Hektoen International

A Journal of Medical Humanities

Experimental evidence for the humoral circulatory system

Mark A. Gray 
Kansas City, Kansas


Illustration showing the four humors and their aspects

Four Humors from the Book of Alchemy by Thurn-Heisser, Leipzig, Germany (1574). Via Wikimedia. Public domain.

Humoralism, otherwise known as Hellenistic or Galenic medicine, posited the existence of four humors that were required to be kept in balance to maintain health. Blood was special among these humors, believed to deliver both physical and spiritual nourishment to the body.1 To a modern scientist, the physiology ancient Greek physicians built around their observations of blood and the anatomy of the circulatory system appears obtuse. However, to dismiss this system entirely is to invalidate the legitimate observations these scientists made of the circulatory anatomy and to discredit the logic that informed their concept of hemo-physiology. While modern biases might have led to different suppositions and scientific rigor to more effective testing, without knowing more than the ancient Greeks did, it is hard to imagine making a more reasonable initial hypothesis.

Galen of Pergamon, the preeminent physician of the second century A.D. and watershed purveyor of humoralism, did not formulate his theory of the circulation de novo. He inherited an older tradition that considered the veins and the arteries as unconnected systems: the veins for the conduction of blood and the arteries for the conduction of pneuma, a metaphysical substance produced from air that animated living things. The arteries never, in a healthy and intact person, contained blood.2 What could have led Galen’s predecessors to such an erroneous conclusion?

While pneuma was a concept deeply engrained in the metaphysics of the region (and would evolve as religions came and went), an uncritical acceptance of it as part of healthy human physiology is not sufficient to explain why ancient Greek medicine placed the pneuma within the body exactly where it did. Instead, a number of anatomical observations were thought to locate, logically, the pneuma in the arteries. First was the observation that when cut, the veins of a corpse contained blood but the arteries did not.3 This was the case when Galen’s predecessor Erasistratus made his observations of bloodless arteries, likely cutting into a body that was at least several hours old and had most likely been stored supine. Blood drains to the lowest point in the body after death and the veins, due to their greater internal volume, receive the majority of it. Erasistratus would have started his anatomical studies somewhere on the anterior surface of the body while the blood had been draining posteriorly since the moment of death, leading him to conclude that the arteries were bloodless.

Additionally, as the dissection progressed, the arteries could be traced to the aorta and finally to the left ventricle of the heart. That chamber would have also been empty, the blood pooled elsewhere according to the whims of gravity. Erasistratus would have observed significant vasculature connecting the left side of the heart to the lungs, which can clearly be seen to connect to the external environment by the trachea. Thus, assuming that anatomy is formed according to its purpose (and the ancient Greeks did4), it would appear logical that the arteries were for the conduction of air, or something like it, from the lungs and the heart through the arteries and to the body at large.

This theory of the contents of arteries presents a rather obvious problem, however. When the artery of a living being is cut, the artery bleeds. This was explained as, essentially, a decompression event. As Aristotle is said to have claimed, nature abhors a vacuum and it was thought that opening an artery in a living being created a space of negative pressure that extracted blood from nearby veins and drew it into the damaged artery. This was one of the larger leaps the system had to make to remain seemingly relevant. It was also the one Galen’s experiments thoroughly did away with.5

Many of Galen’s experiments relied on the use of ligatures to interrupt conduction through vessels. It was a simple matter for him during vivisection to isolate an artery, ligate it at two points, and then open it between the ligatures. The blood he observed leaving the artery could come from nowhere but the artery itself, between his tie-offs. Thus, he proved that arteries as well as veins were filled with blood. This was not, however, enough to convince him to discard the notion that arteries conducted pneuma, but only to modify the theory so that the arteries conducted pneuma and blood. Galen has been criticized ever since for imagining unobservable pores in the septum between the ventricles of the heart through which venous blood flowed into the arterial system in order to mix with pneuma.

It should be noted that in humoral medicine, the system under consideration is not properly a circulatory system but an irrigation system.6 That is, whatever the contents of the arteries and veins, it flowed from a central source outward to the periphery of the body and was consumed there by the tissues. Not even William Harvey had the tools to observe the microscopic termini connecting the two halves of the vasculature, so it is unsurprising the difficulty the ancients had intuiting a system in which blood was constantly recirculated throughout the body.

The central source of arterial pneuma was the left ventricle, which contained an energy source the Greeks knew as “vital heat.” It was the purpose of the vital heat to forge simple air brought in by the lungs into life-giving pneuma. At the same time, this influx of air mitigated the ferocity of the vital heat. The heart was conceptualized as a forge’s bellows, a device familiar to people of the time, rather than as a pump for liquid, which had not yet been invented.

It was thought that the liver manufactured blood and was the headwaters of the veins that conducted it. This was based on anatomical observations.7 Just as the arteries were connected to the left ventricle and the left ventricle to the lungs, the veins were largely found, via the vena cava and hepatic vein, to be connected to the liver and the liver to the intestines by the portal veins. Food in the intestines was supposed to be the raw material from which the liver made blood before sending it outward to the body, including a small amount sent to the lungs through the right heart, strictly for the purpose of nourishment. It was of course known that to not eat was to lose flesh, and to eat too much was to gain flesh, so in the humoral physiology, ingested food was distributed to the body as blood from which tissue was then built. Again, there is a clear logic based on what could be observed by the exploring anatomist who accepted the Aristotelian assumption that nature does nothing in vain: if a venous path exists between intestines and the rest of the body, then the body must find it necessary to transport something important from the intestines to the rest of the body. If at one end of the path there is food and at the other there is blood, some structure must make this conversion because food does not, of its own accord, spontaneously become blood. If the liver intersects this pathway, perhaps it is responsible for the conversion.

Three acute necessities for life have been discussed: the spiritual essence called pneuma, the vital heat of the left ventricle, and the physical nutrition that was blood itself. While the Greeks wrote of these things with all the metaphysical trappings typical of the time, their pinpointing of these life-indicators is hardly random or illogical. One has simply to ask themselves what the primary sensory differences are to an observer between a living body and a deceased one: the cessation of breathing, the cooling of the body, and in the event of trauma, the evacuation of blood; or in the terms of ancient medicine the loss of pneuma, vital heat, and blood. And it must be remembered that in ancient times, before the advent of more destructive modern methods by which to kill, these external changes of the body were almost always available for observation. There was little else to readily indicate to a period observer that death had taken place.

The humoral theory, as theories go, was not the worst. Humoral medicine did certainly incorporate many confounders, including the metaphysical aspects of pneuma and vital heat. However, despite the need for a trim with Occam’s razor, the theory was grounded in legitimate observations and not so outrageously wrong that it could not explain the limited experiences of physicians of the time. The theory’s great sin was that it became a religion clung to by fundamentalists who insisted on its completeness despite evidence to the contrary. However, being wrong in the face of new evidence is different than being wrong from the start, as every scientist knows. The damage wrought by the inflexibility of the humoral theory had a real human cost and should serve as a reminder to us all that theories must be allowed to grow, develop, and change.



  1. Mavrodi, Alexandra and Paraskevas, George. Morphology of the Heart Associated with its Function as Conceived by Ancient Greeks. International Journal of Cardiology. 2014; 172 23-28.
  2. Karamanou M., et al. Galen’s (130-201 AD) Conception of the Heart. Hellenic Journal of Cardiology. 2015; 56: 197-200.
  3. Aird WC. Discovery of the Cardiovascular System: from Galen to William Harvey. Journal of Thrombosis and Haemostasis. 2011; 9 (Suppl. 1): 118-129.
  4. von Staden H. Experiment and Experience in Hellenistic Medicine. Bulletin of the Institute of Classical Studies. 1975; No. 22 pp. 178-199.
  5. Brain P. Galen on Bloodletting: A study of the origins, development and validity of his opinions, with a translation of the three works. Cambridge University Press. 1986. New York.
  6. Boylen M. Galen on Blood, the Pulse, and the Arteries. Journal of the History of Biology. 2007; Vol. 40, No. 2, pp. 207-230.
  7. Ajita R. Galen and His Contributions to Anatomy: A Review. Journal of Evolution of Medical and Dental Sciences 2015; Vol. 4, Issue 26, March 30; Page: 4509-4516, DOI: 10.14260/jemds/2015/651



MARK A. GRAY is currently completing a Master’s thesis at the University of Missouri. His work is focused on the endocrine signals produced by bone and the acute effects of those signals on the cardiovascular system during stress and exercise. In the future, he intends to study the effects of the Anthropocene Era on human health.


Submitted for the 2019–2020 Blood Writing Contest

Winter 2020  |  Sections  |  Blood

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