The “Law Of Urination” And Why You Should Go By The “21 Second Rule”

In 2015, an Ig Nobel Prize for physics was awarded for taking high-speed footage of animals peeing in zoos, and gawping at more videos of animals urinating via YouTube. The team modeled the fluid dynamics involved in peeing for a variety of different sized animals, and found what they termed the “Law of Urination“.

It’s a simple law, but a curious one: animals that are over 3 kilograms (6.6 pounds) empty their bladders over about 21 seconds. In smaller animals – for example, rats – urination can happen in a fraction of a second. The research helped confirm that pee duration was not just to do with bladder pressure, but also related to our good friend gravity. In smaller animals, pee time is constrained by the surface tension of urine.

“How can bladders of both 0.5 kg and 100 kg be emptied in nearly the same duration? Larger animals have longer urethras, and so greater gravitational force driving flow. These long urethras increase the flow rate of larger animals, enabling them to perform the feat of emptying their substantial bladders over approximately the same duration,” the team wrote in their paper.

“In this study, we find the urethra is analogous to Pascal’s Barrel, acting as an energy input device. By providing a water-tight pipe to direct urine downward, the urethra increases the gravitational force acting on urine and so the rate that urine is expelled from the body. Thus, the urethra is critical to the bladder’s ability to empty quickly as the system is scaled up.”

Here’s a recap of Pascal’s Barrel, in which a small amount of water causes a large jug to burst because of that added energy.