by James Morris
Illustration by Sara Haidermota
A few summers ago, I was driving my two sons to a trail head for a hike in the Adirondacks in upstate New York. They were going with a group of kids up a mountain called Iroquois.
“Urine?” they asked incredulously.
I am an author of an introductory biology textbook. That day, I started writing a chapter on the kidneys, which produce urine. One of the many challenges that I encounter is how to make the material interesting and engaging to students of many backgrounds and experiences.
Since my kids showed some interest, or at least disbelief (which I took as a sign of interest), I decided to pursue the topic with them to see if I could find a hook or something that would engage them that I could then use to begin the chapter.
“How do you think I should start?” I asked them.
I tried a few possibilities. “Did you know that the kidneys don’t just remove waste, but also are important for water and salt balance?”
That didn’t do anything for them.
“Did you know that the kidneys produce a hormone that promotes red blood cell production?”
I clearly wasn’t getting anywhere with them.
“Did you know that when you break down proteins and other molecules, you end up with ammonia, which is toxic. Ammonia must either be excreted, or converted to something that is less toxic and then excreted?”
They started putting on their headphones. I had to think quickly.
“Ok, how about this? Have you heard about Gatorade?”
They paused and looked up.
“Well, Gatorade was invented at the University of Florida in the 1960s as a new sports drink to help (aid) the football team (the Gators). Instead of just using water and sugar, the researchers realized that electrolytes, like sodium and potassium, are also important, since water and electrolytes are both lost in sweat. Some credit Gatorade with the Florida Gators’ win over their rivals, the Georgia Tech Yellow Jackets, and the rest is history.”
“Not bad, Dad.” Then the headphones went on. But at least I had them for a couple of minutes.
So that’s how the chapter starts.
It’s not a bad beginning, but as I began to read about the kidneys and think about the chapter, I became more and more fascinated with the kidney itself. In many ways, it doesn’t need a hook at all.
Consider the job that the kidney does. We have two ways to eliminate wastes from the body – some in the form of feces and some in the form of urine. Why two and not one? And aren’t they about the same?
No, they aren’t. If you think about it, you will note a key difference. The gut is a long tube that courses through the body and is open on both ends. In other words, it is continuous with the outside world. Therefore, it is relatively easy to take in food, extract what’s needed, and then simply leave the rest behind, so to speak.
The kidneys have an entirely different task, and a different challenge. They remove wastes that end up in the blood, inside the body. How do wastes get into the blood in the first place? They either come from food we eat, absorbed by the gut into our bloodstream. Or, they are a breakdown product of proteins and nucleic acids. Ammonia is formed, which in mammals like us gets converted to urea and excreted in urine.
In addition to wastes, the blood has all kinds of important components, like red blood cells to deliver oxygen to tissues, cells and antibodies of the immune system to fight infections, proteins for blood clotting or transporting other molecules in the bloodstream, and sugar for energy. All of these ingredients that the body needs get mixed up with wastes that must be removed.
So the task of the kidney is to separate these essential elements from the waste, the wheat from the chaff. That’s a tricky business.
You might imagine that the simplest way to do this is to actively secrete the wastes into the ducts of the kidneys, leaving all of the good stuff behind. While the kidney does do this to some extent, this isn’t the primary way that wastes are excreted.
Instead, the kidneys go about excreting waste in what might seem like a counterintuitive way. There is a filter between the blood and kidneys. This filter is, well, a filter, so it lets some things go through but not others. It blocks large proteins and cells, but lets through everything else. Everything else includes wastes, but also water, electrolytes, sugar, and other key substances.
In other words, it is not a very good filter at all. It’s bit like a strainer you might use to remove pulp from orange juice. Yes, it removes the pulp, but what you have when you are done is still orange juice.
Then what? We now have a filtrate of blood that includes both wastes and essential elements. So the kidney now has to reabsorb all of the essential elements back into the bloodstream.
Why go through all of this trouble? Wouldn’t it be simpler to secrete just the waste, and hold on to everything else? Yes and no. It seems simpler, but it might not work as well, for two reasons.
To secrete waste, you would need to be able to recognize it and pump it out in some way. This is fine if the waste is something you are already familiar with, like urea. But what happens if it’s something novel, something you haven’t seen before, like a poison from a mushroom you just ate? In this case, our kidney has an advantage. By basically discarding everything but then reabsorbing the things we need, the kidneys are able to get rid of any substance that the body doesn’t want or doesn’t recognize.
Another reason goes back to my first response to my kids. The kidneys are remarkable in that they don’t just excrete waste. They are involved in lots of other functions, one of which is the control of the amount of water and electrolytes in the body. By letting them pass through the filter more or less freely, but then selectively reabsorbing them, we can fine-tune the amount of water and electrolytes in the bloodstream, which is important for hydration, blood pressure, and the like.
Pretty interesting, isn’t it? Or maybe I should just stick with the story of Gatorade?
© James Morris and Science Whys, 2017