William Harvey, an English physician and anatomist, made significant contributions to understanding the human body. Born in 1578, Harvey studied medicine at the University of Padua, a prominent medical center. His studies under Hieronymus Fabricius, who discovered valves in veins, laid groundwork for his later investigations. He served as a physician at St Bartholomew’s Hospital in London and as royal physician to King James I and King Charles I.
Unveiling Blood’s Path
Before Harvey’s work, medical understanding of blood circulation was based on the theories of Galen, a Greek physician. Galen proposed blood formed in the liver from consumed food, then distributed throughout the body by veins, where organs consumed it. He also believed arterial blood, carrying “vital spirits” and air from the lungs, operated in a separate system, with blood passing between heart ventricles through invisible pores. This model suggested an open-ended system where blood was continuously produced and consumed, rather than circulating.
Harvey challenged this long-held view by demonstrating blood circulates continuously within a closed system. He proposed the heart acts as a pump, propelling blood throughout the body. He concluded blood flows from the heart through arteries and returns through veins, completing a circuit. This discovery, detailed in his seminal work Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (De Motu Cordis), was published in 1628.
In De Motu Cordis, Harvey described the heart’s contraction (systole) as the force expelling blood, observing that valves in the heart and veins ensure unidirectional flow. He showed the amount of blood expelled by the heart was far greater than what the body could produce or consume, concluding blood must recirculate. His work presented a unified circulatory system, disproving separate venous and arterial blood systems.
His Methodical Approach
Harvey’s conclusions were based on observation, experimentation, and quantitative reasoning, groundbreaking for his era. He studied living animals, including dogs, eels, and frogs, observing their beating hearts to understand mechanics. These vivisections showed how the heart contracted to pump blood and how valves prevented backflow.
He also dissected human cadavers, noting the heart’s structure and the one-way valves in veins, which Fabricius had observed but whose function was unclear. Harvey used ligature experiments on human arms to demonstrate blood in veins flows towards the heart, not away from it, by showing veins would swell on the side away from the heart when compressed.
A significant aspect of Harvey’s methodology was his use of mathematical calculations. By estimating the volume of blood pumped by the heart with each beat and heartbeats per minute, he calculated the heart propelled an immense quantity of blood. He reasoned if Galen’s theory were correct, the body would produce an implausibly large amount of new blood continuously. This quantitative evidence provided a compelling argument for a closed circulatory system.
Shifting Medical Paradigms
Harvey’s discovery of blood circulation altered the understanding of human anatomy and physiology, moving away from ancient beliefs. His work provided a new framework for comprehending the body’s functions, replacing the idea of blood being consumed with its continuous movement. This shift was a significant event in the Scientific Revolution, emphasizing observation and experimentation over traditional dogma.
Harvey’s findings laid the groundwork for modern physiology and cardiology. His hydraulic description of blood flow, propelled by the heart, enabled studies into blood velocity, vascular resistance, and blood pressure. The understanding of continuous circulation became foundational for medical interventions, such as intravenous administration of medicines, blood transfusions, and the development of life support systems like dialysis and heart-lung machines. His insistence on experimental evidence and quantitative reasoning established a methodology that became a standard in biological and medical research.