跟读练习: Why Don't Our Bones Keep Growing Forever? - 通过视频学习英语口语

Fish, amphibians, and reptiles –actually, almost all vertebrates– get bigger throughout their lives; they generally keep on growing until they die.

Then, there are humans, who, by the age of 20 or so, just stop getting bigger… or at least taller.

Why do we stop growing partway through life?

Hi, I'm Cameron, and this is MinuteEarth.

We can think about the question of why humans stop growing –like a lot of other questions about the world around us– in two ways.

Like, on an individual level, what actually stops our bones from getting longer at some critical moment?

But then also, on a grander scale, why do humans stop growing at a certain point, while other animals don’t?

Let's start with the first one.

Many bones have special layers of cells on each end known as "growth plates"; these cells are constantly dividing and turning into brand-new bone.

That's what makes both you and, say, an alligator grow.

And for the alligator, that growth continues throughout its life.

But in humans, once puberty is complete, our bodies release hormones that harden our growth plates permanently, turning them into "no-more-growth" plates.

Occasionally, some medical problem –like a hormone imbalance– will block this normal process from happening; that’s how the tallest people that have ever lived, like nine-foot-tall Robert Wadlow, reach their towering heights.

But, assuming things work as they should, the hardening of your growth plates means you can’t get any taller.

Now, here's where –if you're thinking what I'm thinking– you're contemplating that bigger question of why this whole growth-stopping process happens at all in us humans.

And it has to do with the fact that humans –along with other mammals, and birds– are energy-hungry critters.

Our fast metabolisms allow us to run –or swim or fly– long distances, but they require a lot of energy to sustain, even when we’re at rest.

And burning all that energy produces a lot of heat.

And if you recall, as an animal grows, its volume, and therefore the amount of heat it produces, increases much faster than its surface area does – surface that's needed to vent that heat outward.

Getting too big would burn us up, so it makes sense that there’s a biological process that limits our growth.

But other vertebrates –like alligators– have much slower metabolisms that don’t burn much energy most of the time.

Even as they grow bigger – and their relative surface area decreases – they’re generally able to vent the limited amount of heat their bodies produce.

And since being big does confer a lot of benefits, they might as well keep on getting bigger throughout life.

Bigger might be better for us humans too.

After all, it would be nice to be able to reach the highest fruit on the tree or change lightbulbs without a ladder, or something.

Unfortunately, we just can't take the heat.

If there’s anything that won’t stop growing, it’s the ridiculous amount of browser tabs I have open at any one time.

But that’s alright because I use Tab for a Cause, a simple browser extension that lets users raise money for charity for free just by opening browser tabs.

Right now, Tab For a Cause is working with The Humane League to help end animal cruelty in factory farms.

Simply download Tab for a Cause at the link in the description, and every tab you open will raise money that will go toward improving the lives of animals.

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跟读练习: Why Don't Our Bones Keep Growing Forever? - 通过视频学习英语口语

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