The science of skin - Emma Bryce

Between you and the rest of the world
lies an interface

that makes up 16% of your physical weight.

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

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

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

But a look beyond the surface

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

First, the basics.

Skin is the foundation
of the integumentary system,

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

Made up of three layers,

the epidermis,

dermis,

and hypodermis,

skin’s thickness varies
from 0.5 millimeters at its thinnest

and up to four millimeters
at its thickest.

It also carries out three key functions:

protecting,

regulating,

and sensing the world beyond its limits.

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

of physical sensations,

relying mostly on large,
pressure-sensitive skin components

called Merkel cells.

In your fingertips alone,

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

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

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

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

fatally exposed to the elements.

Skin effectively seals off your insides

and also absorbs pressure and shock

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

The epidermis is made up mainly
of skin cells called keratinocytes

that are completely replaced
every four weeks.

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

When these cells move upwards,

they’re filled with a hardened protein
called keratin.

Once they reach the surface,

they form a tightly-overlapping,
waterproof layer

that’s difficult for invading
microbes to breach.

Any harmful microbes that make it
into the epidermis

will encounter Langerhans cells.

This group of protective skin cells
detects invaders

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

which react by launching
an immune response.

A crucial feature of this immune defense

is the several thousand
species of microorganisms

that inhabit the planes,

folds,

and crevices of your skin.

These microbes,
which include bacteria and fungi,

thrive in the sebum,

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

by sebaceous glands nestled
inside the dermis.

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

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

Beyond this protective role,

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

two roles that are closely interlinked.

Nerves detect whether your skin
is warm or cold

and communicate that
information to your brain.

In return, the brain instructs
localized blood vessels

to either expand if the body is too warm,

releasing heat from
the blood through the skin,

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

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

making this process extremely efficient.

Under warm conditions,

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

transferring heat out of the body.

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

The average human
has 5 million hair follicles

embedded everywhere on the body

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

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

where they help shield
the large surface area of your head

from physical damage and sunburn.

When you're cold, tiny muscles
called arrector pilli

cause hair to stand upright
across the body.

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

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

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

Its multifunctional layer cools us down
and keeps us warm.

The integumentary system
may be many things,

but it’s certainly more than skin deep.