Blood circulation sustains life by delivering oxygen and nutrients to tissues and removing waste products. For centuries, however, the true nature of blood flow remained largely a mystery, with various theories attempting to explain its purpose and mechanics.
Ancient Theories of Blood Flow
Before modern understanding, theories about blood movement were largely based on philosophical and observational rather than experimental approaches. The most influential ideas came from the Greek physician Galen. His theories, which dominated medical thought for over 1,400 years, proposed that blood was continuously created in the liver from digested food and then distributed throughout the body through the veins. He believed this blood was consumed by the organs. Galen also posited that a different type of blood, mixed with “pneuma” (a vital spirit from the air), was carried by the arteries from the heart to the body. He thought blood passed between the heart’s ventricles through invisible pores in the septum, and that arteries possessed an innate pulsatility rather than being propelled by the heart as a pump. This model described blood moving in an ebb-and-flow manner within separate systems, with no comprehensive return loop to the heart.
William Harvey’s Breakthrough
The understanding of blood flow began to transform significantly in the 17th century with the work of English physician William Harvey. In 1628, Harvey published his groundbreaking findings in his book, Exercitatio Anatomica de Motu Cordis et Sanguinis in Animalibus (Anatomical Exercise on the Motion of the Heart and Blood in Living Beings). This publication presented the first complete and accurate description of blood circulation in a closed loop, propelled by the heart. Harvey’s revolutionary idea was that blood is pumped by the heart through arteries to the body and returns to the heart through veins, in a continuous circuit.
He challenged the long-held Galenic view by demonstrating that the sheer quantity of blood pumped by the heart in a short period could not possibly be consumed by the body. Harvey conducted experiments that involved observing the hearts of living animals, noting that systole (contraction) was the active phase of the heart’s movement, pushing blood out. He also performed ligature experiments on human arms, showing that blood flow in veins was unidirectional, towards the heart, due to the presence of one-way valves. His teacher, Hieronymus Fabricius, had previously observed these valves, but Harvey correctly deduced their function in directing blood flow.
Completing the Circulatory Map
While Harvey established the large-scale circulation of blood, he could not visually demonstrate the connection between arteries and veins. The missing link was provided decades later by the Italian anatomist Marcello Malpighi. Using an early microscope, Malpighi discovered capillaries in 1661.
Malpighi observed these minute vessels in the lungs of frogs, noting their network connecting the smallest arteries (arterioles) with the smallest veins (venules). This discovery provided the physical evidence for Harvey’s theoretical continuous circuit, confirming that blood flowed from arteries into these tiny vessels and then into veins. Malpighi’s work, which included detailed microscopic studies of various organs, solidified the understanding of the circulatory system as a closed and continuous network.
Revolutionizing Medicine and Biology
The discovery of blood circulation by William Harvey and its subsequent confirmation with Malpighi’s microscopic observations reshaped the fields of medicine and biology. This new understanding moved medical thought away from ancient humoral theories toward a more mechanistic and evidence-based approach. It provided a coherent framework for understanding how the body functioned, laying the groundwork for modern physiology and anatomy.
This shift in understanding had far-reaching implications, influencing advancements in diagnostics, pharmacology, and surgical procedures. Knowledge of blood flow and pressure became fundamental for diagnosing conditions and developing treatments. Harvey’s emphasis on observation and experimentation also set a new standard for scientific inquiry in medicine.