Thanks to Brilliant for supporting this episode of Journey to the Microcosmos.

Go to Brilliant.org/microcosmos to learn how you can take your STEM skills to the next level this year! Every living thing on this planet has water in them.

Some may have more than others. And some, like our desiccated tardigrade tuns, may be able to survive long periods of time without water. But while those tardigrades may be alive, they’re not quite living.

They’re taking a very protective nap, and they’ll wake up only when water reappears in their life. Water’s chemical structure is the basis of life itself. It shapes the interactions of biological molecules, maintains the shape and structure of cells, and drives reactions that power our entire world.

And so, inside each of us, is an ocean. And that…is why we pee. Getting rid of waste is a complicated, messy business to untangle, and boiling it all down to a simple matter of pooping and peeing is doing a disservice to the variety of organismal junk around the world.

But we are just a humble YouTube channel who has made one episode about how microbes poop. And so now, it feels necessary to give equal due to how they pee. The goal, whether solid or liquid, is the same: removal.

There are things in a body that must come out. But we cannot just wantonly dump things out. That would be like sweeping any small item in your house into a trash bag.

Sure, you get rid of some dust, but you’re also potentially, throwing your wedding ring into the dumpster. In our bodies, we turn to kidneys to filter out toxins and ammonia waste while reabsorbing metabolites. What doesn’t get reabsorbed gets released as urine.

But even simpler organisms need to be able to regulate the water and metabolites inside them. For one, water is a physical thing: it takes up space and creates pressure inside a cell, which is good...up until a point.. And once you get past that point, organisms can end up bursting, like this poor microbe here.

And aside from the terrifying thought that you could water yourself into bursting, there’s the matter of maintaining the right balance of life-sustaining molecules and getting rid of waste. So parameciums and some other single-celled eukaryotes rely on a set of organelles called the contractile vacuole complex that are connected to the outside through a pore. The contractile vacuoles go through cycles of filling and expelling.

As the vacuole fills up with fluid from inside the organism, the pore closes up--keeping everything inside the vacuole. And when that’s done, the pore opens back up. You know, for the peeing.

So there are simple and effective methods on the one end, and the complex organs on the other. So, what comes in the middle? Well, let’s take a look at this Asplanchna.

It looks weird, even for a rotifer. And at around half a millimeter long, it’s on the large size for what we normally see. But it’s still a microscopic metazoan, which means that unlike our single-celled eukaryote friends, its body has organs.

Some of these organs seem roughly familiar, like the Asplanchna’s stomach. And the space around those organs is not empty. It’s full of fluid, giving the rotifer something to monitor and regulate for the sake of its own survival.

To do that, the Asplanchna and other simple multicellular organisms like it rely on a structure called the protonephridium, which begins--oddly enough—at the end: with a set of hollow cells called terminal organs. In Asplanchna or other similar animals like this Polyarthra, those terminal organs are referred to as flame cells because of the flickering movement inside them created by bundles of cilia. Some organisms replace those cilia with flagella, and their terminal organs are called solenocytes instead.

The Asplanchna’s flame cells draw in fluids, acting as an initial filter that blocks the larger molecules from passing through. The cells use their cilia to send the fluid down a coiled tube. But at that point, there still might be some important small molecules that the organism could use in that mixture, like certain sugars or amino acids.

And so as the fluid travels through the tube, those molecules get selectively reabsorbed into the rotifer. What remains in the protonephridium is the urine, which gets collected in the bladder, drained into the cloaca, and then released into the world. A fun sidetone: cloacas act as a sort of multi-purpose opening.

In the case of the rotifer, yes, it is used for peeing. It’s also used for giving birth. So maybe you’re swimming in rotifer pee, or maybe you’re swimming in rotifer babies.

Whichever one it is, it all came from the same place. As we mentioned, other organisms have protonephridia as well, including flatworms. Some segmented worms, or annelids, also have a protonephridium, while others have a metanephridium.

The principle is the same: they take in fluid, and then modify it to reabsorb what’s necessary for the organism and remove what isn’t. But the metanephridia connect directly to the worm’s cavity through a ciliated funnel. The remarkable thing about waste is that, like water, we all have to deal with it.

It is every bit as much a part of life as the living itself is, proof that our bodies have done something, consumed something, made something. The only thing more remarkable is that even when the basic principles of waste management stay the same, the end results can look so different. Thank you for coming on this journey with us as we explore the unseen world that surrounds us.

And thank you again to Brilliant for supporting this video. Sure, we might’ve spent the last several minutes talking to you about bodily fluids, but with Brilliant, you can actually learn about the physics of how toilets flush after you put those fluids into them. In the course Physics of the Everyday, you’ll Investigate everyday physics, from household objects to weather patterns.

Brilliant has courses about science, engineering, computer science and math, and the courses are designed to be hands-on with interactive quizzes and guided problems with explanations. You can access the Daily Challenges for free, but if you sign up to become a Premium member, you’ll get access to the entire archive. And Brilliant courses are now available offline using their iOS and Android app.

So if you’re traveling or have spotty internet connection, you’ll be able to keep learning. If you’re interested in learning more, you can get 20% off an annual premium subscription at Brilliant.org/microcosmos. The people on the screen right now, these people are our Patreon patrons.

They make it possible for us to do this show. So, if you’re thankful for a nice pee episode of Journey to the Microcosmos, they are the people to thank. If you would like to join them, you can go to patreon.com/journeytomicro.

If you want to see more from our master of microscopes James Weiss, check out Jam & Germs on Instagram. And if you want to see more from us, there’s always a subscribe button somewhere nearby.