The urination constant

The urination constant October 22, 2014

I am a squeamish person, and I dislike talking about bodily functions.  But I stumbled upon a finding that is so interesting that I had to share it with you.  Did you know that virtually all animals, from a dog to an elephant–including human beings–take about the same amount of time to urinate?  That would be around 21 seconds.  See why after the jump.

From Jason G. Goldman, Why a rhino urinates at the same rate as a raccoon — and why that’s useful to know – The Washington Post:

You might think that elephants take longer to empty their bladders than humans do, because pachyderms are so much larger. But you’d be wrong. Recent research shows that most animals, including humans, take the same amount of time to pee. . . .

Until now, the urethra was thought to be simply a pipe from the bladder to the genitals that expels urine from the body. But David Hu, an assistant professor of mechanical engineering and biology at Georgia Tech, suspected there might be more to it. What he found was that it is indeed a pipe, but with important size considerations. The key was in its length.

The researchers led by Hu discovered that the smallest critters can’t produce streams of urine. Instead, they pee in drops. But as soon as an animal tops about 6.6 pounds, it can produce jets. And no matter how big that animal is — from Fido to Dumbo — it takes the same amount of time to do it as other animals: roughly 21 seconds. Hu’s team reported its findings in June’s Proceedings of the National Academy of Sciences.

It’s all thanks to the urethra. According to Hu, the time it takes to pee is constant, no matter the animal’s size, because of gravity. “Consider the pressure at the bottom of a swimming pool,” he said. If you sat on the bottom of a 50-foot-long pool that was 10 feet deep, you would feel the same pressure from the water pushing down on you as you would at the bottom of a 500-foot-long pool that was also 10 feet deep. That’s because pressure is determined only by the height of the water above it, not the width of the pool.

An elephant’s bladder is about 11 / 2 feet tall. But its urethra is three feet long. If you could fit inside the end of the elephant’s urethra, you’d feel the pressure of 41 / 2 feet of urine above you: the three-foot urethra plus the 11 / 2-foot bladder.

That’s why a rhino or an elephant can expel urine in roughly the same time as a raccoon or a ferret, even though the larger animals have a lot more urine. “Our model shows that differences in bladder capacity are offset by differences in flow rate,” Yang writes, “resulting in a bladder emptying time that does not change.” In other words, as the bladder size increases, the urethra becomes longer to compensate, allowing gravity to produce a higher flow speed.

Steven Vogel, a biology professor at Duke University who was not involved in the research, was surprised. “Who would have expected urination time to be so nearly constant?” he said. “I wrote a whole book on biological fluid mechanics, even giving some space to one aspect of urination, and I never imagined.”

OK, I know many of you will be taking your stop-watch app into the bathroom and with you when you take your dog for a walk.

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