No one knew how our blood moved—until this doctor realized the heart is a pump

In 'The Wine-Dark Sea Within,' cardiologist Dhun Sethna describes a 17th-century insight that revolutionized medicine.
In 'The Wine-Dark Sea Within,' a cardiologist describes the history of how we understand blood flow.
Photo by Karolina Grabowska

Excerpted from The Wine-Dark Sea Within: A Turbulent History of Blood by Dhun Sethna. Copyright © 2022. Available from Basic Books, an imprint of Hachette Book Group, Inc.

The idea that blood circulates around the body. It may seem a common, even obvious notion. Yet that familiar concept that the heart is an organ that pumps blood and oxygen through the arteries, with the “waste” returning by way of the veins, took over two thousand years to develop. When it was established, it revolutionized the life sciences and inaugurated modern medicine. In importance, it stands alongside the Aristotelian Corpus that laid the foundation for the biological sciences, and Darwin’s theory of natural selection. And like those ideas, its development was largely a solitary effort, conceived, begun, and completed by a single individual, the English physician William Harvey. He published his discovery in 1628 as a slim volume titled Exercitatio anatomica de motu cordis et sanguinius in animalibus (Anatomical Exercise on the Motion of the Heart and Blood in Animals), abbreviated to De motu cordis. Through it, Harvey lived not only one of the greatest adventures of all time in medicine but, in the process, experienced the insecurity, vulnerability, and frailty of the human condition. He is a man of the present; he belongs everywhere.

Blood circulation, as is now understood, is a double system. There is a circuit through the body (the systemic circulation) as well as one through the lungs (the pulmonary circulation). Each circuit is spoken of as a circulation because the circle is a symbol that ends at the point at which it begins. The historical unraveling of each circulation is a theme of this book. The heart, too, is in reality an assembly of two hearts that work in harmony at two different tasks. The right heart chamber propels blood to the lungs; the left chamber distributes it to the other organs and limbs. Because the motion of life-preserving blood was realized to be closely allied to the breath of life and the maintenance of a constant body temperature, those three processes merged to lie at the very core of the new physiology. Hence, the province of this narrative is also the development of a theory of animal heat and the early physiology of respiration.

The discovery of the circulation was a game changer in the history of the life sciences. It ushered in a new quantitative way of thinking that spawned further innovations in disease management without which medicine, as we know it, would have been impossible. Harvey’s hydraulic description of the circulating blood, founded on pumps and pipes, laid the groundwork for a quantifiable, mechanical system of cardiovascular physiology that led to our modern quantitative way of thinking in terms of blood velocity, vascular resistance, blood pressure, pulse waves, and so on, as well as their quantitative changes under varying pathophysiological conditions, and the effects of abnormal velocities and pressures on the body organs. If blood circulated, then new questions needed answers. What was the need for blood to perpetually go round in a circle? What did it carry when flowing in such a fashion and why? How and where did it take up its stuff? How, where, and why did it part with it? Those answers unraveled a comprehensible picture of the working of the human organism and established a physiological basis for modern medical practice.

The correct analysis of the heart’s motion as a mechanical pump, and only as a pump, that ejected blood into the vessels at each contraction was a central innovation essential to his scheme.

It followed that mechanisms of disease were amended and expanded. A circulation throughout the body meant that diseases could result not only from imbalances of internal “humors,” as was believed until our mid-nineteenth century, but also by noxious agents from outside that could enter the bloodstream and travel to all tissues. As a corollary, some diseases could arise from an “insufficiency” of blood circulation to vital organs because of obstructions within the arterial conduits, including those of the heart and brain, which led to our understanding of how heart attacks and strokes occur. They remain the primary causes of disability and death today.

Aspects of today’s therapies, such as intravenous infusions (as in chemotherapy) or subcutaneous injections (like insulin shots), even nasal sprays for allergy, could only have been conceived after it was understood that substances introduced into the bloodstream at one site, or even breathed in, are transported to any and every other site because the blood circulates. Routine interventions such as heart catheterizations and stent placements within arteries, and the flotation of pacemaker and defibrillator electrodes through veins, all require a unidirectional blood flow within blood vessels into, or from, the heart chambers as described by Harvey. Lifesaving support systems such as dialysis units as well as heart-lung machines that allow “open heart” surgeries are essentially extensions of the concept to extracorporeal circulations, and heart-assist devices (artificial hearts) that save lives during extreme acute illness, or serve as alternatives to heart transplantation, too, rely on a circulation model. A cogent outcome of Harvey’s groundbreaking discovery is our present understanding of heart failure, which is our most expensive hospital diagnosis for persons over age sixty-five. Contemporary therapy not only embraces the heart as a failing pump but also addresses the circulating chemical abnormalities that cause the heart muscle to deteriorate.

Scientific discovery is a complex phenomenon. The American philosopher of science Thomas Kuhn provided what is arguably its best description. The process, according to Kuhn, begins with the recognition of an inconsistency in the normal expectations of things. Next comes an extended exploration of that anomaly, and the progression ends only when the new knowledge itself becomes clear. The normal state of things is now adjusted to accommodate that learning, and what Kuhn calls a “paradigm shift” takes place. 

Courtesy of Basic Books

The idea of a circulation was one such paradigm shift. In the case of Harvey’s circulation, the dominant system, which was the Galenic model that had prevailed undisputed for fifteen centuries, could not be redefined but had to be replaced. The Roman physician Galen had pictured blood as flowing back and forth in the vessels, like a tidal ebb and flow. He had imagined two separate systems of vessels, the veins and the arteries, arising from two different organs, the liver and the heart, which offered blood to all parts of the body. Harvey’s revolutionary discovery drove out those obsolete beliefs with new items of fact.

Characteristic, too, in the discovery process is the progress of science by analogy, with certain analogies preventing and others enabling the way to the truth. From earliest times, thinkers have been captivated by analogies to describe the realities of nature. The analogy of blood flow to the ebb and flow of Homer’s “wine-dark sea” and Aristotle’s comparison of the vascular system to an irrigation canal, as well as the analogy between life and respiration or combustion pervades the entire narrative, from Galen to Boyle. Galen invents his “natural faculty of attraction” in the body from the affinity between a magnet and iron. Empedocles in Sicily draws on the workings of the Egyptian clepsydra (water clock) to enunciate his novel theory of cardiorespiratory physiology, as does Erasistratus at Alexandria from the phenomenon of horror vacui (nature abhors a vacuum). Descartes compares fermentation in the heart chamber to log fires. Finally, Harvey seeks comfort in Aristotle’s philosophy of circles, and finds confirmation for the heart’s function as a mechanical pump in Caus’s mechanical fire pump. Harvey’s own work encompassed two simultaneous paradigm shifts: the mechanism of contraction of the heart, and then the circulation of blood. The correct analysis of the heart’s motion as a mechanical pump, and only as a pump, that ejected blood into the vessels at each contraction was a central innovation essential to his scheme. Before Harvey, the accepted process of heart function, which came from Aristotle, was a heat-driven “fermentation” of blood within the heart that caused that organ to expand and, like “boiling milk spilling over,” caused an overflowing of blood into the aorta.

The crowning point of the discovery arrived during the scientific revolution of the seventeenth century, an era of “promise with disappointment, and resilience with despair.” Further development of Harvey’s ideas linked a galaxy of the greatest minds and some of the oddest personalities in British science—John Locke, Christopher Wren, Robert Hooke, Henry Cavendish, Joseph Priestley, and their peers; the Scot Joseph Black; the Anglo-Irish “Skeptical Chymist” Robert Boyle with the Oxford Chemists, as well as the French Europeans René Descartes and Antoine Lavoisier. Together, they dissolved the misconceptions of two thousand years of physiology. They, in turn, stood upon the shoulders of the now forgotten pioneers of the more ancient Ionian, Athenian, and Alexandrian intellectual revolutions, men like Alcmaeon of Croton, Diogenes of Apollonia, Hippocrates and Praxagoras both of Cos, the Sicilian Empedocles, the Alexandrians Herophilus and Erasistratus, and the Roman Galen, who all broke ground to understand the natural world within us. Plato and Aristotle, too, played their significant parts. And, like everything else, it all began with Homer—with the ebb and flow of his “wine-dark sea.”

Buy The Wine-Dark Sea Within by Dhun Sethna here.