The nephron is a tricky concept for many students but it doesn't have to be. In this post, I explain how I've taught it recently, without any slides, to 16-year-olds.
We started with images of the kidney and noted the key distinctions in a small diagram.
We then drew a stock and flow model together. The intention was for students to see the general idea of the system before we got into the concrete biological details.
Seeing the system
I asked the students if the model made sense and let them talk in pairs. At this point it doesn't; it suggests that everything is filtered out of the blood and urinated away. To move, I told them, we'd need to understand what happens during the filtration.
I asked my students about their kitchen colanders (filters) and how they worked. They agreed that they separate by size.
If the nephron filters by size, what passes, and what doesn't? Cells and large proteins remain in the blood plasma, while all small molecules – water, urea, ions, toxins, glucose and amino acids – pass into the nephron.
I then asked the students again if the model made sense. We'd made a crucial distinction, not everything is filtered – some remains in the plasma – but the model suggests we filter out desirable molecules such as amino acids.
I told the students that we do filter these out and must reabsorb desirable molecules – one reason the kidney is so metabolically hungry. We added the "reabsorbing" flow to show this:
What a strange system, they may think. We filter out most of what we want to retain. Why don't we just filter out what should be excreted?
To explain this to students, I must tell them about the law of requisite variety – something I discuss frequently in my lessons (explained in Difference Maker).
Requisite Variety
A system’s variety of options must match the variety of situations encountered.
Requisite Variety: applied to the nephron
An excretion system’s variety of filtration options must match the variety of things to be excreted.
To understand this, I needed to show how the idea could vary – how it could be different (i.e. using principles of the variation theory of learning).
Imagine a human hunter-gatherer population 40,000 years ago. They fit their environment well and persist but soon migrate to another area. Here the food is different and they eat some new berries. These new berries contain a toxin the humans need to excrete, lest it accumulate in their bodies.
If the nephron only filtered out what it wanted to excrete, it would need to be highly specific. It would require evolving protein pumps to remove specific molecules from the plasma. That would be fine for things like urea, which are always present.
It would be much cheaper energetically, but as soon as something new appears in the body, its excretion would be impossible.
In other words, if you only had five protein pumps for filtering the blood, and then came across a sixth molecule, you wouldn't have the variety of options available to persist in that niche.
Our system is better as it gives great flexibility. By filtering by small size, it provides infinite variety of filtering options. We can migrate to new environments, eat new foods, and take new medicines. We just have to reabsorb what we want, and this remains stable over time – we'll always want glucose and amino acids.
Teaching the concrete parts of ultrafiltration
I showed students images of the glomerulus, Bowman's capsule, and the rest of the nephron. We then drew the site of ultrafiltration:
We needed a mechanism for the filtration process, so I showed students images of podocytes and discussed the glomerulus' fenestrations. With high blood pressure passing over these filtration slits, the blood is filtered. We added this to out model.
"+" : More X, more Y; or less X, less Y.
"–" : More X, less Y; or less X, more Y.
We this variable, we can see that the lower the blood pressure, the less is filtered and vice versa.
Teaching the concrete parts of reabsorption
I provoked my students with questions: "What would you expect for the reabsorption? Protein channels or protein pumps?" I had them vote and then justify their reasons (I explain how to do this in Difference Maker).
Protein channels would allow diffusion, but an equilibrium would be reached that would limit how much could be reabsorbed. To go further, active transport is needed.
I provoked them again, "Considering their role in absorption, what traits would you expect of the cells in the proximal convoluted tubule?" We discussed: high surface area, short diffusion distance, many protein pumps, and mitochondria (to provide the ATP).
We drew it together and looked at images of the cells.
We could then add the final variables to our model – making explicit the causal nature of the system. In other words, we can show exactly what causes what, without getting lost in the biological images.
In the last phase of the lesson, I provoked students to act on potential variation, which helps the make meaning of the model. I asked them, and we discussed:
What if the surface area were smaller, say, without microvilli?
e.g. The reabsorption rate would decrease, and some desirable molecules would be found in urine.
What if the diffusion distance were longer, say, two cells thick?
e.g. The rate at which molecules returned to the blood would decrease, possibly causing homeostasis issues (but shouldn't affect the active transport out of the nephron).
What if there were less active transport pumps, or, like in diabetes, the pump's substrate was in excess?
e.g. The pumps would be saturated (would all be occupied at one time) and desirable molecules would appear in urine.
What if, like in glucose in diabetes, the pump's substrate was in excess?
e.g. The pumps would be saturated (would all be occupied at one time) and desirable molecules would appear in urine.
Learn more about teaching through diagrams and dialogue – without lecturing, slide decks, or leaving students to discover for themselves – in my book Difference Maker. Download the first chapters of books here.
Final co-constructed diagram:
