How I introduce the digestive system

In my view, the teaching of the digestive system isn't just another topic of here's how the body works, but another opportunity to establish identity—thinking about what we and others are.

The fundamental insight to the digestive system is how it functions to meet the needs of the whole organism. To address this, we must first begin with the whole, but we must contrast it with other systems—we have to see what we're not to see what we are.

Contrasting fungi, Hydra, and humans.

First, I drew the fungus and established the term of secretion. As this example is very similar to the previous, I moved on to include Hydra.

I searched for images of Hydra on google to show how these species—as with other cnidarians—have a primitive digestive system without an anus. I made explicit how the examples of fungi and Hydra, show the same concept.

But here, there are more questions.

Fungi and animals are both heterotrophs, but one has an digestive system, while the other doesn't? Why?

I had students suggest why it was beneficial to have a digestive system if you could move, and therefore take a digesting meal with you. Other answers I received were that animals can be less vulnerable while digesting food, and that they could have specialised organs for digesting. These discussions took maybe five minutes.

Finally, I added the diagram of the animal with an anus and the interesting question now pertained to the ecological evolution of the anus.

Some students came to the answer—with more or less guidance depending on the class—of how an anus allows several meals to be processed at once, at different stages. Without an anus, it's one meal at a time.

They also saw how Hydra's simple system was a generalist. Having a mouth and anus has favoured the evolution of specialist organs that can process foods in different ways in sequence. With this in mind, we moved on to the human digestive system.

The key from here was to have students see the fundamental pattern—of secretion, digestion, and absorption—in the digestive tract. We began with an iconic drawing of the digestive tract and noted where secretion of enzymes occurred, and where absorption happened.

However, often students can get lost in the idiosyncratic details of organs. I wanted them to see the pattern in an abstracted form: a diagram of the system, which clearly shared my understanding of its structure and principal functioning.

I began by drawing the principal stocks and flows: mouth, stomach, small intestine and large intestine. However, I only named the first one: nutrients in the mouth and asked the students to name the others.

I then focused on the glands. I drew the stocks and flows and asked students to name them using the previous diagram to help them.

Finally, I added the stock of the blood and asked students to identify it before labelling it myself. This completed the diagram and I was able to make explicit the pattern of secretion, digestion, and absorption in the human digestive tract.

From here, I asked students to carry out self-explanation, one of Fiorella and Mayer's eight generative learning strategies (2015). This activity involves students explaining the diagrams to themselves as if they were explaining it to a classmate who had missed a lesson. I then helped when students realised they had missing information. This is teaching with diagrams and dialogue; do you want to co-construct meaning without lecturing, slide decks, or leaving students to discover for themselves? Learn how and why in my books. Download the first chapters of each book here.

References

Fiorella, L., and Mayer, R. 2015. Learning as a Generative Activity: Eight learning strategies that promote understanding. UK: Cambridge University Press.