Circulação sanguínea

In this episode, the supertroupers rehearse a play by Molière, "Le malade imaginaire", about the discovery of blood circulation in the 17th century. A discussion leads the drama teacher to ask the 4 characters to inquiry about the history of blood circulation. They decide to go to the library of the father of one of them, who is a doctor. After consulting several books and the internet, they identify a number of scientists who have been interested in the subject since Antiquity. The arrival of the father led to a new discussion on the controversy between the opponents and supporters of Harvey's discovery of blood circulation in the early 17th century. The opponents defended Galen's irrigation model. The main players in the discovery of blood circulation are identified and shown how they paved the way for Harvey to propose his model of blood circulation.

The aim of this episode and the associated teaching materials is to get pupils thinking about the two explanatory models proposed, the irrigator model and the circulator model. It is also an opportunity for them to become aware of their own explanatory model and its limitations.

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Pupils' initial conceptions correspond to the knowledge and representations that pupils mobilise when faced with a subject, whether or not it is taught. These conceptions are linked to the fact that each person forms a representation of the world around them based on their personal experience, their culture and their schooling. These conceptions are not there ex nihilo but are the response to a problem, a question posed. These conceptions are interesting to note in class because they are often erroneous and can reveal the presence of obstacles that can prevent learning if they are not taken into account. On the subject of blood circulation, one of the main obstacles is that of irrigation, which leads pupils not to envisage blood returning to the heart. This is not always easy to spot, as pupils frequently use the term circulation to mean something other than a closed circuit. More often than not, it is associated with the idea of movement.... Although the historical and educational contexts are different, work on the different historical models should enable pupils to work on their own representations and overcome this obstacle.

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Prototypical examples of students' initial conceptions

The two ideas put forward leave no doubt, either in the diagram or in the text, as to how they see the blood flowing through the body. But many of the pupils' productions are less explicit. Only the direction of the blood flow represented by the arrows makes it possible to categorise a pupil's concept with any certainty. However, the other clues, especially when they are numerous and concordant, can enable a probable categorisation to be made. Here is a list of clues to look for both in the text and in the diagram to analyse students' ideas about blood circulation.

Indications of an irrigation consception

• One-way or two-way blood flow
• Vessels open to the periphery
• Only one type of vessel reaching the organs
• Presence of only one type of blood

Indications of a circulatory conception

• One-way blood flow, returning to the heart
• Existence of two types of vessels in the organs (entry and exit)
• Existence of continuity between vessels in the periphery (directly, via other vessels, via organs)
• Several types of blood are considered (rich in 02, CO2, glucose, etc.)

Blood circulation was a concept that took a long time to develop. Although the movements of the heart and blood have been studied since ancient times, it was not until the 17th century that blood circulation was discovered by the English physician William Harvey. In fact, since the second century, the path of the blood has been envisaged according to an irrigator model: blood is distributed from the heart via the veins and arteries to the organs where it is consumed. Despite important discoveries such as the passage of blood through the lungs between the two ventricles (in the 13th century in the Middle East, in the 16th century in the West), this model was not challenged by the work of Harvey (see historical milestones).

The Grandiloquent episode 1 comic strip evokes the great names of the scientists associated with this discovery. It has two major aims:

Problematise blood flow in the body

The comic shows that there was a major controversy in the 17th century between Harvey's supporters, the circulators, and his opponents, who defended Galen's irrigation model. This controversy is repeated in Molière's play that the children worked on in episode 1. The comic does not explicitly describe the two models in question. It is designed to encourage pupils to ask themselves: what is irrigation? What does it mean to circulate? It aims to get pupils thinking about the possible models for blood movement in the body. A debate between the pupils on the two models mentioned in the comic allows them to think about their own explanatory model and to consider the limits of each. Combined with other resources (see the section on additional resources), the comic strip can highlight the need for a circulatory model.

The construction of scientific knowledge

The comic strip associates a number of scientists with the discovery of blood circulation. Indeed, the work of his predecessors strongly influenced William Harvey's work. Alongside these earlier discoveries, experiments and calculations are mentioned. The development of biological knowledge is therefore not reduced to experimental proof. Finally, the controversy between circulator and irrigator shows the importance of the scientific community in validating a new discovery. All these elements make it possible to approach the construction of scientific knowledge with pupils.

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Aristotle (384 BC - 322 AD): Greek philosopher

He imagined three ventricles in the heart and made no distinction between veins and arteries. He established that all blood vessels originate from the heart. For him, blood is consumed by the organs, which transform it into flesh. He proposed a pathway for blood in the body from the heart to the organs, based on an analogy with the concept of irrigation.

Galen (129-210): Greek physician

He demonstrated the presence of blood in the arteries and gave a precise description of the structure of the heart (atria, ventricles, valves) and its movements. He described a dual system for distributing blood to the organs: in the veins from the liver and in the arteries from the heart. Venous blood is dark and contains food, while bright red venous blood is rich in vital spirit. The liver is seen as the blood-forming organ: it makes blood. The blood leaving the liver is sent partly in the veins to the organs and partly in the right ventricle. Some of the venous blood then passes into the left ventricle and mixes with the inspired air: it is transformed into vital spirit and then distributed to the organs with the arterial blood. This communication between the two ventricles takes place through invisible pores in the interventricular wall. This idea would prevail for many centuries.

Ibn Al Nafis (1208-1288): Syrian physician practising in Cairo.

He is known for having described what we now call the small circulation in his Anatomical Commentary on Avicenna's Canon. Blood is sent from the right ventricle to the lungs via the pulmonary artery before returning to the left ventricle via the pulmonary vein. Despite the importance of this discovery, the irrigator model was not called into question: arterial blood flows from the heart to the organs and venous blood flows from the liver to the organs. Ibn Al Nafis' writings were not disseminated in the West.

Michel Servet (1509-1553): Spanish physician, anatomist and theologian

He wrote several controversial books on theology. In 1553, in Restitutio christianismi, he described small circulation. Based on dissections, he explained the large calibre of the pulmonary artery by the large quantity of blood it sends to the lungs. The blood thus mixes with the air and returns to the left ventricle via the pulmonary vein. He was burnt for heresy in 1553 and most of his works were destroyed. As with Ibn Al Nafis, it was the passage of blood through the interventricular wall that was called into question, but not the irrigator model.

Vesalius (1514-1564): Flemish anatomist and physician

A convinced Galenist, his numerous dissections nevertheless led him to question the passage of blood through the interventricular wall: in the second edition of his book De corporus fabrica, he indicated that the partition between the two ventricles was impermeable.

Realdo Colombo (1516-1559): Italian physician and professor of anatomy

In 1558, he published De re anatomica, in which he used dissections to describe the passage through the lungs between the two ventricles. He was the person who disseminated this discovery, which was also made by Ibn Al Nafis and Servet. Nor did he question the irrigator model. Blood is always consumed by the organs.

William Harvey (1578-1657): English physician

His discovery of blood circulation challenged the irrigator model that had been accepted for 1400 years. He was the first to demonstrate that blood flows from the heart into the arteries and back to the heart in the veins. He demonstrated his theory using rigorous reasoning based on dissections, observations and calculations. His main arguments are based on :

• Calculating the amount of blood ejected by the heart into the aorta in a day. This quantity is too large to be contained in all the blood vessels or to be consumed by the organs. The blood reaching the organs therefore has to return to the heart, and it is the same blood that passes through the heart several times over a 24-hour period in a circular movement.
• The publication of his book De motu cordis in 1628, in which he explained his discovery, gave rise to a major controversy between supporters and opponents of circulation.
• Ligature experiments in the arm demonstrate the movement of blood from the heart to the organs in the arteries and from the organs to the heart in the veins.
• Observation of the arrangement of venous valves that prevent blood flowing back to the organs.

The publication of his book De motu cordis in 1628, in which he explained his discovery, gave rise to a major controversy between supporters and opponents of circulation.

Marcello Malpighi (1628-1694) - Italian physician

In 1661, in his work De pulmonibus observationes anatomicae, Malpighi used the microscope to describe the blood capillaries connecting the veins and arteries in the lungs. This discovery, which completed the blood circuit, validated the circulation model and put an end to the controversy.


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Knowing what your organs need

In middle school, blood circulation is seen as a systemic function that links the body's major functions (respiration, digestion, excretion, etc.) and enables organs to function. To understand blood circulation, you need to know what your organs need in terms of nutrients and oxygen.

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