WEBVTT

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Between you and the rest of the world 
lies an interface

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that makes up 16% of your physical weight.

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

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

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

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But a look beyond the surface

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

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First, the basics.

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Skin is the foundation 
of the integumentary system,

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

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Made up of three layers,

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the epidermis,

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dermis,

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and hypodermis,

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skin’s thickness varies 
from 0.5 millimeters at its thinnest

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and up to four millimeters 
at its thickest.

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It also carries out three key functions:

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protecting,

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regulating,

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and sensing the world beyond its limits.

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

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of physical sensations,

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relying mostly on large, 
pressure-sensitive skin components

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called Merkel cells.

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In your fingertips alone,

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

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

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

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

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fatally exposed to the elements.

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Skin effectively seals off your insides

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and also absorbs pressure and shock

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

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The epidermis is made up mainly 
of skin cells called keratinocytes

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that are completely replaced 
every four weeks.

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

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When these cells move upwards,

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they’re filled with a hardened protein 
called keratin.

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Once they reach the surface,

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they form a tightly-overlapping, 
waterproof layer

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that’s difficult for invading 
microbes to breach.

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Any harmful microbes that make it 
into the epidermis

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will encounter Langerhans cells.

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This group of protective skin cells 
detects invaders

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

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which react by launching 
an immune response.

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A crucial feature of this immune defense

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is the several thousand 
species of microorganisms

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that inhabit the planes,

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folds,

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and crevices of your skin.

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These microbes, 
which include bacteria and fungi,

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thrive in the sebum,

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

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by sebaceous glands nestled
inside the dermis.

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

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

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Beyond this protective role,

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

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two roles that are closely interlinked.

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Nerves detect whether your skin 
is warm or cold

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and communicate that
information to your brain.

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In return, the brain instructs
localized blood vessels

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to either expand if the body is too warm,

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releasing heat from 
the blood through the skin,

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

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

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making this process extremely efficient.

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Under warm conditions,

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

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transferring heat out of the body.

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

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The average human 
has 5 million hair follicles

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embedded everywhere on the body

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

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

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where they help shield
the large surface area of your head

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from physical damage and sunburn.

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When you're cold, tiny muscles 
called arrector pilli

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cause hair to stand upright
across the body.

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

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

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

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Its multifunctional layer cools us down
and keeps us warm.

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The integumentary system 
may be many things,

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but it’s certainly more than skin deep.