Tag: Physics

All mammals bigger than 3 Kg pee in 21 seconds

The Ig Nobel is a prize awarded for “research that makes people LAUGH, then THINK — real research, about anything and everything, from everywhere”. Although it started as a parody of the real Nobel prize, over the past two decades it gained much respect and is coveted by many researchers, almost – but not quite – like the real Nobels.

The 2015 Ig Nobel Prize in Physics went to a group of researchers who established, once and for all, how long it takes for a mammal to pee. That’s right, pee.

Sometimes, the time spent procrastinating on YouTube, where you start by looking for something specific and end up 3 hours later watching funny cat video compilations, may not be a complete waste. For example, Yang et al. (2015) gathered 28 YouTube videos of various animals peeing. They also went to the local zoo and videotaped 16 more animals at the Atlanta Zoo emptying their bladders and collected their urine.

Their analysis showed that any mammal larger than 3 Kg pees in an amazing constant amount of time: 21 ± 13 s. That’s right: the cat and the elephant, the dog and the zebra, all pee in about 21 seconds. Too bad they didn’t include the humans in this experiment. I guess there would have been serious questions regarding the videotaping of a man or a woman’s privates…(but they could have sent their students to the toilet with a stopwatch…) Nevertheless, the knowledge about human urethra diameter, bladder size and flow rate (mL/s) can give an estimate of human urination duration, which is about the same as other mammals.

So…. why is that? After all, you would expect that emptying the 160 L of urine (that’s the capacity of an elephant bladder ) will to take longer than 1.4 L (the dog’s bladder capacity). The answer lies in the specifications of the urethra. The longer and wider the urethra is, the faster the urine flow. As the researchers put it, “the urethra is analogous to Pascal’s Barrel: by providing a water-tight pipe to direct urine downward, the urethra increases the gravitational force acting on urine and therefore, the rate at which urine is expelled from the body” (p. 11936). Therefore, the urethra is not just a tube between bladder and genitals, as the medical textbooks define it, but is and amazingly adaptive, robust and efficient system. Contrary to previous thinking, the peeing time is not dictated by muscular contraction resulting in bladder pressure, but by the length and width of the emptying tube.

Of important note, animals smaller than 1 Kg do not follow the what is probably called by now the Hu Constant Urination Law (Hu is the Principal Investigator of the lab and last author of the paper). Why? Because their urethrae are so small that gravity may not help them much. In other words, capillary and viscous forces force them to expel urine in drops in less than 2 seconds. (So astronauts in space pee in drops?)

The researchers also produced a mathematical model of their data (of course). Such model can be used for studying urological disorders in humans. Until now, these studies have been conducted in rodents, which turns out that may not be as good models after all. Another application is in engineering, when designing draining that does not depend on the size of the system.

Overall, a funny, very graphic (has videos attached), highly mathematical and interesting paper to read. The prize is well-deserved.

95 pee - Copy

Reference: Yang PJ, Pham J, Choo J, & Hu, DL (19 Aug 2014, Epub 26 Jun 2014). Duration of urination does not change with body size. Proceedings of the National Academy of Sciences of the United States of America, 111(33): 11932–11937, doi: 10.1073/pnas.1402289111 Article | FREE FULLTEXT PDF

By Neuronicus, 9 March 2016