哺乳动物尿尿平均需要几秒？

Researchers at Georgia Institute of Technology in Atlanta, USA, have compiled footage of animals peeing to prove their "law of urination", which states that mammals take about 21 seconds to pee.

Male or female, small bladder or big bladder, it doesn't matter. Elephants, farm animals, dogs and any mammal above 3 kilograms in weight require a similar time frame to relieve themselves, give or take 13 seconds.

Dr. David Hu, Associated Professor of Mechanical Engineering and Biology at the Georgia Tech is the study's co-author. He talks about his inspiration for the research.

"The whole thing started when I was changing my son's diaper, while he urinated on my face. And then I realized he was urinating for a really long time. You know babies are really small. He is maybe 10 to 20 pounds. And the time it takes is about 20 seconds. And it was the same when I go to the bathroom later; it is as the same time as I took. What made me wonder is but I am much bigger than him, shouldn't I take more time?"

The researchers headed over to Zoo Atlanta to watch the animals do their business. Dr. Hu says, watching animals relieve themselves came with its own unique challenges.

While timing each pee session was easy, measuring the flow rate was hard, because they had to collect the urine. That's no easy task. First off, it's hard to catch an animal in the act. Second, they have to catch all the urine in order to get an accurate measurement of how much liquid is coming out per second.

"The elephant, the zoo trainers know that they stand on the stool and step down from the stool, stand on the stool and step down, eventually they want to urinate. They got the urine which is filled the whole garbage can, 20 liters. The Cow, to get them to urinate; you actually have to (give them a) massage. And then, dogs, you can't have a pot underneath them, so you take beneath them a big mat, and you weight the mat. So when you have a goat urinating, it's more similar to us, because they are about the same size to us."

Plus, since they had to collect the urine by hand, it was unavoidable for the high-speed camera and the scientists themselves to occasionally be splattered by the liquid.

After they collected all kinds of data, they found something very strange or even surprising. While some of those larger animals had really capacious bladders, as Dr. Hu mentioned just now that an elephant can fill a kitchen garbage can with pee. From the cat to the goat to the elephant, they all seem to empty in about the same time: 21 seconds, give or take 13 seconds. That range may partly be because the animals often urinate for different reasons, for example, males often use urine to mark territory and some rodents use the urine as a defense.

Dr. Hu says it's a remarkably tight window. After all, even though an elephant's 18-liter bladder is nearly 3,600 times larger than a cat's, it doesn't take 3,600 times longer to empty. This means that the elephant must be spewing out a massive torrent of liquid at high speeds in order to empty itself in the same time frame as a relatively tiny feline bladder.

The secret seems to lie in the urethra's design, which uses gravity to its advantage. Dr. Hu gives out 2 resons to answer the question why almost all the mammals urinate for the same time.

"First, imagine the urethra is a highway. On a highway, if you have more lanes, you have more cars travelling through. So the urethra for elephant for example, it's thicker than my leg. And that allows having a huge area to allow the urine to pass through. The other reason is much more subtle. The elephant's urethra, the height is one meter; it's about as tall as my leg. What that does is to make the urine travel much faster, because it has more gravitation energy. "

The longer the urethra, the faster the flow, because just as the pressure builds toward the bottom of a deep swimming pool, the pressure in a liquid-filled urethra also builds toward the bottom of the tube. And since the urethra is also proportionally wider in larger animals, it basically adds extra lanes to this fast-moving pee freeway, or "pee-way".

Tiny animals, like rats, appeared to urinate in a very different way from us: in little "gumballs" of water dispensed one by one. Small animals don't have enough liquid in their system to make it flow. But animals bigger than 3 kilograms, including humans can store a large amount of urine and used essentially the same mechanism: it basically came out in a jet or a stream.

"Urination is a very important area of research, because for humans, there can be a lot of problems with the urinary system. So maybe you heard about kidney stones, or urinary tract affections. A lot of these medical problems are difficult to diagnose. And the reason is because the urinary system is inside the body. So in our study we wanted to figure out a way to characterize the health of urinary system, just by looking on the outside."

Understanding urination duration has important health implications given that illnesses like prostate cancer or conditions like obesity can put pressure on the urethra, it can make the process of voiding lengthier. If it takes too long to pee, chances are there's a health problem that needs attention.

The findings have been released by the Proceedings of the National Academy of Sciences, shed light on the mysterious fluid dynamics of urination.

Understanding the urinary tract, a mechanism that works at many different size scales, could also help engineers build better devices that rely on fluid flow, from above-ground water tanks to water-efficient toilets.

"So the urethra is a magical tool. You add something in the container, and you can control how much time it empties. It doesn't matter how big the container's, it could be as big as a cup, like the dog, or as big as a kitchen garbage can, can be one liter, 20 liters, or can be a hundred liters or thousand liters or million liters, as long as you have this urethra, it will be 21 seconds. We think this might have applications in building large water towers or backpacks, anything you want to control the time for things to come out, you don't have to add a pump, all you have to do is to design a tube very carefully, just as the animals do."