Law of urination: It takes 20 seconds to pee!

Jul 01 2014, 17:07 IST
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 The longer the tube, the faster it empties.(thinkstock) The longer the tube, the faster it empties.(thinkstock)
SummaryThe study investigated how quickly 32 animals urinate. It turns out that it's all about the same.

All animals - from elephants to cats - that weigh more than 3 kilogrammes take about 20 seconds to pee, scientists have found.

Even though an elephant's bladder is 3,600 times larger than a cat's, both animals relieve themselves in about 20 seconds, researchers found.

The study investigated how quickly 32 animals urinate. It turns out that it's all about the same. In fact, all animals that weigh more than 3 kilogrammes urinate in that same time span, they said.

"It's possible because larger animals have longer urethras," said David Hu, the Georgia Institute of Technology assistant professor who led the study.

"The weight of the fluid in the urethra is pushing the fluid out. And because the urethra is long, flow rate is increased," Hu said.

For example, an elephant's urethra is one metre in length. The pressure of fluid in it is the same at the bottom of a swimming pool three feet deep.

An elephant urinates four meters per second, or the same volume per second as five shower-heads.

"If its urethra were shorter, the elephant would urinate for a longer time and be more susceptible to predators," Hu explained.

The findings conflict with studies that indicate urinary flow is controlled on bladder pressure generated by muscular contraction.

Hu and graduate student Patricia Yang noticed that gravity allows larger animals to empty their bladders in jets or sheets of urine. Gravity's effect on small animals is minimal.

"They urinate in small drops because of high viscous and capillary forces. It's like peeing in space," said Yang.

"Mice and rats go in less than two seconds. Bats are done in a fraction of a second," said Yang.

The research team went to a zoo to watch 16 animals relieve themselves, then watched 28 YouTube videos. They saw cows, horses, dogs and more.

The more they watched, the more they realised their findings could help engineers.

"It turns out that you don't need external pressure to get rid of fluids quickly. Nature has designed a way to use gravity instead of wasting the animal's energy," said Hu.

Hu envisions systems for water tanks, backpacks and fire hoses that can be built for more efficiency.

As an example, he and his students have created a demonstration that empties a teacup, quart and gallon of water in the same duration using varying lengths of connected tubes.

In a second experiment, the team fills three cups with the same amount of water, then watches them empty at differing rates.

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