The other "Golden Rule" - David L. Hu

翻译人员: Zizhuo Liang 校对人员: Helen Chang

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

猫的膀胱容量只相当于 一个高尔夫球的体积。

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

人类膀胱大约与咖啡杯容量相同， 而大象则相当于厨房垃圾桶的容量。

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

大象膀胱的大小 是猫膀胱大小的400倍，

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

但是大象小便的时间 可不是猫的400倍。

So, how does this work?

这到底是为什么呢？

The answer lies in what scientists call the “Other Golden Rule.”

答案在于科学家 所谓的“另一条黄金法则”中。

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

它适用于用尿液来 排除身体中废物的哺乳动物们。

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

其他种类的动物通过不同的 系统和方法来代谢废物。

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

该规则解释了泌尿器官和压力是如何 因为身体大小的不同而改变的。

enabling a range of mammals to achieve similarly rapid relief.

从而使一系列哺乳动物 能够实现类似的快速排尿。

There are two main factors contributing to urination speed:

影响排尿速度的主要因素有两个：

bladder pressure and gravity.

膀胱压力和重力.

Bladder pressure is created when mammals contract their muscles

当哺乳动物肌肉收缩时 就会对膀胱产生压力

to squeeze urine out.

从而将尿液挤出。

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

这种压力等同于 每单位面积内的肌肉的力量，

since these factors scale proportionally,

但由于这些因素是成比例的，

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

所以膀胱压力实际上 并不随身体大小而变化。

Meanwhile, the effects of gravity do vary across species.

同时，重力的影响 确实因物种而异。

Mammals have a pipe-like organ called the urethra

哺乳动物有一个被称为 尿道的管状器官，

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

它将尿液从高处输送到低处—

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

特别是从膀胱输送到 尿液流出身体的地方。

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

这样，尿道遵循了 托里切利定律，

which states that flow rate increases with container height.

托里切利定律认为流速 随容器高度的增加而增加。

Take these two containers holding the same volume of liquid.

这两个盛有相同体积液体的容器。

When identically punctured at the bottom,

当底部以同样形式被刺穿时，

the vertical container drains nearly twice as fast.

垂直容器的排水速度 几乎是原来的两倍。

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

这是因为水是从一个 更大的高度差下降的。

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

尿道的作用与其类似， 通过管道将尿液从高处输送到低处。

The urethra is probably not perfectly vertical,

尿道可能不是完全垂直的，

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

所以它的长度并不能确切地 告诉我们尿液下落的距离。

However, it’s a good approximation.

然而，这可以提供 一个很好的近似值。

The urethra’s diameter also affects flow rate.

尿道的直径也会影响流速。

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

当垂直容器的出口孔直径较大时，

the water drains out even faster.

水排得会更快。

The length and diameter of an animal’s urethra varies

动物尿道的长度和直径各不相同，

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

这取决于它的物种 以及它是否有阴茎或阴道。

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

例如，一头雌象有一米长的尿道

with a thirty-five millimeter diameter.

直径为35毫米。

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

有阴道的人的尿道要 比其短20倍

and 5 times thinner.

薄5倍以上。

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

大象更长、更宽的尿道使它能够

for a comparable amount of time as a human.

与人类排尿时间相同。

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

事实上，如果大象有家猫的尿道，

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

它可能需要2个小时 才能排空膀胱。

It’s hypothesized that the Other Golden Rule

据推测，另一条黄金法则

is conserved because quicker pees may both reduce vulnerability to predators

则是保命用的，因为更快地排尿 既可以减少被捕食者攻击的概率，

and flush out disease-causing pathogens.

也可以清除致病病原体。

但这一规则有其局限性。

But the Rule has limitations.

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

这只被证明适用于 非水生哺乳动物。

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

且这条规则不适用于 3公斤以下的哺乳动物。

They actually pee in droplets, not continuous streams,

它们事实上是以水滴的形式排尿， 而不是以水流的形式，

because surface tension breaks up their tiny jets of urine.

因为表面张力破坏了 它们微小的尿液喷射。

Even within non-aquatic mammals above three kilograms,

即使在3公斤以上的 非水生哺乳动物中，

different behaviors disrupt the Other Golden Rule.

另一种行为破坏了 这条黄金法则。

Male dogs don’t always fully empty their bladders

雄性狗并不总是 完全排空它们的膀胱

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

相反，他们会在短时间内 喷洒尿液来标记自己的领土。

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

雄性大熊猫有时会倒立撒尿， 在树上的高处留下记号

in order to broadcast their scent to potential mates.

以便向潜在的配偶 传播他们的气味。

Handstand pees take longer because urine must work against gravity.

倒立排尿需要更长的时间 因为尿液必须抵克服重力。

Underwater pees are also lengthier

水下排尿的时间也会更长，

because the external water pressure counteracts the internal forces

因为外部水压抵消了

由高度差引起的内力。

resulting from elevation difference.

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

无论是消防栓的强度 还是水枪的强度，

this system swiftly delivers our urine to the outside world.

这个系统都能迅速将我们的 尿液输送到外部世界。