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It was recently brought to our attention that there's a category here that we have covered quite a lot, and that people have… watched. Here's a selection of our absolute favorite SciShow Pee episodes!

Check out our new channel, SciShow Pee, at

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Hank: Hello, SciShow viewers. I'm Hank Green. It has been a long time since we launched SciShow and it's been a magical time. Thousands of videos uploaded, more than a billion total views on the channel, and then we launched SciShow Space, two episodes a week all about things happening not on our planet. Instead, just everywhere else. And then we went smaller, from the whole cosmos down to just the inside of you, with SciShow Psych. We're always on the lookout for topics that we cover a lot on SciShow. Topics that do really well, like psychology and outer space. Big topics. Topics we love that our community loves.

Well, it was recently brought to our attention that there is a category here that we have covered quite a lot and people have watched, and so without further ado, I'm extremely excited to announce our new channel, SciShow Pee. Since launching SciShow, we have had no less than 14 episodes about pee, totaling more than 5 million views on our channel, but we've been around for nearly nine years now. That's barely two pee episodes per year. This dribbling release was fine for 2012, but it's 2019 now. It's time to turn that leak into a firehose.  

Now, of course, we've already learned why waterfalls make you have to pee and what you can learn from your pee color and what the heck urinal cakes are for. We've discussed the myths of cranberry juice and UTIs, and on SciShow Space, we've even talked about turning astronaut pee into plastic, but lest you think we are done, we are not. We have not even begun to plumb the depths of urine, so get ready for another two episodes every week. Head on over to SciShow Pee right now and subscribe.  

Or you can just keep watching right here for a selection of our absolute favorite SciShow Pee episodes. First, the eternal question: what happens when you hold your pee.  Considering how long we've been holding in this channel idea, it's only a fitting place to start.

Michael: When you gotta go, you gotta go. And by “go,” I mean relieve yourself. You know... of urine. But sometimes -- and we’ve all done this -- you’ll hold your pee to finish one more email, or so you don’t miss the end of a movie, or just because you’re too lazy to walk to the bathroom from the couch. You’ve probably heard people say that that’s bad for you. And it is! But only if you do it frequently and for a really long time. And it almost always isn’t life-threatening.

If you’re an adult, your bladder can hold up to about half a liter, or two full cups, of pee, before you’ll feel the need to... let it go, as Elsa might say. Your bladder wall is lined with receptors that can measure how full your bladder is, and when it’s reached capacity, these receptors send your brain a signal that it’s time to hit the can. Most adults have control over their bathroom urges, meaning you can choose to pee right away after receiving these signals—or to hold it for a bit if you’re not near facilities.

If you do decide to hold it, the cylindrical sphincters in your bladder close up tightly to keep all of the urine from leaking through your urethra. But if you hold it in for long periods of time a lot—like if you’re a trucker who’s on the road for hours on end, for years—then you might find yourself facing some not-so-fun long-term effects, like urinary retention and increased risk of infection. Urinary retention is the inability to empty your bladder completely. Constantly holding in your pee can weaken your bladder muscles, which can be the cause of urinary retention as you age. Your bladder can also become a breeding ground for bacteria if it’s constantly holding large amounts of urine, which slightly increases your chance of getting a bladder or urinary tract infection. Which sounds bad enough, but what if you held it in for longer?

Think about Tycho Brahe, the 16th-century astronomer who supposedly died from a bladder that burst after he held his pee for too long? Well, long before you get anywhere near your bladder bursting, odds are your body will ignore your brain's attempts to hold in the pee, and you’ll just wet yourself. However, in certain very rare, very extreme cases, bladders do burst. When this happens, it’s almost always in people who already had a damaged bladder, like from a pelvic injury -- though there are a few reports of bladders bursting in people who seemed perfectly fine before. In those cases, it’s generally because the person was been drinking a lot, and the alcohol dampened the signal to their brain that was telling them they needed to pee, like RIGHT NOW. But again, it’s incredibly rare. So if you do hold your pee in a few hours longer than you should, probably nothing super-terrible will happen. Unless you consider wetting yourself in public super-terrible. Totally understandable if you do!

Hank: That feels like a relief, especially because sometimes, you have to pee in really inconvenient places, like in the middle of that five hour hike to a waterfall or on a long country road in the middle of a rainstorm, far away from the nearest exit. Actually, come to think of it, really anytime there's running water, there's a good chance you'll have to go, but why? Here is another one from Michael.

Michael: Listen to this. [water flowing] The sound of a bubbling stream can be pretty relaxing. But, can I ask a personal question...? Do you kinda have to pee now? ‘Because if you do, you’re not alone. Just the sound of running water -- whether it’s a leaky faucet, some light rain, or a gushing waterfall -- sends some people running for the nearest bathroom. But why?

Psychologists and urologists, or scientists who study the urinary tract, chalk it up to the power of suggestion. Basically: running water kind of sounds like urination, so just hearing the noise can make you feel like you need to pee. This sort of unconscious association is called a conditioned response, and the psychological theory behind it has been around for a while. In fact, you’ve probably heard of this classic example -- the Russian psychologist  Ivan Pavlov and his experiments with dogs.

Pavlov would set some delicious meat powder in front of a dog, and it would start salivating -- which is a normal doggy reaction to food, or an unconditioned response. And then, he would ring a bell, and then feed the dog. After repeating this for months, Pavlov could ring a bell without offering any meat powder, and the dog would still start drooling. So his pup had learned to associate a specific sound -- the bell -- with a specific act -- being fed, the dog began slobbering -- that was its conditioned response.

This same effect could explain our strange peeing urges. Ever since potty training, we learn to associate the sound of running water with urinating and other bathroom things, like flushing a toilet and washing our hands. So instead of hearing a bell, being reminded of food, and drooling... we hear running water, are reminded of all things bathroom, and need to pee. That’s the theory at least.

The thing is, this phenomenon hasn’t been officially studied in much detail, so scientists aren’t positive that this is the whole cause. But it is a pretty common thing. In fact, doctors have successfully used running water sounds to help prostate surgery patients and people with paruresis [par-you-REE-sis] -- also known as “shy bladder syndrome” -- to turn on the waterworks.

Some researchers even think this power of suggestion isn’t limited to audio cues -- so even just looking at pictures of gushing waterfalls could give you the urge to pee. But that’s probably enough urine talk for now. We’ll wrap it up, in case some of you need to take a bathroom break.

Hank: If you had to take a break after that last video, welcome back. And if you shivered while you were on the potty break, good news. We have made a video about that, too. I told you, we have a lot of videos about pee! Shivering while you pee is totally a real thing, but if you've read a bunch of online articles about why it happens, well, those might not be true. Here is Olivia with more.

Olivia: It's one of those questions that a lot of you have asked: Why do I shiver when I pee? If you haven't experienced this, you're probably wondering if it's even a real thing, or if it's just some big practical joke online. I don't blame you, because urinating and shivering don't seem that closely related. Well, it turns out that shivering after you pee is something a lot of people seem to experience, and it seems to affect males more than females. But nobody really know why it happens. There actually hasn't been any peer-reviewed research on it.

Now, we're not the first ones on the internet to try to answer this question. If you look around, you'll find plenty of articles giving a few possible explanations. Here's the thing though: lots of those articles are wrong. They seem to be getting their information from a letter-response article written in 1994, which itself was based on a discussion about pee shivers a bunch of people were having in a forum -- not exactly a legitimate scientific source.

So, you might've heard people say that these shivers happen because your body temperature suddenly lowers after you lose all that warm urine. But that idea just comes from the discussion forum, and it doesn't really make any sense anyways. You don't shiver when you vomit, even though that's also a case where you're losing a lot of warm fluid.

Even the supposedly-official scientific name for the phenomenon, post-micturition convulsion syndrome, was made up by someone in that forum. So, remember to check those sources before you believe things online.

When journalists have asked excretory system experts about this, they kind of have an explanation: the shivers might have to do with an interaction between two different parts of your nervous system. When you choose to start urinating, you body also lowers your blood pressure. That's the parasympathetic nervous system at work -- the part responsible for involuntary processes that are more about resting, like digestion and lowering you heart rate. But peeing also triggers the sympathetic nervous system, which is involved in other involuntary processes like in fight-or-flight response. It's possible that you get the pee shivers when there's an especially strong interaction between these two responses.

And that would explain why this seems to affect males more than females. When you stand up to pee, your blood pressure will be slightly lower than if you're sitting, which could lead to a stronger interaction between the two parts of the nervous system.

But again, there's no research on this, so it's hard to know for sure. So, get on this scientists! The world wants to know! 

Hank: I can't really blame scientists for not studying why people shiver when they pee. Then again, there have also been studies looking at how long humans and other animals urinate, which isn't any less weird. What's even stranger is that these studies actually found something interesting. Most mammals, including humans, will only pee for 21 seconds. If that sounds too weird to be true, here's more detail.

Mammals, whether they be cats or people or wildebeests, have a few things in common you know, like hair, milk... and peeing for 21 seconds. This is an actual thing!

Lots of mammals take about 21 seconds to empty their full bladder. It doesn’t matter if their bladder contains 5 milliliters of urine or 18 liters. 21 seconds seems to be the magic number. And that’s thanks to a combination of physiology and fluid dynamics.

The time it takes to tinkle depends on the amount you need to expel and the flow rate — how fast you expel it. That rate, in turn, depends on things like the width of your urethra — the pipe that delivers urine from the bladder to the outside world — and the pressure pushing the pee out. And it just so happens that those pipes are fairly proportional: that is, the larger the animal, the longer and wider the urethra.

The pressure created by an animal’s bladder muscles is remarkably constant— about 5.2 kilopascals, if you were wondering. You know you have that number in your brain. I'm sure it'll stay there for a long time.

But the overall pressure pushing the urine out also depends on both that and the urine’s hydrostatic pressure: the pressure exerted by the fluid itself. So if you're a horse, there's a lot of weight in the bladder, and all of that mass is pushing down on the urine at the bottom and so it gets pushed the same amount by the bladder but much more by the hydrostatic pressure, whereas if you're a cat you have less liquid pushing down on the liquid and the same amount of bladder pressure but less hydrostatic pressure. All of that means that urine in a large animal is being pushed out more forcefully through a tube that’s proportionally wider. That makes it flow more quickly, which makes up for the difference in volume.

We know all this because some researchers actually did the math in a 2014 paper in Proceedings of the National Academy of Sciences. They also filmed and watched a bunch of animals pee to validate their results, which, you know, that's science!

There are a few caveats, though. The 21 second rule only counts for full bladders in mammals heavier than 3 kilograms. For smaller animals like little bats, rats, or mice, something different happens.

They urinate in a series of quick droplets because the thinness of their urethras and the friction created by the flowing fluid actually pushes up on the liquid, opposing the force of gravity. This makes the urine droplets stick to the walls of the urethra at first, and grow bigger in size until they’re heavy enough for gravity to take over. Then, they fall out quickly— usually, in less than a second.

It also might not apply to aquatic mammals because the water, especially at depth, applies pressure to the urethra and pushes the pee back up the pipe. The same actually goes for swimmers— it takes you longer to pee in the pool. Not that you’d ever do that.

And, to be clear, none of this counts for reptiles or birds because they’ve got a whole different excretion system. Now while this might sound kind of like… silly, to study animal peeing, understanding the physics and mathematics of urine flow actually has some practical applications. Engineers could apply these findings to design systems where reservoirs, water towers or even water backpacks of different sizes take the same amount of time to empty.

And this also suggests that studying urinary diseases in small animals like rodents might not be applicable to humans because their whole peeing process is just fundamentally different than what we do, even though we have the same basic body parts. It just goes to show that some curious research can sometimes lead to useful results.

Okay, so I mentioned in that video that swimmers pee a little differently because of the water pressure, but don't pee in the pool, please, for the love of all things sanitary, just go to the toilet. Urine plus the chlorine makes some really nasty stuff. Just in case you have any doubt, here is Stefan to tell you why it's a terrible idea.

Stefan: If you’ve ever been a competitive swimmer, or just spent a lot of time at the pool – you might have peed in the water once or twice.

Or maybe a lot. According to interviews, Olympic swimmers pee in the pool all the time.

Even though it sounds disgusting, a lot of people say urine is sterile, plus chlorine is a disinfectant. So what’s a little pee between friends… right? Well, you might actually wanna take your business to the bathroom.

For one, it turns out that pee isn’t all that sterile. But there’s another problem, too: Mixing urine with the chlorine in your pool can make chemicals that might cause respiratory and nervous system problems. Urine is mostly water, but it contains a lot of junk your body doesn’t want anymore, including uric acid and urea, both nitrogen-containing molecules.

Uric acid is produced when your body breaks down molecules called purines, like the ones in some of your DNA bases. And urea is made from the breakdown of proteins. When these waste molecules mix with chlorine in the pool that’s meant to destroy bacteria and viruses, they react to form disinfection by-products, or DBPs for short.

Specifically, urea reacts to create a type of chemical called chloramines, swapping out its hydrogen atoms for chlorine atoms. Trichloramine, especially, is pretty reactive, and can corrode the metal in and around pools. And you might know it by its smell – that classic chemical “pool smell” is caused by chloramine gases, not chlorine.

A lot of people, like lifeguards, have reported they get red eyes, a runny nose, or a scratchy voice after being around the pool too much, which could potentially have something to do with irritation from lots of trichloramine. Some researchers think chloramines could cause respiratory problems in swimmers, too, since they probably breathe in a bunch more than your average pool-goer. But we’ll need to do more research to really understand the health effects.

We’ve known about the connection between urea and trichloramine for a while now, but we’ve pretty recently found a link between uric acid and a molecule called cyanogen chloride. Cyanogen chloride gas doesn’t have a familiar smell, but it’s real bad news and can cause respiratory, cardiovascular, and central nervous system problems. It’s part of a group of chemicals called cyanides, which all have a carbon atom bonded to a nitrogen atom.

They’re toxins, and nasty ones at that. These chemicals mess with how your cells use oxygen, so your cells struggle to produce energy, and if the concentration is high enough, all kinds of things can go wrong. In one study, published in the journal Environmental Science & Technology in 2014, researchers created synthetic urine and combined it with various concentrations of chlorine.

And within an hour, the uric acid created some cyanogen chloride. The amount varied based on the chlorine concentration they used, but it was around 2 to 8 milligrams per liter. Now, there aren’t many official guidelines about what concentration of liquid cyanogen chloride is dangerous, but some sources recommend avoiding exposure to more than 0.6 milligrams per cubic meter of the gas form.

So getting 2 to 8 milligrams per liter of cyanogen chloride sounds like a huge deal. But it’s pretty unlikely you’ll ever find that much in your swimming pool, because this experiment used higher concentrations of chlorine than you’d find outside of a lab. So, you probably don’t need to panic if you go to a pool party every once in a while.

DBP levels might be an issue at large swim meets, where hundreds of swimmers are probably peeing in the water. Especially if a lot of people are peeing in the same spot – say, at the foot of the diving board – that area will have higher concentrations of DBPs like trichloramine or cyanogen chloride. And researchers are trying to figure out if long-term exposure to DBPs are related to the unusually high amounts of asthma reported among competitive swimmers, which has been documented in multiple surveys.

For now, it looks like DBPs probably aren’t a life-or-death issue in the pool, although they’re not harmless either. So getting out of the pool, drying off, and going inside to use the bathroom is kind of a pain, but your lungs – and probably your friends – will thank you.

Hank: I warned you.  Just clench those good old bladder muscles and make a run for the nearest toilet.  Everybody will thank you, and thanks for watching this special pee-themed compilation.  If this video was like super your cup of--you can head on over to the link in the description.  Find our new channel, SciShow Pee, available now, two episodes a week, forever.  Don't pay any attention to what day it is.