that makes up 16% of your physical weight.

佔了人體 16 % 的體重

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

也就是皮膚，人體最大的器官

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

皮膚展開可達 1.7 平方公尺

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

皮膚的作用很簡單，保護體內器官

But a look beyond the surface

除此之外

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

皮膚在我們生活中扮演很驚人的角色

First, the basics.

首先，最基本的

Skin is the foundation of the integumentary system,

皮膚是皮膚系統的基礎

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

連接毛髮、指甲、特殊腺體和神經細胞

Made up of three layers,

皮膚構造可分三層

the epidermis,

表皮

dermis,

真皮

and hypodermis,

皮下組織

skin's thickness varies from 0.5 millimeters at its thinnest

皮膚厚度因部位而異，最薄是 0.5 公釐

and up to four millimeters at its thickest.

最厚有 4 公釐

It also carries out three key functions:

表皮有三個重要功能

protecting,

防禦

regulating,

調節

and sensing the world beyond its limits.

感知外在情況

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

每天皮膚的廣闊表面進行著千百個

of physical sensations,

體感過程

relying mostly on large, pressure-sensitive skin components

完全仰賴龐大、壓力感知的皮膚構造

called Merkel cells.

稱為梅克爾細胞

In your fingertips alone,

單憑指尖

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

皮膚每 1 平方公分就有 750 個梅克爾細胞

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

內含 2,500 個感知細胞，賦予我們觸覺

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

表皮也是身體的第一道防線

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

若沒有皮膚，你會變成一攤摻和組織細胞的水

fatally exposed to the elements.

致命地暴露於外在環境

Skin effectively seals off your insides

皮膚有效地包裹身體

and also absorbs pressure and shock

吸收壓力和衝擊

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

具彈性的膠原蛋白佈滿在真皮層

The epidermis is made up mainly of skin cells called keratinocytes

表皮層主要由角質細胞組成

that are completely replaced every four weeks.

每四周會淘汰老舊細胞

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

新的細胞在表皮層生成的同時，老舊細胞會被往上推

When these cells move upwards,

當這些老舊細胞往上推時

they're filled with a hardened protein called keratin.

稱為角質的硬化蛋白質會覆蓋這些細胞

Once they reach the surface,

一旦細胞移至表面

they form a tightly-overlapping, waterproof layer

細胞緊密交結，形成防水層

that's difficult for invading microbes to breach.

細菌難以入侵

Any harmful microbes that make it into the epidermis

任何有害細菌進入表皮層

will encounter Langerhans cells.

必須與朗格漢細胞對抗

This group of protective skin cells detects invaders

這類型的防禦皮膚細胞會偵測外在威脅

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

並且會發出訊號呼叫免疫系統 T 細胞取代自己

which react by launching an immune response.

T 細胞隨即進行免疫反應

A crucial feature of this immune defense

這個免疫防禦細胞的關鍵特徵

is the several thousand species of microorganisms

就是擁有數千種微生物種

that inhabit the planes,

分佈於皮膚表面

folds,

皺褶溝

and crevices of your skin.

以及皮膚細縫

These microbes, which include bacteria and fungi,

這些微生物含有細菌及真菌

thrive in the sebum,

大量生長在毛囊中

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

分泌至皮膚表面的油脂來自

by sebaceous glands nestled inside the dermis.

真皮層內的皮脂腺

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

這些皮膚微生物讓免疫系統維持長期的備戰狀態

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

確保身體承受風險時，免疫系統能夠馬上進行反應

Beyond this protective role,

除了扮演禦敵的角色之外

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

皮膚同時是感應器官，能夠調節體溫

two roles that are closely interlinked.

兩者角色密切相連

Nerves detect whether your skin is warm or cold

神經系統偵測皮膚的冷熱

and communicate that information to your brain.

傳遞訊息至大腦

In return, the brain instructs localized blood vessels

接著，大腦指示該處血管

to either expand if the body is too warm,

若體溫過高，則將血管擴張

releasing heat from the blood through the skin,

透過皮膚幫助血液散熱

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

若體溫低，則收縮血管保持溫度

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

任何時候，人體有 25% 的血液流經真皮層

making this process extremely efficient.

使得調節體溫過程極具效率

Under warm conditions,

在溫熱環境下

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

皮膚的汗腺會透過管路分泌汗水至皮膚表面

transferring heat out of the body.

幫助身體散熱

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

毛髮也能夠調節體溫

The average human has 5 million hair follicles

人類平均有 5 百萬個毛囊

embedded everywhere on the body

分佈於全身

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

除了手掌以及腳底沒有

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

頭皮有 9 萬至 15 萬的毛囊

where they help shield the large surface area of your head

保護大面積的頭頂表面

from physical damage and sunburn.

免遭外傷和曬傷

When you're cold, tiny muscles called arrector pilli

感到寒冷時，稱為「豎毛肌」的微小肌肉

cause hair to stand upright across the body.

使身體的毛髮豎起

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

這就是所謂的雞皮疙瘩，將體溫保留在皮膚表面

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

皮膚廣大的面積不僅具有防護作用

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

也讓我們能夠與外在世界連結或互動

Its multifunctional layer cools us down and keeps us warm.

多功能的皮膚層能夠使身體降溫或升溫

The integumentary system may be many things,

皮膚系統扮演許多角色

but it's certainly more than skin deep.

其重要性絕不「膚淺」