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  • A cat's bladder can only store a golf ball's worth of urine.

  • For humans, it's a coffee mug and for elephants, a kitchen trash can.

  • An elephant's bladder is 400 times the size of a cat's,

  • but it doesn't take an elephant 400 times longer to pee.

  • So, how does this work?

  • The answer lies in what scientists call theOther Golden Rule.”

  • It applies to mammals, which pee out some of their wastes in a yellowy liquid.

  • Other classes of animals use different systems and methods to excrete waste.

  • The Rule describes how urinary organs and forces change with body size,

  • enabling a range of mammals to achieve similarly rapid relief.

  • There are two main factors contributing to urination speed:

  • bladder pressure and gravity.

  • Bladder pressure is created when mammals contract their muscles

  • to squeeze urine out.

  • This pressure is equal to the applied muscular force per unit area but,

  • since these factors scale proportionally,

  • bladder pressure doesn't actually change much depending on body size.

  • Meanwhile, the effects of gravity do vary across species.

  • Mammals have a pipe-like organ called the urethra

  • that transports urine from a higher elevation to a lower one

  • specifically, from the bladder down to where urine exits the body.

  • In doing so, the urethra harnesses the power of Torricelli's Law,

  • which states that flow rate increases with container height.

  • Take these two containers holding the same volume of liquid.

  • When identically punctured at the bottom,

  • the vertical container drains nearly twice as fast.

  • This is because the water's falling from a greater elevation difference.

  • By transporting urine from high to low, the urethra has a similar effect.

  • The urethra is probably not perfectly vertical,

  • so its length doesn't tell us exactly how far the urine falls.

  • However, it's a good approximation.

  • The urethra's diameter also affects flow rate.

  • When that vertical container's exit hole has a larger diameter,

  • the water drains out even faster.

  • The length and diameter of an animal's urethra varies

  • depending on its species and whether it has a penis or vagina.

  • For example, a female elephant has a meter-long urethra

  • with a thirty-five millimeter diameter.

  • A person with a vagina has a urethra that is more than 20 times shorter

  • and 5 times thinner.

  • The elephant's longer, wider urethra is what allows it to urinate

  • for a comparable amount of time as a human.

  • Indeed, if an elephant had the urethra of a housecat,

  • it might take it about 2 hours to drain its bladder.

  • It's hypothesized that the Other Golden Rule

  • is conserved because quicker pees may both reduce vulnerability to predators

  • and flush out disease-causing pathogens.

  • But the Rule has limitations.

  • It's only been shown to apply to non-aquatic mammals.

  • And the Rule doesn't apply to mammals under three kilograms.

  • They actually pee in droplets, not continuous streams,

  • because surface tension breaks up their tiny jets of urine.

  • Even within non-aquatic mammals above three kilograms,

  • different behaviors disrupt the Other Golden Rule.

  • Male dogs don't always fully empty their bladders

  • and will instead tinkle in short spurts to mark their territory.

  • Male pandas sometimes pee in handstands, marking bark higher up on trees

  • in order to broadcast their scent to potential mates.

  • Handstand pees take longer because urine must work against gravity.

  • Underwater pees are also lengthier

  • because the external water pressure counteracts the internal forces

  • resulting from elevation difference.

  • Whether it's with the intensity of a fire hydrant or a squirt gun,

  • this system swiftly delivers our urine to the outside world.

A cat's bladder can only store a golf ball's worth of urine.

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B2 urethra urine bladder pee elephant diameter

Do larger animals take longer to pee? - David L. Hu

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    林宜悉 posted on 2020/11/05
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