“Elucidating the hydrodynamics of urination”

Oh man. This is too much. I don’t even know how to write this without it sounding like a joke.

A group of physicists walk into a zoo to watch the animals pee.

After what must surely have been an awkward trip to the zoo, a team of Georgia Tech physicists report that the most mammals urinate for approximately 21 seconds despite very different bladders sizes. Unfortunately the standard deviation for their experiment is 13 seconds, so perhaps it would be more conservative (and accurate) to say that about 70% of all the animals observed urinated for between 8 and 34 seconds. While that doesn’t really sound as concise as a nice universal figure like 21 seconds, it still highlights an amazing similarity: even an elephant, with a 100L bladder, still urinates in about half a minute.

Here’s a video the team produced to illustrate this concept. It might be considered NSFW if your workplace doesn’t appreciate slow motion videos of animals peeing.

Since they’re physicists, the Georgia Tech researchers have modeled the fluid dynamics associated with urination…because that seems like a normal thing to do. Their paper, which (bizarrely?) consists of only one paragraph, asserts that a variance in urethral length is to blame for the convergent rates of urine flow. In other words, an elephant’s longer urethra enables him to void a 100L bladder in approximately a quickly as a mouse voids a 2mL bladder. The team claims that the longer urethra allows gravity to play a larger role in urination, increasing the flow rate.

I have so many questions.

  1. If a longer urethra equals faster urination, do human men (urethra length 8 inches) urinate faster than human women (1.5 inches)? Wait, did I just solve the mystery of why there’s always a line for the women’s bathroom?
  2. Did the research team determine that the animals had full bladders before they started urinating? How full? I can promise that the length of time I spend at the urinal at the end of a long car ride is significantly greater than when I’m sitting in my apartment writing utterly ridiculous blog posts. You can’t really ask a goat to quantify how badly he has to pee.
  3. How much does the diameter of the urethra vary as animals get bigger? Surely that plays a role too?
  4. There’s no muscle layer associated with causing bladder contraction, but the rate of urine discharge can certainly be increased by tightening the abdominal muscles which push on the bladder. Is this being accounted for? For single guys, this is a frequently used tactic for cleaning spots off the toilet bowl.

This paper is funny, and the Youtube video which accompanies it is downright hysterical, but the science seems questionable at best. I appreciate that an increase in urethra length allows gravity to play a larger role in urination, but that feels like a small part of the equation to me. If you’re reading this Patricia J. Yang, Jonathan C. Pham, Jerome Choo, or David L. Hu, I demand answers!


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