Researchers go with the flow in checking out how animals pee


What do horses, elephants and cats have in common when it comes to emptying their bladders?

Fluid dynamics at work in a mare.
Fluid dynamics at work in a mare.

Their diverse systems all rely on the fundamental principles of fluid mechanics, according to United States researchers.

The study at the Georgia Institute of Technology study investigated how fast 32 mammals urinated. It turns out that it is all about the same.

Even though an elephant’s bladder is 3600 times larger than a cat’s – 18 liters versus 5 milliliters – both animals relieve themselves in about 20 seconds.

In fact, all animals that weigh more than 3 kilograms (6.6 pounds) urinate in that same time span.

“It’s possible because larger animals have longer urethras,” explained David Hu, the 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.”

For example, an elephant’s urethra is one meter 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 showerheads.

“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 suggesting urinary flow is controlled by 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, who is pursuing her doctoral degree in the George Woodruff School of Mechanical Engineering.

“Mice and rats go in less than two seconds. Bats are done in a fraction of a second.”

The research team went to Zoo Atlanta to watch 16 animals relieve themselves, then watched 28 YouTube videos. They observed horses, cows, dogs and more.

They used high-speed video recording and flow-rate measurements obtained at the zoo to discover that all mammals above 3kg emptied their bladders over nearly constant duration of 21 seconds, plus or minus 13 seconds.

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

“It turns out that you don’t need external pressure to get rid of fluids quickly,” Hu said.

“Nature has designed a way to use gravity instead of wasting the animal’s energy.”

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.

Even though an elephant’s bladder is a massive 18 liters, they still relieve themselves in about the same time it takes smaller animals.
Even though an elephant’s bladder is a massive 18 liters, they still relieve themselves in about the same time it takes smaller animals.

In a second experiment, the team fills three cups with the same amount of water, then watches them empty at differing rates. The longer the tube, the faster it empties.

“Nature has shown us that no matter how big the fire truck, water can still come out in the same time as a tiny truck,” Hu added.

“Our findings reveal that the urethra is a flow-enhancing device, enabling the urinary system to be scaled up by a factor of 3600 in volume without compromising its function.

“This study may help to diagnose urinary problems in animals as well as inspire the design of scalable hydrodynamic systems based on those in nature,” Hu said.

The findings of the study have been published in the journal, Proceedings of the National Academy of Sciences. It is titled, “Duration of urination does not change with body size”.

The abstract of the study can be read here.

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