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

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The concept of blood circulation throughout the body. It may appear to be a common, even obvious idea. Yet it took over two thousand years to create the notion that the heart is an organ that pumps blood and oxygen via the arteries, with the "waste" returning through the veins. It revolutionized the life sciences and ushered in modern medicine when it was founded. It ranks with the Aristotelian Corpus, which set the groundwork for biological sciences, and Darwin's theory of natural selection in terms of importance. And, like other concepts, it was essentially the result of a lone endeavor, conceived, initiated, and completed by a single person, English surgeon William Harvey. In 1628, he wrote Exercitatio anatomica de motu cordis et sanguinius in animalibus (Anatomical Exercise on the Motion of the Heart and Blood in Animals), shortened to De motu cordis. Harvey experienced not just one of the greatest medical adventures of all time, but also the instability, susceptibility, and weakness of the human condition. He is a modern man who fits in everywhere.

Blood circulation is now recognized as a two-part system. There is a circuit that runs through the body as well as one that runs through the lungs (the systemic circulation) (the pulmonary circulation). Because the circle is a sign that finishes where it started, each circuit is referred to as a circulation. This book's central premise is the historical unraveling of each circulation. The heart, too, is made up of two hearts that work together to accomplish two separate duties. The left chamber distributes blood to the other organs and limbs, whereas the right chamber pushes blood to the lungs.Because the movement of life-sustaining blood was discovered to be intimately linked to breathing and maintaining a steady body temperature, the three processes combined to form the foundation of the new physiology. As a result, the development of a theory of animal heat and the early physiology of breathing are also covered in this story.

The discovery of the circulation was a watershed moment in the history of biology. It ushered in a new quantitative way of thinking that fostered many disease management advances without which medicine as we know it would be impossible. Harvey's hydraulic description of circulating blood, based 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 other parameters, as well as their quantitative changes under varying pathophysiological conditions, and the effects of abnormal velocities and pressures on body organs.

If new questions arose as a result of the circulation of blood, responses were required. What was the purpose of blood circling in a circle all the time? What was it carrying, and why was it flowing in this way? What method did it use to collect its belongings, and where did it go? What happened to it, how did it get rid of it, and why did it do it? These answers provided a clear picture of how the human body works and provided a physiological foundation for contemporary medicine.

A fundamental breakthrough necessary to his concept was the proper interpretation of the heart's motion as a mechanical pump that expelled blood into the arteries at each contraction.

As a result, illness mechanisms were changed and enlarged. Diseases might arise not just from internal "humors" imbalances, as was previously thought until the mid-nineteenth century, but also from noxious chemicals from the outside that could enter the bloodstream and spread to all tissues. As a result of blockages inside arterial conduits, including those of the heart and brain, various disorders might form as a result of "insufficiency" of blood circulation to essential organs, which led to our knowledge of how heart attacks and strokes occur. They are still the leading causes of disability and mortality in today's world.

They are still the leading causes of disability and mortality in today's world.

Aspects of today's therapies, such as intravenous infusions (as in chemotherapy) or subcutaneous injections (as in insulin shots), and even allergy nasal sprays, could only have been conceived after it was realized that substances introduced into the bloodstream at one site, or even breathed in, are transported to any and every other site because blood circulates. As explained by Harvey, routine operations such as cardiac catheterizations and stent placements inside arteries, as well as the floating of pacemaker and defibrillator electrodes through veins, all need unidirectional blood flow within blood channels into or out of the heart chambers.

Extracorporeal circulations, such as dialysis units and heart-lung machines that allow "open heart" surgeries, are essentially extensions of the concept, and heart-assist devices (artificial hearts) that save lives during extreme acute illness or serve as alternatives to heart transplantation, rely on a circulation model as well. Our current understanding of heart failure, which is the most expensive hospital diagnosis for those over the age of sixty-five, is a fruit of Harvey's remarkable finding. Modern treatment recognizes the heart as a failing pump while also addressing the circulating chemical imbalances that cause the heart muscle to degenerate.

Scientific advancement is a complicated process. Thomas Kuhn, an American philosopher of science, provides possibly the greatest account of it. According to Kuhn, the process begins with the realization of a discrepancy in one's customary expectations of things. The progression then moves on to a more in-depth examination of that anomaly, with the development ending only when the new information becomes evident. The usual condition of things is now altered to suit that learning, and a "paradigm shift" occurs, as Kuhn describes it.

One such paradigm shift was the concept of circulation. The dominating system in Harvey's era, the Galenic model, which had ruled unchallenged for fifteen centuries, could not be reinterpreted and had to be replaced. Blood, according to the Roman physician Galen, flows back and forth in the vessels like a tidal ebb and flow. He imagined two distinct systems of vessels, veins and arteries, arising from two separate organs, the liver and the heart, and supplying blood to all parts of the body. Harvey's breakthrough discovery disproved those outdated assumptions and replaced them with fresh facts.

The growth of research via analogy is also characteristic of the discovery process, with certain similarities obstructing and others helping the path to the truth. Analogies to depict the reality of nature have intrigued intellectuals from the dawn of humanity. From Galen to Boyle, the connection of blood flow to the ebb and flow of Homer's "wine-dark sea" and Aristotle's reference of the vascular system to an irrigation canal, as well as the relation of life to respiration or combustion, pervades the whole tale. Galen derives his “natural faculty of attraction” in the body from a magnet's attraction to iron. Empedocles in Sicily uses the Egyptian clepsydra (water clock) to explain his unique theory of cardiorespiratory physiology, while Erasistratus in Alexandria uses the occurrence of horror vacui to explain his novel theory of cardiorespiratory physiology (nature abhors a vacuum). Descartes compares heart chamber fermentation to log fires. Finally, Harvey seeks solace in Aristotle's philosophy of circles, and in Caus's mechanical fire pump, he finds proof for the heart's role as a mechanical pump. Harvey's own research included two parallel paradigm shifts: the mechanics of cardiac contraction and then blood circulation.

A crucial novelty in his concept was the proper interpretation of the heart's motion as a mechanical pump, and solely as a pump, that expelled blood into the veins at each contraction. The acknowledged method of heart function before to Harvey was a heat-driven "fermentation" of blood within the heart, which caused the organ to expand and, like "boiling milk spilling over," generated an overflow of blood into the aorta.

During the seventeenth-century scientific revolution, an era of “promise with disappointment, and resilience with despair.”  the discovery reached its pinnacle. Harvey's ideas were further developed by a galaxy of the greatest minds and some of the strangest personalities in British science—John Locke, Christopher Wren, Robert Hooke, Henry Cavendish, Joseph Priestley, and their peers; Scot Joseph Black; Anglo-Irish "Skeptical Chymist" Robert Boyle with the Oxford Chemists; and French Europeans René Descartes and Antoine Lavoisier. They dispelled two thousand years of physiological myths by working together. 

They, in turn, stood on the shoulders of the now-forgotten forefathers of the Ionian, Athenian, and Alexandrian intellectual revolutions, men like Alcmaeon of Croton, Diogenes of Apollonia, Hippocrates and Praxagoras of Cos, Sicilian Empedocles, Alexandrians Herophilus and Erasistratus, and Roman Galen, who all paved the way for us to understand the Plato and Aristotle also had key roles to play. And it all started with Homer—with the ebb and flow of his "wine-dark sea," as it were.