0:00:07.266,0:00:10.428
Between you and the rest of the world [br]lies an interface

0:00:10.428,0:00:13.437
that makes up 16% of your physical weight.

0:00:13.437,0:00:17.087
This is your skin, the largest organ [br]in your body:

0:00:17.087,0:00:22.468
laid out flat, it would cover close to [br]1.7 square meters of ground.

0:00:22.468,0:00:26.527
Its purpose may seem obvious—[br]to keep our insides in.

0:00:26.527,0:00:28.317
But a look beyond the surface

0:00:28.317,0:00:32.177
reveals that it plays a surprising [br]number of roles in our lives.

0:00:32.177,0:00:33.978
First, the basics.

0:00:33.978,0:00:37.557
Skin is the foundation [br]of the integumentary system,

0:00:37.557,0:00:44.158
which also incorporates your hair, [br]nails, and specialized glands and nerves.

0:00:44.158,0:00:45.788
Made up of three layers,

0:00:45.788,0:00:46.948
the epidermis,

0:00:46.948,0:00:47.997
dermis,

0:00:47.997,0:00:49.229
and hypodermis,

0:00:49.229,0:00:53.738
skin’s thickness varies [br]from 0.5 millimeters at its thinnest

0:00:53.738,0:00:57.019
and up to four millimeters [br]at its thickest.

0:00:57.019,0:00:59.789
It also carries out three key functions:

0:00:59.789,0:01:00.746
protecting,

0:01:00.746,0:01:01.714
regulating,

0:01:01.714,0:01:04.768
and sensing the world beyond its limits.

0:01:04.768,0:01:09.579
On a daily basis, its huge surface [br]processes hundreds, if not thousands,

0:01:09.579,0:01:11.747
of physical sensations,

0:01:11.747,0:01:15.746
relying mostly on large, [br]pressure-sensitive skin components

0:01:15.746,0:01:17.959
called Merkel cells.

0:01:17.959,0:01:19.631
In your fingertips alone,

0:01:19.631,0:01:24.599
there are 750 Merkel cells [br]per each square-centimeter of skin,

0:01:24.599,0:01:29.709
coupled with over 2,500 receptors[br]that give you your sense of touch.

0:01:29.709,0:01:34.038
This surface is also the body’s first [br]major line of defense.

0:01:34.038,0:01:38.349
Without it, you’d be a soggy mass [br]of tissue and fluids,

0:01:38.349,0:01:41.059
fatally exposed to the elements.

0:01:41.059,0:01:44.341
Skin effectively seals off your insides

0:01:44.341,0:01:47.036
and also absorbs pressure and shock

0:01:47.036,0:01:51.120
with flexible collagen [br]that makes up most of its dermal layer.

0:01:51.120,0:01:55.791
The epidermis is made up mainly [br]of skin cells called keratinocytes

0:01:55.791,0:01:58.812
that are completely replaced [br]every four weeks.

0:01:58.812,0:02:04.071
As new cells form at the base of the[br]epidermis, older ones are pushed up.

0:02:04.071,0:02:06.322
When these cells move upwards,

0:02:06.322,0:02:09.991
they’re filled with a hardened protein [br]called keratin.

0:02:09.991,0:02:11.511
Once they reach the surface,

0:02:11.511,0:02:15.261
they form a tightly-overlapping, [br]waterproof layer

0:02:15.261,0:02:18.772
that’s difficult for invading [br]microbes to breach.

0:02:18.772,0:02:21.864
Any harmful microbes that make it [br]into the epidermis

0:02:21.864,0:02:24.332
will encounter Langerhans cells.

0:02:24.332,0:02:28.136
This group of protective skin cells [br]detects invaders

0:02:28.136,0:02:32.785
and communicates their presence [br]to resident immune system T-cells,

0:02:32.785,0:02:36.172
which react by launching [br]an immune response.

0:02:36.172,0:02:38.838
A crucial feature of this immune defense

0:02:38.838,0:02:42.335
is the several thousand [br]species of microorganisms

0:02:42.335,0:02:43.695
that inhabit the planes,

0:02:43.695,0:02:44.694
folds,

0:02:44.694,0:02:47.085
and crevices of your skin.

0:02:47.085,0:02:50.023
These microbes, [br]which include bacteria and fungi,

0:02:50.023,0:02:51.575
thrive in the sebum,

0:02:51.575,0:02:54.975
an oily substance that’s secreted [br]onto the skin’s surface

0:02:54.975,0:02:58.595
by sebaceous glands nestled[br]inside the dermis.

0:02:58.595,0:03:03.596
These skin microbes keep the immune system[br]in a state of constant surveillance,

0:03:03.596,0:03:08.647
ensuring that it’s ready to react [br]if the body really is at risk.

0:03:08.647,0:03:10.297
Beyond this protective role,

0:03:10.297,0:03:15.206
your skin is also a sensory organ that[br]helps regulate your body’s temperature,

0:03:15.206,0:03:17.916
two roles that are closely interlinked.

0:03:17.916,0:03:21.266
Nerves detect whether your skin [br]is warm or cold

0:03:21.266,0:03:24.126
and communicate that[br]information to your brain.

0:03:24.126,0:03:27.327
In return, the brain instructs[br]localized blood vessels

0:03:27.327,0:03:29.947
to either expand if the body is too warm,

0:03:29.947,0:03:32.426
releasing heat from [br]the blood through the skin,

0:03:32.426,0:03:36.937
or to constrict if the body is cold,[br]which retains heat.

0:03:36.937,0:03:42.327
At any given time, up to 25% of the body's[br]blood is circulating through the dermis,

0:03:42.327,0:03:45.267
making this process extremely efficient.

0:03:45.267,0:03:46.638
Under warm conditions,

0:03:46.638,0:03:51.257
the skin’s sweat glands will secrete sweat[br]via ducts onto the surface,

0:03:51.257,0:03:53.449
transferring heat out of the body.

0:03:53.449,0:03:58.318
Hair can also be stimulated[br]to conserve or release body warmth.

0:03:58.318,0:04:01.440
The average human [br]has 5 million hair follicles

0:04:01.440,0:04:03.496
embedded everywhere on the body

0:04:03.496,0:04:06.705
except the palms of your hands [br]and soles of your feet.

0:04:06.705,0:04:10.285
Ninety to 150,000 of those [br]are on your scalp,

0:04:10.285,0:04:13.506
where they help shield[br]the large surface area of your head

0:04:13.506,0:04:16.426
from physical damage and sunburn.

0:04:16.426,0:04:19.788
When you're cold, tiny muscles [br]called arrector pilli

0:04:19.788,0:04:23.376
cause hair to stand upright[br]across the body.

0:04:23.376,0:04:29.017
That’s the phenomenon known as goosebumps[br]and it traps body heat close to your skin.

0:04:29.017,0:04:31.648
Skin’s vast surface isn’t just a shield;

0:04:31.648,0:04:35.416
it also enables us to interact [br]and connect with the world.

0:04:35.416,0:04:39.297
Its multifunctional layer cools us down[br]and keeps us warm.

0:04:39.297,0:04:42.270
The integumentary system [br]may be many things,

0:04:42.270,0:04:44.737
but it’s certainly more than skin deep.