The science of skin - Emma Bryce

譯者: Chen Chi-An 審譯者: Helen Chang

【「皮膚是人最好的衣裳。當然， 社會要你不只穿著皮膚。」馬克吐溫】

Between you and the rest of the world lies an interface

在你和世界間有一個界面，

that makes up 16% of your physical weight.

構成體重的 16%。

This is your skin, the largest organ in your body:

這是你的皮膚，全身最大的器官，

laid out flat, it would cover close to 1.7 square meters of ground.

如果攤開來，它能覆蓋 將近 1.7 平方公尺的地面。

Its purpose may seem obvious— to keep our insides in.

目的很顯然：把體內構造維持在體內，

But a look beyond the surface

但當我們看得更深，

reveals that it plays a surprising number of roles in our lives.

會發現皮膚在我們的生命中 扮演讓人驚奇的角色。

First, the basics.

首先，最基本的，

Skin is the foundation of the integumentary system,

皮膚是「外皮系統」的基礎，

which also incorporates your hair, nails, and specialized glands and nerves.

這個系統包括你的頭髮、 指甲、特化的腺體和神經。

Made up of three layers,

皮膚組成有三層：

the epidermis,

「表皮層」、

dermis,

「真皮層」

and hypodermis,

和「下皮層」，

skin’s thickness varies from 0.5 millimeters at its thinnest

其厚度最薄從 0.5 公分，

and up to four millimeters at its thickest.

到最厚 4 公分都有。

It also carries out three key functions:

皮膚執行三個關鍵的功能：

protecting,

保護、調節和感受外在世界。

regulating,

and sensing the world beyond its limits.

On a daily basis, its huge surface processes hundreds, if not thousands,

每天，人的體表處理 數以百計的身體感覺，

of physical sensations,

relying mostly on large, pressure-sensitive skin components

主要依靠相對較大 且對壓力敏感的構造。

called Merkel cells.

叫做「默克細胞」 。

In your fingertips alone,

僅在你的指頭上，

there are 750 Merkel cells per each square-centimeter of skin,

每平方公分就有 750 個默克細胞，

coupled with over 2,500 receptors that give you your sense of touch.

同時結合超過 2500 個 能接受觸覺的受體。

This surface is also the body’s first major line of defense.

人的體表也是身體的 第一道主要防線，

Without it, you’d be a soggy mass of tissue and fluids,

如果沒有皮膚，你就只是 暴露在致命物質下的

fatally exposed to the elements.

一灘組織和液體。

Skin effectively seals off your insides

皮膚能有效的將 身體組織密封在內部，

and also absorbs pressure and shock

且能吸收外部壓力和衝擊，

with flexible collagen that makes up most of its dermal layer.

因為真皮層的主成分是 富含彈性的膠原蛋白。

The epidermis is made up mainly of skin cells called keratinocytes

表皮主要是由「角質細胞」所構成，

that are completely replaced every four weeks.

每四週完全更新一次，

As new cells form at the base of the epidermis, older ones are pushed up.

每當新細胞從底部生成， 舊細胞就會被推向上。

When these cells move upwards,

當這些細胞向上移動時，

they’re filled with a hardened protein called keratin.

表皮會被堅硬的「角蛋白」填充。

Once they reach the surface,

當細胞到達皮膚表面時，

they form a tightly-overlapping, waterproof layer

會形成一層緊密堆疊的防水構造，

that’s difficult for invading microbes to breach.

使欲入侵的微生物難以突破皮膚。

Any harmful microbes that make it into the epidermis

任何有害的微生物一進入表皮，

will encounter Langerhans cells.

就會遇到「蘭格漢氏細胞」。

This group of protective skin cells detects invaders

這群保衛性皮膚細胞能偵測入侵者，

and communicates their presence to resident immune system T-cells,

並傳遞入侵的訊息給免疫 T 細胞，

which react by launching an immune response.

後者啟動免疫反應。

A crucial feature of this immune defense

這免疫防衛機制的一項重要特色，

is the several thousand species of microorganisms

在於有數千種「微生物」

that inhabit the planes,

存在於皮膚表面、

folds,

皺褶

and crevices of your skin.

及裂縫中。

這些微生物包含「細菌」和「真菌」，

These microbes, which include bacteria and fungi,

thrive in the sebum,

在「皮脂」裡茁壯成長，

an oily substance that’s secreted onto the skin’s surface

皮脂是一種油性物質，

透過真皮內的「皮脂腺」 分泌到皮膚表面。

by sebaceous glands nestled inside the dermis.

These skin microbes keep the immune system in a state of constant surveillance,

這些皮膚上的微生物 讓免疫系統隨時維持在監視狀態，

ensuring that it’s ready to react if the body really is at risk.

確保當身體真的有風險時， 免疫系統能隨時做出反應。

Beyond this protective role,

除了保護的角色，

your skin is also a sensory organ that helps regulate your body’s temperature,

你的皮膚也是一個感受器官， 能幫助身體調節體溫。

two roles that are closely interlinked.

有兩個角色互相協同運作：

Nerves detect whether your skin is warm or cold

「神經」偵測你的皮膚是熱是冷，

and communicate that information to your brain.

接著將訊息告訴你的大腦。

In return, the brain instructs localized blood vessels

大腦就會回傳指示給局部血管，

to either expand if the body is too warm,

如果身體太熱，血管舒張，

releasing heat from the blood through the skin,

使體熱從血液經皮膚釋放；

or to constrict if the body is cold, which retains heat.

如果身體太冷， 血管會收縮來維持體熱。

At any given time, up to 25% of the body's blood is circulating through the dermis,

任何時候，超過 25% 的 血液會流過真皮，

making this process extremely efficient.

因此這過程非常有效率。

Under warm conditions,

感到熱時，

the skin’s sweat glands will secrete sweat via ducts onto the surface,

皮膚的「汗腺」會分泌汗液， 經管狀結構釋放到皮膚表面，

transferring heat out of the body.

藉此將熱能釋出體外。

Hair can also be stimulated to conserve or release body warmth.

毛髮也能被刺激 用以保存或釋放體熱。

The average human has 5 million hair follicles

人類全身平均有 500 萬個「毛囊」，

embedded everywhere on the body

except the palms of your hands and soles of your feet.

除了手掌和腳掌外。

Ninety to 150,000 of those are on your scalp,

頭皮有 9 萬至 15 萬個毛囊，

where they help shield the large surface area of your head

它們幫助屏蔽大面積的頭部表面，

from physical damage and sunburn.

避免物理性傷害或曬傷。

When you're cold, tiny muscles called arrector pilli

當你覺得冷時，

細小的「豎毛肌」會讓 全身的毛髮豎直挺立，

cause hair to stand upright across the body.

That’s the phenomenon known as goosebumps and it traps body heat close to your skin.

這現象就是「雞皮疙瘩」，

能將體熱鎖在皮膚裡。

Skin’s vast surface isn’t just a shield;

廣大面積的皮膚不僅是屏蔽，

it also enables us to interact and connect with the world.

它也能讓我們和外界互動和連結。

Its multifunctional layer cools us down and keeps us warm.

它的多功能層能降低或維持體溫。

The integumentary system may be many things,

「外皮系統」有許多功能，

but it’s certainly more than skin deep.

很顯然它並不「膚淺」。