If You Sprinkle When You Tinkle

One of the things that I love about science is that it's often incredibly bizarre, which often makes for the most interesting table conversations! As I was scanning the most current issue of the Proceedings of the National Academy of Sciences (which is one of the most highly respect journals in the scientific community), one paper in particular popped out at me: "Duration of urination does not change with body size".

Francis, you've lost your mind. Urination? Really? Someone funded this?

Before you cast judgment, let's delve a little into what these researchers at the Georgia Institute of Technology looked at.

Patricia Yang et al. used a combination of high-speed videography and flow-rate measurements to discover whether or not the size of a mammal influences the rate at which it empties its bladder. There were a plethora of images and videos, which I will spare for the sake of decency. There were several fun facts and findings listed in the paper, a few of which I've summarized here:

1. An elephant's bladder, at 18 liters, is nearly 3,600 times large in volume than a cat's bladder which can only hold 5 milliliters.
2. Most small mammals (Less than 3 kg) take between 0.01-2 seconds to urinate, while larger mammals take about 21 seconds.
3. The peak pressure in your bladder is consistently measured 5.2 kPas, or about 1 X10^-6 pounds per square foot.
4. Larger animals tend to have continuous bladder streams, while small animals tend to urinate in small droplets

Figure 1. These animals take the same amount of time to pee!

So then why do smaller mammals take just as long to empty their bladders than larger mammals? Don't they have less volume to release? As it turns out, all of this can be explained via basic physics. As you would imagine, larger animals contain larger urethras, the part of the body that carries urine from the bladder to the outside world. These longer urethras have greater hydrostatic pressure driving the flow of liquid forward, allowing for high flow rates. The narrower urethras of smaller animals causes urine to only exist in small drops due to high viscous and capillary forces. Liquid tends to slow down in narrower pipes because more of this liquid is interacting with the interior of the pipe. In contrast, large pipes also have liquid interacting with the interior of the pipe but this leaves the majority of liquid to flow in the middle at maximum velocity.

Figure 2. Laminar flow. Note the faster flow is towards the center!

You're probably experienced this phenemonan in rivers in streams. If you step into a river, you'll note that water is moving pretty slowly near the shore. But if you were to walk directly into the water, you will notice that the water is moving much faster in the middle of the river. This is called laminar flow. Pretty snazzy!

In summation, all mammals above 3kg empty their bladder in 21 +/- 13 seconds. This is incredible, as this same rate is applicable for animals as small as a cat to animals as large as elephants. So in the end, who cares? What's the significance?

Based on the average time it takes to urinate, the average mammal spends about two minutes a day urinating. Given that this is such a small proportion of a mammal's day, it likely does not influence reproductive fitness, or the ability for an organism to pass on its genes (especially in comparison to activities such as eating and sleeping). The newly found significance in the geometry of the urethra, however, may play a role in more important activities such as ejaculation. This knowledge may also be used to find urinary problems in animals and in biological-based hydrodynamic systems.

Remember that all science is significant, you may just need to adjust your perspective!