Australasian Science: Australia's authority on science since 1938

The Law of Urination

By Magdeline Lum

Why do bats, dogs and elephants take the same time to urinate?

Dogs, goats, cows and elephants are all mammals that can empty their bladders in around 21 seconds, even though their bladders range in size from 100 mL to 100 L. Researchers at the Georgia Institute of Technology in Atlanta wanted to know why mammals of varying size empty their bladders in the same amount of time, and used high-speed video to study how urine is ejected from various animals. The team also gathered footage of animals urinating from YouTube.

Data from the footage was combined with information of the animals’ mass, bladder pressure and urethra size to create a mathematical model of urinary systems of mammals. The resulting “Law of Urination” applies to both males and females.

Previous mathematical model of urinary systems had only considered the effect of bladder pressure. It turns out that the length of the urethra also plays a role in the call of nature.

The team found that the longer the length of the urethra, the greater the effect of gravity and the flow rate of urine, particularly in larger animals. They also suggest that the urethra evolved as a flow-enhancing device that allows the urinary system to be scaled up without compromising its function.

Medium-sized animals like dogs and goats have shorter urethras. Their system isn’t affected by gravity as much, so the flow of urine is slower. However, their bladders are smaller than that of the elephant, which has a larger bladder and urethra. The result is that they all empty their bladders in the same amount of time.

However, this model does have its limitations. When it comes to smaller mammals like rats and bats, gravity is a small component of how urine is ejected. These small mammals urinate in just under a second. The viscosity of the urine and surface tension effects dominate, and urine is released as a series of drops instead of a stream of liquid like in larger mammals.

Stopping Toilet Splash

A team of physicists from the Splash Lab at Bringham Young University has been studying peeing at a urinal, and may have devised a way to save bathroom floors from drops and puddles.

The researchers used 3D printer to create a model of the male urethra that was attached to a pressurised container and tubing to simulate the male urinary system. A steady stream of dyed water was sent through the tubing at 21 mL/second – equivalent to the urination rate of a middle-aged man. The streams of urine hitting the surface of a urinal were filmed using high-speed cameras and analysed.

The best way to reduce splash-back is to ensure that the stream hits the wall of the toilet or urinal at a gradual angle. The closer the angle of impact gets to 90 degrees, the worse the splash-back.

Other ways of reducing splash-back is to not urinate directly into the water. Placing a layer of toilet paper in the urinal before urinating can do this. Obstacles like urinal cakes and the toilet rim can also reduce splash back.

However, what works against preventing splash-back is the very material used to make toilets and urinals. Porcelain is hydrophilic, which is perfect for repelling water and aqueous liquids like urine and hence increases splashback.

Of course, another method to reduce splash-back is to sit down when urinating.