Models (14–16): Circulatory system

Circuits and blood vessels

Most students refer to “veins” when what they mean is “blood vessels”. They haven’t yet distinguished between forms of blood vessels. As I explain in Difference Maker and Teaching Meaning, distinguishing is the essence of meaning making; seeing differences that make a difference and which trigger us to recognise things we previously hadn’t perceived.

The meaning of the blood vessels, therefore, isn’t just in their differences, but what difference those differences make. In other words, how they contribute differently to the whole circulatory system. The lesson begins with the direction of flow and the idea of the closed circuit. In this circuit, a variety of functions are carried out; the most important being bulk transport and diffusion. We’ll need, therefore, a variety of structures fit for the job. The final and most important piece is asking students where they expect capillaries to be in the body. They see the circuit diagrams and say things like “in the legs”. It’s important not to move on until everyone understands why capillaries are found everywhere.

Circulatory system function model

The previous lesson focused on blood vessel structure and function, and the system as a whole fell into the background. Now that students have distinguished the blood vessels, the system comes back to the foreground. Following my typical moves, I begin with the stocks and label them before asking students to name the flows. Then I add the variables and ask students to name them and decide whether the connection is + or –. (See how to teach with stock and flow diagrams in my book Difference Maker).

During this process, we check if the model can explain lived experiences. For example, when running and suddenly stopping and feeling lightheaded. The model suggests that, when running, heart and muscle contractions keep blood flowing rapidly. But when suddenly stopping, the heart continues beating quickly while the skeletal muscle contractions stop. Blood, therefore, is still flowing to capillaries rapidly, but is now returning much more slowly and accumulates in the extremities. Less blood reaches the heart and is available to be sent elsewhere. After these discussions, I add the flows of gas exchange and ask students to label the flows.

Components of blood

Much of this model is descriptive: what’s in blood? Explanation comes as we explore the reasons for the structure of red blood cells. This requires contrasting enucleated mammalian red blood cells with nucleated red blood cells of the other vertebrates. And, asking why other possible shapes didn’t evolve. As I explain in Biology Made Real and Difference Maker, these are evolutionary questions. We don’t ask why something is the way it is; we ask why the other possibilities didn’t make it.

The heart

This model requires two lessons. In the first lesson, we draw the picture on the right and name all the parts that students need to know to be able to discuss the heart. Then we focus on the key distinctions: the atria, ventricles, and valves, by asking “What if?” questions. For example, what if this part weren’t here? What if it were weaker? The meaning, therefore, comes from asking students to think about possible variations: a key move I explain in Teaching Meaning.

In the second lesson, we build the stock and flow diagram on the left. Following the usual moves explained in Difference Maker, I begin by drawing the stocks and flows and label just one of the stocks. I then ask the students to identify the others. We don’t name the flows here, so I ask students to identify the variables that affect the flows. To help, I draw a red and green circle (without a label) next to each flow. Then we discuss as a class their ideas and figure out whether they make sense. For example, does it make sense that valves are causes of blood flow? What if the valve weren’t there?

Coronary heart disease

The end goal of this lesson is to understand the positive feedback loop that drives heart attacks. Yet, one of the largest problems my students have is firstly understanding (and remembering) that the coronary arteries supply blood directly to the heart. Therefore, the first half of the lesson is spent building the stock and flow structure and establishing where the coronary circuit is in the body. And, crucially, making sense of why the heart needs its own supply when blood is already flowing through its chambers.

Adaptation to altitude

I begin by asking students if they would win a running race in Nepal against a class of students from Nepal. Most agree that they wouldn’t; the lesson is about why and for which we need a good model. This is a model we first constructed in a previous topic on gas exchange. Therefore, I draw the stock and flow structure and have students fill in what they can from memory. Then we discuss short and long-term adaptations. Note that the feedback loop runs both ways. Athletes who train at altitude will have lower blood oxygen, causing structural adaptation. This adaptation causes blood oxygen to rise again, a negative feedback loop. Yet, when they return to sea-level, blood oxygen levels will now be high, which causes the production rate of red blood cells to decrease, so they return to their previous state.

Discover how to teach this way in my books: