评论
打印
Mammals, whether they be cats or people or wildebeests, have a few things in common you know, hair, milk and peeing for 21 seconds.
哺乳动物，不论是猫、人类还是牛羚，都有一些共同点，比如毛发、哺乳，还有尿尿时长21秒。
This is an actual thing! Lots of mammals take about 21 seconds to empty their full bladder.
这是真的！很多哺乳动物需要约21秒清空它们满满的膀胱。
It doesn't matter if their bladder contains 5 milliliters of urine or 18 liters. 21 seconds seems to be the magic number.
不论它们的膀胱里存了5毫升还是18公升尿液，这都无所谓。21秒似乎是个幻数。
And that's thanks to a combination of physiology and fluid dynamics.
这多亏了生理机能和流体动力学的结合。
The time it takes to tinkle depends on the amount you need to expel and the flow rate— how fast you expel it.
尿尿的时间取决于你需要排出的量以及流速，即排尿的速度。
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.
这个速度反过来取决于尿道的宽度（尿道是从膀胱向外排尿时流经的管道）以及排尿时的压力。
And it just so happens that those pipes are fairly proportional: that is, the larger the animal, the longer and wider the urethra.
正是如此，那些尿道相当均衡：因此，动物体型越大，它们的尿道就越宽越长。
The pressure created by an animal's bladder muscles is remarkably constant—about 5.2 kilopascals, if you were wondering.
动物膀胱肌肉所创造出的压力是恒定的，约5.2千帕。
You know you have that number in your brain. I'm sure it'll stay there for a long time.
在脑袋里想一下，我保证这个数字准让你吃惊。
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.
但是排尿的整体压力也取决于此以及尿液的液体静压力，即流体本身的压力。
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
所以如果你是一匹马，膀胱积攒很多尿液，并且所有液体的重量都在底部挤压
and so it gets pushed the same amount by the bladder but much more by the hydrostatic pressure,
因此尿液受到了同重量膀胱的推挤，但是更多的是来自液体静压力的挤压，
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...
但如果你是一只猫，就会受到较少液体以及同重量膀胱压力的挤压，但所受液体静压力就更少。
All of that means that urine in a large animal is being pushed out more forcefully through a tube that's proportionally wider...
所有这一切意味着，大型动物尿道更宽，所受压力更大，
...that makes it flow more quickly, which makes up for any difference in volume.
流速更快，弥补了容积差。
We know all this because some researchers actually did the math in a 2014 paper in Proceedings of the National Academy of Sciences.
我们知道这一点是因为一些研究人员确实在《美国科学院论文集》2014年的一篇论文中进行了计算。
They also filmed and watched a bunch of animals pee to validate their results, which, you know, that's science!
他们还拍了影片并观察了一群动物尿尿以验证他们的结果，你懂得，这就是科学嘛！
There are a few caveats, though. The 21 second rule only counts for full bladders in mammals heavier than 3 kilograms.
但也有一些忠告。21秒规则仅适用于体重3公斤以上且憋尿的哺乳动物。
For smaller animals like little bats, rats, or mice, something different happens.
对体型小的动物，如小蝙蝠、大鼠或老鼠来说，情况有所不同。
They urinate in a series of quick droplets because the thinness of their urethras
它们是以一系列迅速水滴形式排尿，因为它们尿道的薄度
and the friction created by the flowing fluid actually pushes up on the liquid, opposing the force of gravity.
以及流动液体推动液体时产生的摩擦和重力相左。
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.
因此，起初尿滴是附在尿道壁上的，并越积越大直到尿液重到可以由重力接管。
Then, they fall out quickly—usually, in less than a second.
然后，它们迅速落下—通常不足一秒。
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.
21秒规则可能也不适用于水生哺乳动物，因为水，尤其是深水，会将压力作用于尿道并将尿液挤压回尿道。
The same goes for swimmers— it takes you longer to pee in the pool. Not that you'd ever do that.
游泳者也一样—在水中尿尿的时间更长。（不要这么做）
And, to be clear, none of this counts for reptiles or birds because they've got a whole different excretion system.
明确来说，这些都不适用于爬行动物或鸟类，因为它们的排泄系统完全不同。
Now while this might sound kind of like…silly to study animal peeing,
虽然研究动物尿尿听起来有点蠢，
understanding the physics and mathematics of urine flow actually has some practical applications.
但了解尿流的物理现象和数学其实有些实际应用。
Engineers could apply these findings to design systems where reservoirs,
工程师可以应用这些发现设计系统让水库
water towers or even water backpacks of different sizes take the same amount of time to empty.
水塔，甚至不同大小的水背包排空时间相同。
And this also suggests that studying urinary diseases in small animals like rodents might not be applicable to humans
其还表明研究小型动物，如啮齿动物的泌尿系疾病，或许不适用于人类
because their whole peeing process is just fundamentally different than what we do even though we have the same basic body parts.
因为它们的整个排尿过程和我们不同，即便我们有同样的基本身体部位。
It just goes to show that some curious research can sometimes lead to useful results.
这说明一些古怪的研究有时可以得出有用的结果。
Thanks for watching this episode of SciShow!
感谢收看本期《科学秀》！
Now if you're here thinking :Well that was good but I want to know more about my pee,
现在如果你在想：这个研究不错，但是我想了解更多关于自己尿液的知识，
well, we have a whole episode on what your pee says about you....
好吧，我们有一期讲的是尿液反映人的视频...
