William Harvey’s discovery of the circulation of the blood was one of most important discoveries of the Scientific Revolution and of the history of medicine. Quickly accepted by the medical community, it led doctors and scientists to rethink the blood and heart’s physiology, as well as general therapeutic strategies.
William Harvey’s discovery of the circulation of the blood was not a punctual event. Rather, it was the product of a decade of observations, reflections, speculations, and writings, beginning in 1617 and ending in 1628, when Harvey published Exercitatio anatomica de motu cordis et sanguinis in animalibus (1628; Anatomical Exercise on the Motion of the Heart and Blood in Animals, 1653; commonly known as De motu cordis
The dissections Harvey performed while lecturing on surgery at the Royal College of Physicians gave him the opportunity to study the heart, although he originally intended to focus his research on the heartbeat, and convinced him that the Galenic understanding of the function and motion of the heart and blood needed to be revised. In fact, while Renaissance dissections had considerably expanded the knowledge of human anatomy—epitomized by Andreas Vesalius’s De humani corporis fabrica (1543; On the Fabric of the Human Body, books I-IV, 1998; better known as De fabrica)—they had not revealed how or why the heart moves, nor elucidated the circulation of the blood. Indeed, the cardiovascular system and cardiac movements are very complex phenomena that cannot be easily observed.
Although the Renaissance physician Realdo Colombo (1516?-1559) had already raised doubts about Galen’s description of the heart’s function, when Harvey first considered the anatomical evidence, he was a convinced Galenist, like most of his colleagues. In Galenic medicine, blood was considered to be one of the four humors whose balance determined the health of an individual. It was thought of, not as circulating in a closed system, but rather as being continuously produced and destroyed. Galen believed that the liver produced blood to provide nourishment to the body; therefore, blood was consumed and had to be regenerated. This nourishing plasma was identified with the darker, venous blood.
Red, arterial blood, according to Galen, had instead the function of carrying pneuma, the vital spirit that was infused when the blood mixed with air in the lungs. A minimal exchange between the two blood types was believed to occur only in the heart’s septum. The lack of a circulatory process in this model demanded that blood flow very slowly, allowing time for its generation and destruction. Similarly, the heart was not considered responsible for the motion of blood, which was instead thought to be attracted by the various organs and passageways, each at its own rate.
One reason Galen’s model had endured so long was that oxygen-rich arterial blood in fact transforms into depleted venous blood in the capillaries, which are so small that they are visible only under a microscope. Since this process was beyond contemporary observational abilities, it neither required explanation nor stood as a recognizable contradiction to Galen’s system. The capillaries would not be observed until Marcello Malpighi
The chain of ideas and observations that led Harvey to challenge and undo the Galenic system remains a matter of speculation, because his notes and papers were lost during the Great Fire of London (1666). Any reconstruction of the exact history of Harvey’s discovery must therefore rely primarily on circumstantial evidence. Harvey wrote the crucial chapters of De motu cordis between 1617 and 1619, and in the 1628 work’s introduction, he claims to have discovered the circulation of the blood nine years earlier.
Harvey’s introduction also indicates that it was thinking about the properties of venous valves that originally compelled him to doubt Galenic notions, study the heart’s function and movement, and hypothesize that blood circulates through the body. Discovered by Hieronumus Fabricius ab Aquapendente
Harvey’s book uses metaphoric language and relies repeatedly on analogies of microcosm to microcosm and of the meteorological cycle of water. Beyond such abstract reasoning, however, Harvey devised a series of clever experiments to support his daring claim that blood circulates and is somehow transferred from the arteries to the veins. For example, by ligating an arm and regulating the blood flux in veins and arteries with a finger, he presented a visual demonstrations that valves prevent the blood from flowing away from the heart. These experiments are the only ones illustrated in De motu cordis, a book that includes only two plates.
Harvey also calculated the amount of blood that is expelled by the heart per hour: Even with conservative estimates for the average heartbeat and for the ventricles’ size and volume, he concluded that about 1,000 fluid ounces, or almost 8 gallons (29.5 liters), of blood emerge from the heart each hour. Since it seemed impossible for the body to produce and consume fluid at that rate, this figure seemed to prove that the same blood must pass through the heart repeatedly. Harvey’s combined rhetorical arguments, experimental evidence, and quantitative observations ultimately proved convincing.
The fact of the blood’s circulation correlates with the notion that the heart must function as a pump. Harvey therefore studied the heart’s motion and function, although he never actually used that often repeated comparison between the organ and the machine. Human cardiac movements remain unclear when observed in a heart beating at a normal rate, so Harvey performed vivisections of cold-blooded animals, whose hearts beat more slowly. He also examined dying mammals’ hearts as they slowly stopped beating. These experiments allowed him to understand systolic and diastolic movements and thus to revise Galenic notions about cardiac anatomy and the function of the heart’s four valves.
Harvey’s claims and ideas were bold and challenged the status quo of medicine. Not surprisingly, they gave rise to heated debates in which he himself took part, as did prominent intellectuals of the seventeenth century, including Robert Fludd,
Conversely, Descartes fully supported circulation, but he disputed Harvey’s account of the relationship of the blood to the heart. Following his belief that the body is a machine, Descartes tried to theorize the cardiovascular system and cardiac motions in mechanical terms, with the heart powered by the passage of the blood. Although significant, Riolan’s hybrid system and Descartes’s mechanical reinterpretation did not have a lasting impact; they were quickly disproved by experimental evidence.
Harvey’s discovery of the circulation of the blood is often compared in tenor and importance to Galileo’s discoveries and to Sir Isaac Newton’s theories. Although it was certainly revolutionary, however, it is important to underscore that Harvey’s discovery did not reflect or incorporate the basic tenets of the Scientific Revolution. In fact, Harvey remained a convinced Aristotelian; his explanations are generally qualitative rather than quantitative, and they all embody the causal scheme proper in Aristotelian natural philosophy. In fact, he conceived his research to be part of a program, which he shared with many of his colleagues, aimed at reviving Aristotelian medical theories. It is within this framework that his attitude toward dissection and experimentation should be viewed.
Despite many authoritative attempts to disprove it, Harvey’s discovery became mainstream medical knowledge within four decades of its publication. His discovery had both theoretical and practical implications. Accepting that the blood circulates not only demanded that physicians rethink their notions of blood and cardiac physiology, but it also proved a death blow to Galenic medicine. Therapeutic strategies were also affected. For instance, the practice of bloodletting, previously meant to rid the body of a presumed excess of blood and to reestablish the balance among the four humors, lost meaning.
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Bylebyl, Jerome. “Harvey.” In Dictionary of Scientific Biography, edited by C. C. Gillispie. Vol. 4. New York: Charles Scribner’s Sons, 1972-1990. Seven pages of this informative article focus on Harvey’s discovery. -
Bylebyl, Jerome, ed. William Harvey and His Age: The Professional and Social Context of the Discovery of the Circulation. Baltimore: Johns Hopkins University Press, 1979. Important collection of essays. -
Conrad, Lawrence, et al., eds. The Western Medical Tradition, 800 B.C. to A.D. 1800. New York: Cambridge University Press, 1995. Andrew Wear’s chapter, “Medicine in Early Modern Europe, 1500-1700,” is the best introduction to early modern medicine; it dedicates fifteen pages to Harvey’s achievements. -
French, Roger. William Harvey’s Natural Philosophy. New York: Cambridge University Press, 1994. The best analysis of Harvey’s work and ideas. Dedicates several chapters to Harvey’s discovery and its reception in the medical and philosophical communities. Includes an extensive bibliography. -
Fuchs, Thomas. The Mechanization of the Heart: Harvey and Descartes. Rochester, N.Y.: University of Rochester Press, 2001. Detailed study of the controversies of the circulation of the blood. -
Gregory, Andrew. Harvey’s Heart: The Discovery of Blood Circulation. London: Totem Books, 2001. Short, clear, and precise: The best monograph on this subject. It also offers a useful glossary. -
Harvey, William. The Circulation of the Blood and Other Writings. Translated by Kenneth J. Franklin. London: J. M. Dent, 1990. The best translation of Harvey’s works, with an insightful introduction by Andrew Wear.
René Descartes; Robert Fludd; Galileo; Thomas Hobbes; Sir Isaac Newton.