It's your skin, with a surface area of about 20 square feet in adults.

而是你的皮膚。成人的皮膚表面積可達 20 平方英尺 (1.85平方公尺)。

Though different areas of the skin have different characteristics, much of this surface performs similar functions,

儘管不同部位的皮膚有不一樣的特性，大部分皮膚表層都有相同的作用，

such as sweating, feeling heat and cold, and growing hair.

例如排汗、感覺溫度冷熱，以及生長毛髮。

But after a deep cut or wound, the newly healed skin will look different from the surrounding area,

但是如果有很深的傷口，癒合後的新皮膚與其周圍看起來會有所不同，

and may not fully regain all its abilities for a while, or at all.

並且有可能暫時性或永久性地失去機能。

To understand why this happens, we need to look at the structure of the human skin.

若要了解為何會如此，我們需要一窺人類皮膚的構造。

The top layer, called the epidermis, consists mostly of hardened cells, called keratinocytes, and provides protection.

皮膚的最表層稱為表皮，主要由硬化的細胞－「角質」所組成，而表皮可以提供保護作用。

Since its outer layer is constantly being shed and renewed, it's pretty easy to repair.

由於皮膚外層不斷地脫落與再生，所以很容易就可以修補傷口。

But sometimes a wound penetrates into the dermis,

但有時候傷口穿透了真皮，

which contains blood vessels and the various glands and nerve endings that enable the skin's many functions.

真皮包含了血管、各種腺體及神經末梢賦予皮膚許多功能。

And when that happens, it triggers the four overlapping stages of the regenerative process.

而當傷口傷至真皮時，皮膚會啟動四個相互重疊的階段來進行再生過程。

The first stage, hemostasis, is the skin's response to two immediate threats:

第一階段：止血期，是皮膚對於兩種直接威脅的反應，

that you're now losing blood and that the physical barrier of the epidermis has been compromised.

其一為人開始失血，其二為表皮的物理屏障受到損害。

As the blood vessels tighten to minimize the bleeding,

此時血管開始繃緊以抑制出血，

in a process known as vasoconstriction, both threats are averted by forming a blood clot.

這個過程稱為血管收縮，接著血塊的形成減緩了前述兩種威脅。

A special protein known as fibrin forms cross-links on the top of the skin,

一種稱為纖維蛋白的特殊蛋白質，會在皮膚表層形成交叉連結，

preventing blood from flowing out and bacteria or pathogens from getting in.

以避免血液流出皮膚，細菌或病原體進入皮膚。

After about three hours of this, the skin begins to turn red, signaling the next stage, inflammation.

大約三小時過後，皮膚開始轉為紅色，表示下個階段的開始：發炎期 。

With bleeding under control and the barrier secured,

在出血受到控制，以及有屏障掩護的情況下，

the body sends special cells to fight any pathogens that may have gotten through.

人體會派出特殊細胞，來對抗任何可能入侵的病原體。

Among the most important of these are white blood cells, known as macrophages,

其中最重要的是白血球細胞，也就是巨噬細胞，

which devour bacteria and damage tissue through a process known as phagocytosis,

此細胞會進行吞噬作用，來吞噬細菌以及受損的組織，

in addition to producing growth factors to spur healing.

除此之外，此細胞也會製造生長因子來促進傷口癒合。

And because these tiny soldiers need to travel through the blood to get to the wound site,

而因為這些小戰士們需要透過血液移動到受傷處，

the previously constricted blood vessels now expand in a process called vasodilation.

先前緊縮的血管會開始擴張，此過程稱為血管舒張。

About two to three days after the wound, the proliferative stage occurs, when fibroblast cells begin to enter the wound.

傷口形成後的二至三天增生期發生，此時纖維母細胞開始進入傷口。

In the process of collagen deposition, they produce a fibrous protein called collagen in the wound site,

在膠原蛋白沉積的過程中，纖維母細胞會在傷口處產生一種稱為膠原蛋白的纖維性蛋白質，

forming connective skin tissue to replace the fibrin from before.

形成皮膚結締組織來取代原先的纖維蛋白。

As epidermal cells divide to reform the outer layer of skin, the dermis contracts to close the wound.

當表皮細胞分裂，重建外層皮膚之時，真皮便收縮以閉合傷口 。

Finally, in the fourth stage of remodeling,

最後，在第四階段的重塑期，

the wound matures as the newly deposited collagen is rearranged and converted into specific types.

隨著新沉積的膠原蛋白重新排列及轉換成特殊形態，傷口逐漸成熟。

Through this process, which can take over a year, the tensile strength of the new skin is improved,

這個階段需時超過一年，在此過程新皮膚的張力增加，

and blood vessels and other connections are strengthened.

血管與其他連接處也會增強。

With time, the new tissue can reach from 50-80% of some of its original healthy function,

漸漸地，新組織可以恢復約五成至八成其原有的健康功能，

depending on the severity of the initial wound and on the function itself.

恢復程度取決於原有傷口的嚴重性，以及原有的功能為何。

But because the skin does not fully recover,

但因為皮膚並不會完全恢復，

scarring continues to be a major clinical issue for doctors around the world.

傷疤一直是全世界醫師需要處理的重要醫療問題。

And even though researchers have made significant strides in understanding the healing process,

即使相關研究人員對於傷口癒合過程已經有明顯的進步，

many fundamental mysteries remain unresolved.

許多基本的謎團仍未被解開。

For instance, do fibroblast cells arrive from the blood vessels or from skin tissue adjacent to the wound?

例如，纖維母細胞是由血管還是由皮膚組織到達鄰近的傷口？

And why do some other mammals, such as deer, heal their wounds much more efficiently and completely than humans?

以及為何有些哺乳類動物，例如麋鹿傷口癒合的速度及完整度都優於人類？

By finding the answers to these questions and others, we may one day be able to heal ourselves so well that scars will be just a memory.

藉由找到這些問題與其他問題的答案，人類或許終有一天能夠完全治癒好傷口，到時傷疤將只是記憶 。