Claudius Galenus’s theory of the circulation of blood, the authority on physiology from the Middle Ages through the Renaissance, was long accepted uncritically by the medical community of the time, despite its flawed assumptions. After the 16th century, scholars emerged who challenged the established theory through empirical dissection and experimentation, eventually establishing the modern understanding of the circulation of blood.
From the Middle Ages through the Renaissance, the absolute authority in the field of physiology was Claudius Galenus, who summarized Greek medicine around the 2nd century. “According to Claudius Galenus, venous blood is produced in the liver and consumed as it travels through the veins carrying nutrients throughout the body. Some of the venous blood travels from the right ventricle to the left ventricle through a hole in the septum, the wall of the ventricle, where it meets air from the lungs through the pulmonary veins, a passageway for air, and becomes arterial blood. The arterial blood then travels through the arteries to the rest of the body, where it is consumed as it delivers vital energy. Although this theory contained fundamental errors about how blood traveled, it was accepted as authoritative as part of Claudius Galenus’ comprehensive system of physiology.
Throughout the Middle Ages, human dissection became available, but such errors were not corrected because of the academic culture of deference to ancient authority. Rather than questioning the authority of Claudius Galenus, scholars tried to interpret anatomical findings in ways that supported his theories, for example, discovering the structure of the heart but ignoring the absence of a hole in the septum, or trying to force the pulmonary veins to fit into Claudius Galenus’s description without properly understanding their role.
By the 16th century, however, the winds of academic transformation were blowing. The humanism of the Renaissance gradually began to break down the unconditional trust in ancient authorities, and a new academic approach based on experience and observation emerged. Andreas Vesalius was one of the most prominent figures of this period. He is considered the godfather of anatomy, and after a thorough dissection, he discovered that the septum does not have a hole and that the pulmonary veins are channels for blood, not air. This discovery greatly challenged the previous theory of Claudius Galenus, but his theory still prevailed.
Later, the discovery of the pulmonary circulation, in which blood leaves the heart, passes through the lungs, and returns to the heart, further challenged Galenus’s theory of blood loss. However, physicians of the time were still bound by Galenus’s theory and thought that the pulmonary circulation could explain the problem caused by the absence of a septal hole-the inability of blood to move from the right ventricle to the left ventricle.
It was William Harvey who changed this dominant theory of blood loss. He introduced modern quantitative methods to physiology and was not afraid to challenge established authority. He measured the volume of the heart to estimate the amount of blood it pumps. As it turned out, the amount of arterial blood leaving the heart was much greater than the amount of food being consumed. Since more blood cannot be made than is eaten, William Harvey thought that blood must be circulating.
He conducted a series of experiments to test this hypothesis. “William Harvey tied his arm with a string, compressing the arteries and veins together. When the blood stopped flowing, the non-blooded hand became cold. When he slowed the string slightly to allow only arterial blood to flow, his hand quickly revived, and after a while the veins at the ends of the string, still compressed by the string, swelled. When the strap was finally released, the swollen vein receded. This confirmed that the blood that had left the artery was returning to the vein through the hand.
Based on this experiment, William Harvey proposed in 1628 the following circulatory pathway for blood: left ventricle → aorta → trachea → vena cava → right atrium → right ventricle → pulmonary artery → lung → pulmonary vein → left atrium → left ventricle. Opponents pointed out that they could not find any passages connecting the ends of the arteries and veins through dissection. However, the theory of blood circulation gained widespread acceptance a short time later when Marcello Malpighi discovered capillaries under the newly invented microscope.
Throughout the 17th and 18th centuries, the medical field continued to conduct various experiments based on the theory of blood circulation, and the foundation of modern physiology was established. “William Harvey’s contribution was not limited to the discovery of the circulation of blood, but also to the introduction of the scientific method into medicine, which helped to make it a more precise and systematic discipline. In particular, his work paved the way for many physiologists to follow and marked a major turning point in the systematic understanding of the complex functions of the human body.
This new physiology, which combined scientific thinking and empirical research, moved beyond the medieval medical tradition and laid the foundation for modern medicine, and laid the groundwork for physiology as we understand it today.
