In this blog post, we examine why medieval and Renaissance physicians did not question Galen’s physiology. We explore the reasons why his theories maintained their authority for so long.
From the Middle Ages through the Renaissance, the undisputed authority in the field of physiology was Claudius Galenus, who systematized Greek medicine around the 2nd century. According to Claudius Galenus, venous blood is produced in the liver and is consumed as it travels through the veins, delivering nutrients throughout the body. Some of this venous blood passes through an opening in the septum—the wall separating the ventricles—from the right ventricle to the left ventricle. There, it mixes with air entering from the lungs via the pulmonary veins and becomes arterial blood. The arterial blood then spreads throughout the body via the arteries, delivering vitality and being consumed in the process. Although this theory contained fundamental errors regarding the pathways of blood circulation, it was accepted as authoritative as part of Claudius Galenus’s comprehensive physiological system.
Although human dissection was possible throughout the Middle Ages, such errors were not corrected due to an academic climate that revered ancient authority. Rather than questioning the authority of Claudius Galenus, scholars of the time sought to interpret anatomical discoveries in a way that supported his theories. For example, even after discovering the structure of the heart, they overlooked the fact that there was no hole in the septum, or, failing to properly understand the role of the pulmonary veins, they attempted to force their findings to fit Galenus’s explanations.
However, by the 16th century, winds of academic transformation began to blow. Renaissance humanism gradually eroded the unconditional trust in ancient authority, giving rise to a new scholarly approach based on experience and observation. A prominent figure of this period was Andreas Vesalius. Known as the “father of anatomy,” he discovered through meticulous dissection that there was no hole in the septum and that the pulmonary veins carried blood, not air. This discovery posed a major challenge to the existing theories of Claudius Galenus, yet his theories remained dominant.
Later, with the discovery of the pulmonary circulation—in which blood leaving the heart passes through the lungs and returns to the heart—Claudius Galenus’s theory of blood consumption faced an even greater challenge. However, medical practitioners of the time remained bound by Claudius Galenus’s theories. Therefore, they believed that the problem arising from the absence of a foramen in the septum—namely, that blood could not flow from the right ventricle to the left ventricle—could be explained by the pulmonary circulation.
The person who changed this situation, in which the theory of blood consumption was mainstream, was William Harvey. He introduced modern quantitative methods into physiology and did not hesitate to challenge established authority. He measured the volume of the heart to estimate the amount of blood it pumped out. As a result, the volume of arterial blood leaving the heart was far greater than the amount of food consumed. Since more blood cannot be produced than the amount of food eaten, William Harvey concluded that blood must circulate.
He conducted a series of experiments to verify this hypothesis. William Harvey tied a cord around his arm to compress both the artery and the vein. When blood flow stopped, the hand, deprived of blood, grew cold. When he loosened the string slightly to allow only arterial blood to flow, the hand quickly regained its vitality, and shortly thereafter, the distal veins—which were still compressed by the string—began to swell. When he released the string completely, the swollen veins immediately subsided. This confirmed that the blood that had left through the arteries had circulated through the hand and returned via the veins.
Based on this experiment, William Harvey proposed the following path of blood circulation in 1628: left ventricle → aorta → various organs → vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary veins → left atrium → left ventricle. Opponents pointed out that dissection revealed no passage connecting the ends of arteries and veins. However, shortly thereafter, Marcello Malpighi discovered capillaries using a newly invented microscope, and the theory of blood circulation was widely accepted.
Throughout the 17th and 18th centuries, various experiments based on the theory of blood circulation were conducted in the field of medicine, and this laid the foundation for modern physiology. William Harvey’s achievements went beyond simply elucidating blood circulation; by introducing the scientific method to medicine, he made a significant contribution to the development of medicine into a more precise and systematic discipline. In particular, his research paved the way for many future physiologists and marked a crucial turning point in the systematic understanding of the human body’s complex functions.
This new physiology, born of the combination of scientific thinking and empirical research, transcended medieval medical traditions to lay the foundation for modern medicine, becoming the basis for the physiology we understand today.
