Circulación de la sangre

Episode 1, At the heart of the matter, is based on the history of the concept of blood circulation and the existence of a similar obstacle among scientists and pupils, the irrigator obstacle. The aim of the resources is therefore to explain the main difficulties encountered by pupils when learning this concept, as well as the main historical elements that can help them work through them.

WORK IN PROGRESS (automatic translation, for now)

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.

WORK IN PROGRESS (automatic translation, for now)

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.)

There are also back and forth conception, in which students envisage blood flowing from the heart to the organs and back again via the same vessel.

The aim of this episode is to get students thinking about the concept of blood circulation. Although pupils are familiar with the term blood circulation, they don't always understand it to mean the flow of blood back and forth between the heart and the organs. For most students, the term circulation refers to movement. Some pupils may not even think of blood returning to the heart. The episode is based on the controversy that took place at the time of William Harvey's discovery of blood circulation in the 17th century. It pitted Harvey's supporters, the circulators, against his opponents, the irrigators, who defended Galen's model, which was accepted at the time. For the circulators, blood travels from the heart to the organs via the arteries and returns to the heart via the veins. For irrigators, the blood reaches the organs via the veins and arteries, where it is consumed. No return is envisaged. This was an important controversy at the time, which spread throughout society, as in Molière's Le Malade Imaginaire. This play that the Grandiloquents are working on will lead them to discover the history of the discovery of blood circulation.

Episode 1 has two major teaching objectives :

Problematise blood flow in the body

The strip 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 students thinking about the possible models for blood movement in the body. A debate between the pupils on the two models mentioned in the comic enables 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.

WORK IN PROGRESS (automatic translation, for now)

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 had 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 by Ibn al Nafis, in the 16th century in the West by Servet and Colombo), this model was not called into question by Harvey's work. Although based on numerous observations, experiments and calculations, the circulatory model was hotly debated at the time because it left a number of problems unresolved, particularly that of the passage of blood between arteries and veins. In 1661, Marcello Malpighi used a 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.

Here we present the work on blood movements of the great scientists mentioned in the comic strip.

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.

Here we present a few ideas for using comics in the classroom with pupils. A more detailed proposal is presented in the next section.

• Gather and compare the pupils' initial conceptions, identify the irrigating obstacle.

• Read the comic strip (in full or in part) to pose the problem: what does it mean to circulate?

• Compare Galen's and Harvey's models: how are they different? What are the consequences for organ nutrition? In Galen's model, constant renewal of blood is necessary because it is consumed by the organs. In Harvey's model, oxygen and nutrients need to be replenished. This allows the students to reflect on the functional organisation of blood circulation.

• Bibliographical research: contribution of each scientist to the history of the concept of blood circulation.

• Make a model (scoubidou, modelling clay) of the blood flow according to Galen and Harvey to understand the differences.

• Work on Harvey's calculations (a key element of his demonstration).

• Work on the construction and nature of science: non-linear, pauses, advances, setbacks, etc.

Extracts from the works of William Harvey

The extracts on this page set out Harvey's main arguments for demonstrating blood circulation.

• Extract 1 presenting the path of the blood according to William Harvey

"Considering the great quantity of blood which I found in the vivisections and the openings of the arteries (...), reflecting (...) on the abundance of blood set in motion, on the rapidity of this movement, I wondered whether the juice of the food ingested could suffice to renew incessantly the blood incessantly exhausted. I realised that the veins would be emptied and exhausted, and that the arteries would be ruptured by this continual influx of blood, if the blood did not return by some route from the arteries to the veins and return to the right ventricle of the heart. I recognised that the blood leaving the heart was thrown by the contraction of the left ventricle of the heart into the arteries and all parts of the body, just as by the contraction of the right ventricle, into the pulmonary artery and the lungs. Similarly, passing through the veins, it returns to the vena cava and into the right atrium, and passing through the pulmonary veins, it returns to the left atrium. We can therefore call this movement of the blood a circular movement...".

Extract from De motus cordis - William Harvey (1628).

• Extract 2 on calculating the quantity of blood passing through the heart

"Let us admit by reasoning or by experience that the left ventricle, dilated and filled with blood, contains one, two or three ounces of blood: I myself have found more than three ounces on a corpse. We can admit that the heart loses a certain quantity of blood by contracting: in fact, the ventricle, by constricting, contains less blood than before: thus a certain quantity of blood passes into the aorta artery. (...) Thus, in man, we suppose that with each contraction of the heart, an ounce or three drachmas of blood passes into the aorta. This blood cannot return to the heart because of the obstacle posed by the valves. In half an hour, the heart has more than a thousand contractions; in some people it even has two, three or even four thousand. Multiplying by drachmas, we see that in half an hour three thousand drachmas, or five hundred ounces, pass through the heart into the arteries; in short, a much greater quantity than could be found in the whole body (...). Thus, by calculating the quantity of blood that the heart sends out with each contraction and by counting these contractions, we see that the entire mass of blood passes from the veins into the arteries via the heart and also via the lungs. What's more, let's not take half an hour, or an hour, but a day: it's clear that the heart, by its systole, transmits more blood to the arteries than food could give, more than the veins could contain."

*Note: An ounce corresponds to 28.35g and a drachma to 1.77g.

Extract from De motus cordis - William Harvey (1628).

• Extract 3 on the direction of blood flow in veins and arteries

"If we cut a fairly large artery and the vein that accompanies it, we can clearly see that the part of the vein that is close to the heart does not give any blood, whereas blood, and only blood, flows from the other part (...). On the other hand, in the case of the artery, little blood flows from the peripheral part, while from the other side, as if from a siphon, an impetuous stream of pure blood gushes out. This experiment shows where the blood comes from and where it goes when it circulates through the parts. We also see that it flows rapidly, that it is animated by an impetuous movement and that it does not flow slowly and drop by drop".

Extract from Two anatomical dissertations on the circulation of the blood addressed to Jean Riolan - William Harvey - (1649)

Diagrams of the various historical models

The works of Galen and Harvey do not contain diagrams of the blood flow in the body. The two diagrams below are didactic versions of their two models. Although the liver plays a central role in Galen's model, it has been left out so that students can focus on the direction of the blood flow between the heart and the organs.

A model of blood circulation to build

Here we propose a possible teaching scenario for a cycle 4 class. It is based on the various resources presented in the other sections. This sequence can be planned over two or three sessions.

Phase 1 - Gathering the pupils' initial ideas about the path of the blood

This work can be done before the lesson on blood circulation, which can give the teacher time to carry out the analysis.

Instructions given to the pupils: How does blood nourish the organs? How does blood travel through the body? Present your answer in the form of a text and a diagram.

The pupils can be given a silhouette of the human body to make the work easier.

Phase 2- Introduction to the session

Read chapter 1 of the comic strip. This introduces the problem with the pupils: what does it mean to circulate?

The pupils (3-4) are put into groups with their work from phase 1 and discuss it with each other.

Phase 3 - Reading chapters 1 and 2 of the comic strip

Presentation of the diagrams of the two historical models (without the names) and comparison with the pupils' models.

Instructions given to the pupils: Explain the differences between the two historical models and compare them with your own productions. What are the consequences of each model for organ nutrition ?

Phase 4 - The history of blood circulation

Problem worked on: what discoveries were made between Galen's model and Harvey's model ?

Working in groups, the pupils look for a discovery made by a scientist about blood circulation before Harvey (Ibn Al Nafis, Colombo, Vésale).

Instructions given to pupils: identify the discovery made by a scientist mentioned in BB

Create a timeline showing the various discoveries. This can be used to initiate a discussion with the pupils on the construction of scientific knowledge.

Phase 5 - Justifying Harvey's circulatory model

Reading of extract 1 from Harvey

Problem to be addressed: what arguments does Harvey use to justify his circulatory model?

Reading of extract 2 from Harvey.

Task given to the pupils: Calculate, in kg, the amount of blood leaving the heart in an hour and a day.

Reading of extract 3 from Harvey.

Task given to the pupils: Identify the direction of circulation in the veins and arteries.

Phase 6 - Review

Problem worked on: How does blood circulation enable oxygen and nutrients to be constantly supplied to the organs ?

Make the model with the pupils.

Debriefing with the pupils: return to Harvey's model where the functional aspects are not present.