Discoveries about the circulatory system and how it works

Discovery of how the circulatory system is structured and how it works took a long time. Many scientists contributed to human knowledge of the blood system. Research scientists continue to learn more about it today.

Who (person, nationality, approx dates) discovered what about the human blood system ?

Herophilus (active c. 340 BC)

The Greek physician Herophilus improved Aristotelian anatomy by distinguishing arteries from veins. At that time it was known that arteries originated from the heart. It was thought that veins emerged from the liver. Herophilus also noticed that the coverings 'coats' around the arteries were thicker than those around the veins and studied the pulse, including variations in the magnitude, rate and rhythm of pulses.

Erasistratus (approx. 330-250 BC)

The Greek anatomist and royal physician Erasistratus wrote many studies about anatomy, practical medicine and pharmacy but only titles and fragments survive, retained and preserved by other writers. He is described in The Greatest Benefit to Mankind: A Medical History of Humanity as "nebulous and controversial".

Erasistratus suggested that veins and arteries carried different substances. He believed that veins carried blood but arteries carried only 'pneuma' which as been translated a 'spirit', 'air' or a type of 'animal spirit'.

Galen of Pergamon* (approx. 130-200)
*modern-day Bergama, Turkey

Galen of Pergamon*, a physician, surgeon and philosopher was a prominent Roman (of Greek ethnicity) who has been described as "arguably the most accomplished of all medical researchers of antiquity". He was a private physician to the Roman Emperor Marcus Aurelius and is famous for his contributions to many scientific disciplines, especially in medicine.

Galen's Theory of Circulation

Galen believed that the circulatory system consisted of two separate one-way systems of distribution, rather than a single unified system of circulation. His understanding was that venous blood was generated in the liver, from where it was distributed and consumed by all organs of the body, while arterial blood came from the heart, from where it was distributed and consumed by all organs of the body. As blood was consumed by the organs of the body, it was believed to be regenerated in either the liver or the heart, according to the type of blood.

  • Galen used patient's pulses to help him assess their illness.
  • He acknowledged differences between venous (dark) and arterial (bright) blood.
  • He understood that blood went from one side of the heart to the other side of the heart but he didn't know how that happened. He thought there were tiny holes in a wall between two sides of the heart.
  • He believed that blood passed 'backwards and forwards' along the blood vessels

Galen's theories dominated medicine as taught and practised in Europe for about 1,500 years. However, eventually some scientists made observations that did not fit with Galen's thoughts about blood circulation. They included the Arab physician Ibn Al-Nafis (1210-1290), the Italian scientist and inventor Leonardo da Vinci (1452-1519) and the Spanish scholar and physician Michael Servetus (1509-1553).

Michael Servetus (*1509-1553)- described pulmonary circulation
*some texts state 1511-1553 'exact birthdate unknown', others give specific info (see below)

Accurate description of pulmonary circulation, a description that was unacceptable to the religious authorities at that time.

The scholar and Spanish physician Michael Servetus studied and made important contributions to many fields of knowledge including human anatomy, medicine, pharmacology, geography and theology. His views about Christian theology were more radical than those of the protestant reformer Martin Luther because Servetus developed a nontrinitarian study of Christian theology, i.e. one that rejected mainstream teachings about the holy trinity - as accepted by both catholics and 'mainstream' protestants. In 1531 he published the "De Trinitatis Erroribus" (trans. 'On the Errors of the Trinity'), in which he stated that people who believed in the trinity were really tritheists, i.e. believers in three gods, an argument that both protestant and catholic churchmen considered blasphemous. Servetus' first known medical book "In Leonardum Fuchsium Apologia" was published in 1536 then expanded in 1537.

Michael Servetus is considered by many to be the first European to describe pulmonary circulation accurately. He traced the flow of blood around the pulmonary circulation, i.e. to and from the heart along the veins and arteries between the heart and lungs. He did not think that blood entered the muscular walls of the heart. His controversial account of pulmonary circulation was included in the 700 page book "Christianismi restitutio" (trans. 'The Restoration of Christianity') that he published anonymously in 1553 and in which he continued to deny the trinity. This is the work for which he is most famous as a physician, although it seems that his description of pulmonary circulation was primarily a theory that he developed in order to explain how 'the divine spirit' reached all parts of the body - given that the bible was understood to say that 'blood is the seat of the soul' and that 'the soul is breathed into man by God' - quotes according to The Greatest Benefit to Mankind: A Medical History of Humanity by Roy Porter.

The views of Servetus differed from those of Galen:

Galen had taught that ...

  • The body is 'the instrument of the soul', which matched the teachings of the church
  • There are three 'spirits' in the human body, namely
    • natural - contained in the veins, controlled by the liver
    • vital - contained in the arteries, controlled by the heart
    • animal - contained in the nerves, controlled by the brain

Servetus' account added / countered that ...

  • Servetus agreed that the energy and light of God (some writers might use the word 'Life' rather than of God) is present in all 3 of the 'spirits' identified by Galen.
  • Servetus argued that the soul is located in the blood, citing Genesis 9, Leviticus 7 and Deuteronomy 12.
  • Servetus argued that the vital spirit (i.e. the fluid contained in the arteries) is "composed of a very subtle blood nourished by the inspired air".

Galen and Servetus gave different explanations of how blood passes from the right ventricle (RV) of the heart to the left ventricle (LV) of the heart.

  • Galen said that blood passes from the RV to the LV through tiny openings or pores in the septum of the heart. He argued that most of the blood passed via this route, as opposed to travelling through arteries then veins
  • Galen taught that blood mixed with air in the heart - rather than in the lungs, as argued by Servetus and as taught today.
  • Servetus said that blood passes from the RV to the LV via a route that passes through the lungs.
  • Servetus also argued that the movement of blood from the heart into the lungs at birth (see also persistent ductus arteriosus for more about the fetal vs neonatal - adult heart) serves the purpose of mixing the 'Divine Spirit' (of inhaled air) with blood in the lungs.

Christianismi restitutio was not the only aspect of Servetus' work that displeased the religious authorities. They considered it a serious offence in addition to his previous lectures and publications that had also met with opposition from the church. Despite escaping from the Spanish Inquisition, he was arrested in Geneva, condemned for heresy and burnt at the stake later in October 1553. Consequently, Servetus' description of pulmonary circulation did not have much influence on the development of understanding of human anatomy at that time. That was partly because the description appeared in a book about theology rather than about medicine and also because most of the copies were burnt along with him or soon afterwards. Three copies are known to have survived but were kept hidden for many years.

Was Servetus really the first to describe pulmonary circulation accurately ?

Although Michael Servetus is considered by many to be the first European to describe pulmonary circulation accurately, it has been suggested that others might have done so earlier and from a purer scientific as opposed to theological point of view.

They include:

  • Ibn Al-Nafis (1210-1290), an Arab physician whose book "Commentary on the Anatomy in the Canon of Ibn Sina" includes some arguments in common with those of Servetus, such as blood not passing through the septum of the heart and the mixing of air with blood occurring in the lungs - as opposed to the heart, as argued by Galen. However, the accounts of Al-Nafis and Servetus also differ concerning anatomical details such that more recent scholars have concluded that neither Servetus nor Colombo (below) were aware of Al-Nafis' description.
    For more about Ibn Al-Nafis see "Ibn al-Nafis, the pulmonary circulation, and the Islamic Golden Age" at§
  • Realdo Colombo (1516-1559), an Italian scholar who describes pulmonary flow in his book "De Re Anatomica"
  • Juan Valverde de Hamusco (c.1525-c.1587), a Spanish former student of Colombo whose "De humani corporis fabrica" was published in 1556.

Finally, although there is no suggestion that Leonardo da Vinci (1452-1519) described pulmonary circulation, he did describe many parts of what is now known as the cardiovascular system, including his observations of the muscular action of the heart. His works might have influenced subsequent progress in the understanding of the human blood circulatory system.

*e.g. Bosmia A, et al, "Michael Servetus (1511-1553): Physician and heretic who described the pulmonary circulation", Int J Cardiol (2012), doi:10.1016/j.ijcard.2012.06.046 states that Michael Servetus was born in Spain on 20 Sept 1511.
§ Full ref. John B. West, "Ibn al-Nafis, the pulmonary circulation, and the Islamic Golden Age", J Appl Physiol 105: 1877–1880, 2008.

Fabricius ab Aquapendente (1537-1619)

The pioneering anatomist and surgeon Fabricius ab Aquapendente has become known as "The Father of Embryology" and also made advances in other fields of medicine. He revolutionized the teaching of anatomy when, in 1594, he designed the first permanent theatre for public anatomical dissections.

Discovery of valves in veins

Fabricius ab Aquapendente was the first to describe the membranous folds that he called "valves" in the interior of veins. These are now widely known as valves and are understood to prevent back-flow of blood within the veins, though Fabricius did not explain their purpose.

Fabricius ab Aquapendente also taught William Harvey (see below), who went on to develop Aquapendente's ideas, thereby increasing understanding of the human blood circulation system.

William Harvey (1578-1657) - described systemic circulation via arteries & veins

The English physician William Harvey used his learning from Fabricius ab Aquapendente, especially Aquapendente's discovery of valves in the veins to investigate further into how blood flows. (The structure of the valves in veins is consistent with their function, which is to support the flow of blood in one direction through the system of blood vessels, blood flow in one direction only being Harvey's important contribution to understanding of the circulatory system.)

Harvey's investigations into blood flow - He investigated blood flow by

  1. Blocking an artery by tying a cord around it.
    This caused the side of the blockage closest to the heart to swell-up due to the accumulation of blood that could not pass along the artery due to the blockage (tied cord).
  2. Blocking a vein by tying a cord around it.
    This caused the side of the blockage away from the heart to swell-up due to the accumulation of blood that could not pass along the artery due to the blockage (tied cord).

How would the evidence from (a.) and (b.) be explained today ?

  1. Re. Direction of blood flow along arteries
    Blood accumulated on the side of the blockage closest to the heart, therefore blood passing along the artery is coming from the heart.
  2. Re. Direction of blood flow along veins
    Blood accumulated on the side of the blockage away from the heart, therefore blood passing along the artery is moving towards the heart.

These deductions may seem obvious today but that was not the case when the observations were first made in the 1620s because at that time physicians were taught Galen's theories, as outlined above.

Calculations of blood flow

Harvey calculated the amount of blood pumped by the heart in one hour. He found that the amount of blood pumped by the heart was equivalent to three times (3x) the weight of the whole person. However, that seemed strange because the size of the body did not change significantly. It certainly didn't increase by a factor of three during the time period of one hour.

One explanation for the amount of blood pumped by the heart without significant change in body size was that the heart created this amount of blood in an hour, while another organ destroyed approximately the same amount of blood during the same time period. It is now understood that blood is not created by the heart but until at least the late 1620s this type of 'explanation' was considered reasonable and plausible. However, William Harvey did not think that so much blood could be created and destroyed so quickly, so he proposed the new idea that "blood must move around the body in only one direction".

William Harvey published his theories about blood circulation in a book in 1628. Initially his work was not well-received by the scientific and medical community of the day. Instead, his ideas were ridiculed because they conflicted with the, then 'established' explanations proposed by Galen.

One problem that remained was that Harvey could not explain how blood reached the veins from the arteries.
Remember that by then it had been proposed that the heart pumped blood outwards - along the arteries, and that blood then travelled back to the heart along veins. Therefore the route by which blood passed from arteries to veins was an important 'missing link' in the understanding of blood circulation.

Marcello Malpighi (1628-1694)

Discovery of capillaries

In 1661 the Italian physician Marcello Malpighi studied a bat's wing under a microscope and observed connections between the arteries and the veins in the bat's wing. The connections were tiny vessels that could not be seen with the naked eye. Blood could be seen flowing through these vessels, which are now known as capillaries.

Malpighi's observations of these connections supported Harvey's theory by demonstrating how blood flowed from arteries to veins in the bat's wing and therefore how it might also flow from arteries to veins in other animals, including people. By showing how blood flowed from arteries to veins in one case (that of a bat), Malpighi's work indicated that Harvey's theory of blood flowing in only one direction could be true for bats. Although that did not, on its own, show that Harvey's theory was correct in humans and other animals, it did increase the credibility of the idea that blood might flow in only one direction by showing how that might be possible.

See also brief notes about the history of 'Western' medicine and the histories of some specific therapies, including the history of aromatherapy, the history of Bach flower remedies, the history of homeopathy and the history of reiki (traditional story of reiki).

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