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Mammals, whether they be cats or people or wildebeests, have a few things in common you know, 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 any 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 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. Thanks for watching this episode of SciShow!

Now If you're here thinking "Well that was good but I want to know more about my pee, well, we have a whole episode on what your pee says about you.... { ♪OUTRO }.