But fortunately, there's one important thing we don't have to worry about remembering: breathing.

不過幸運的是，有一件事雖然非常重要，但我們不必特別去記它——那就是呼吸。

When you breathe, you transport oxygen to the body's cells to keep them working and clear your system of the carbon dioxide that this work generates.

呼吸的時候，會將氧氣輸往體內的細胞，以維持細胞的運作，並將產生的二氧化碳排出體外。

Breathing, in other words, keeps the body alive.

也就是說，呼吸維持了身體的基本機能。

So, how do we accomplish this crucial and complex task without even thinking about it?

呼吸如此重要又複雜無比，我們是如何下意識做到的呢？

The answer lies in our body's respiratory system.

答案是靠我們的呼吸系統。

Like any machinery, it consists of specialized components, and requires a trigger to start functioning.

呼吸系統就像機器一樣，有專門的零件以及特定的運作機制。

Here, the components are the structures and tissues making up the lungs, as well as the various other respiratory organs connected to them.

呼吸系統由一些組織及構造組成，形成了肺及其餘相連的呼吸器官。

And to get this machine moving, we need the autonomic nervous system, our brain's unconscious control center for the vital functions.

自主神經系統則能使呼吸系統正常運作，讓我們的頭腦下意識地運行重要的功能。

As the body prepares to take in oxygen-rich air, this system sends a signal to the muscles around your lungs, flattening the diaphragm and contracting the intercostal muscles between your ribs to create more space for the lungs to expand.

當身體準備吸入富含氧氣的空氣時，自主神經系統會傳送信號給肺部附近的肌肉，拉平橫膈膜並收縮肋間肌，如此一來，吸氣時，肺才有更多空間擴張。

Air then whooshes into your nose and mouth, through your trachea, and into the bronchi that split at the trachea's base, with one entering each lung.

接著空氣便咻地進入鼻子與嘴巴，通過氣管再進入氣管底部分支出的兩個支氣管，兩個支氣管各自通往一個肺。

Like tree branches, these small tubes divide into thousands of tinier passages called bronchioles.

支氣管跟樹枝很像，會分化成數千個小管道，稱作「小支氣管」。

It's tempting to think of the lungs as huge balloons, but instead of being hollow, they're actually spongy inside, with the bronchioles running throughout the parenchyma tissue.

我們很容易認為肺是顆巨大的氣球，但其實肺的內部並非空心，而是海綿狀，有小支氣管貫穿薄壁組織。

At the end of each bronchiole is a little air sack called an alveolus, wrapped in capillaries full of red blood cells containing special proteins called hemoglobin.

每一個小支氣管的末端都有一個稱作「肺泡」的小氣囊，肺泡被充滿紅血球的微血管包覆，而紅血球夾帶著一種特殊蛋白：血紅素。

The air you've breathed in fills these sacks, causing the lungs to inflate.

吸入的空氣會填滿肺泡，造成肺膨脹。

Here is where the vital exchange occurs.

而重要的氣體就是在這裡交換。

At this point, the capillaries are packed with carbon dioxide, and the air sacks are full of oxygen.

這個時候，微血管會充滿二氧化碳，而肺泡則會充滿氧氣。

But due to the basic process of diffusion, the molecules of each gas want to move to a place where there's a lower concentration of their kind.

但由於擴散作用的基本原理，所以每個氣體分子都會蠢蠢欲動，想移往同類氣體濃度較低的一邊。

So as oxygen crosses over to the capillaries, the hemoglobin grabs it up, while the carbon dioxide is unloaded into the lungs.

因此，當氧氣進入微血管與血紅素結合時，二氧化碳會在肺部被卸載。

The oxygen-rich hemoglobin is then transported throughout the body via the bloodstream.

富含氧氣的血紅素便可以透過血液，將氧氣運送至全身。

But what do our lungs do with all that carbon dioxide?

那肺要怎麼處理二氧化碳呢？

Exhale it, of course.

當然是呼出去囉！

The autonomic nervous system kicks in again, causing the diaphragm to ball up, and the intercostal muscles to relax, making the chest cavities smaller and forcing the lungs to compress.

自主神經系統會再次啟動，使橫膈膜回彈、肋間肌放鬆，胸腔的空間會變小，迫使肺部壓縮。

The carbon dioxide-rich air is expelled, and the cycle begins again.

二氧化碳被排出體外後，循環會再次開始。

So that's how these spongy organs keep our bodies efficiently supplied with air.

肺部的運作模式就是如此，有效地為身體提供氧氣。

Lungs inhale and exhale between 15 and 25 times a minute, which amounts to an incredible 10,000 liters of air each day.

肺每分鐘吸氣吐氣 15 至 20 次，每天都有高達一萬公升的空氣進出肺部。

That's a lot of work, but don't sweat it.

這個量十分驚人，不過別怕！

Your lungs and your autonomic nervous system have got it covered.

你的肺和自主神經系統會幫你搞定一切。