient Greek physician Galen of Pergamon (c. 2nd century AD). Galenic physiology posited that blood was continuously created and consumed by the body, rather than circulated. The process began in the liver, where blood was generated from ingested food and then distributed through the veins to provide nutrition.
This model described blood moving outward in a slow, non-returning ebb and flow, where it was simply used up by the tissues. The arterial and venous systems were seen as two largely separate networks, with the heart not acting as a pump. Galen believed that a small amount of blood passed from the venous right side of the heart to the arterial left side through invisible pores in the septum, a theory Harvey would later challenge.
William Harvey and the Revolutionary Publication
William Harvey received a sophisticated medical education, earning a Doctor of Physic degree from the University of Padua in Italy in 1602. Padua was a leading center for anatomical study, and Harvey studied under Hieronymus Fabricius, who had noted the presence of one-way valves in veins but did not understand their purpose. After returning to England, Harvey became a physician at St. Bartholomew’s Hospital and later served as physician to both King James I and King Charles I.
His groundbreaking theory was formally presented in his 1628 publication, Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus, commonly abbreviated as De Motu Cordis. The treatise was a landmark in physiology because it relied on methodical observation, dissection, and quantitative reasoning. Harvey spent years developing his ideas, presenting his case for circulation in lectures as early as 1616, before finally publishing his work.
Proving the Systemic Loop
The core of Harvey’s argument was that the heart is a muscular pump that propels blood in a continuous, closed loop. He observed the contractions of the heart, noting that systole, or contraction, was the active phase that ejected blood. By analyzing the heart’s valves, he demonstrated they were structured to ensure blood flowed in only one direction.
Harvey used a quantitative argument to dismantle the old theory. He estimated that the heart expels a minimum of one ounce of blood with every beat. Even with a conservative heartbeat of 72 beats per minute, he calculated that the heart pumped over 8 gallons (about 30 liters) of blood in an hour.
This immense volume was far greater than the entire blood volume of a human body or what the liver could constantly produce from food. This calculation proved that blood had to be recirculated in a closed system, as it could not be consumed as quickly as it was supplied. He also performed ligation experiments on limbs, demonstrating that pressure applied to a vein caused blood to pool only on the side away from the heart, confirming the valves ensured one-way flow back toward the heart.
Immediate Scientific Response and Later Validation
Harvey’s declaration that blood was in perpetual, circular motion was met with considerable skepticism and resistance from the established medical community. His radical theory directly contradicted the Galenic model, which had been accepted dogma for over a millennium. Some contemporary physicians openly mocked the idea, and Harvey reportedly lost private medical patients who viewed his discovery as evidence of poor judgment.
Despite the experimental evidence, Harvey’s theory had one missing structural link: he could not demonstrate how the blood transitioned from the smallest arteries into the smallest veins. He postulated the existence of minute, unseen connections, but the technology to observe them did not yet exist. This final piece of evidence came 33 years later in 1661, four years after Harvey’s death.
The Italian physician Marcello Malpighi used the newly invented microscope to examine the lungs of a frog. This microscopic observation revealed a network of tiny, hair-like vessels connecting the arteries and veins, which he named capillaries. Malpighi’s discovery of the capillaries provided the anatomical proof of the systemic loop, confirming Harvey’s theory.