FreeSchool 101!
You probably don't spend much time thinking about it, but skin is the largest organ of
the human body.
Skin is also the largest part of the integumentary system, the set of organs that form the external
coverings of the body.
This system includes hair, nails, and various glands, and plays a major role in maintaining
homeostasis - or keeping the internal conditions of the body steady.
From the outside skin looks simple, but it is actually a complex structure.
One square inch (that's about 6.5 square centimeters) of skin contains twenty feet of blood vessels,
100 oil glands, 650 sweat glands and 1,000 nerve endings.
As we mentioned before, it is also large.
The skin of an average adult has a surface area of about 21 square feet or 2 square meters,
and makes up ten to fifteen percent of your body weight.
It's big!
So let's talk about the structure of skin.
Skin is about two millimeters thick and is divided into three main layers.
The thin, outer layer is called the epidermis, and the thicker, inner layer is called the
dermis.
Below that lies subcutaneous tissue, also called the hypodermis.
The epidermis is made mostly of epithelial cells and does not contain any blood vessels
or glands.
However, it replaces itself every four to six weeks.
Cells at the bottom of the epidermal layer are constantly dividing to create new cells.
These new skin cells gradually move upwards and produce keratin - a tough protein that
is found in hair and fingernails.
As the skin cells fill with keratin, they die.
Once the dead cells reach the surface, they form a strong, waterproof layer that protects
the body.
The outermost skin cells are shed over time and are replaced with more cells from below,
over and over and over.
The base of the epidermis contains special cells called melanocytes.
Melanocytes make melanin, the pigment that gives your skin its color.
Everyone has about the same number of melanocytes, but those belonging to people with darker
skin produce more melanin.
Increased exposure to ultraviolet light will also make your skin produce more melanin,
which is the reason going out in the sun can cause your skin to tan.
Below the epidermis is the dermis.
The dermis is much thicker, making up about 90% of the thickness of your skin, and is
composed of tough connective tissue.
This layer is where you will find blood vessels and nerve endings, so any time you get a cut
that hurts and bleeds that means you have punctured the dermis.
The dermis is also where you will find hair follicles, the structures that produce hairs,
as well as two types of glands.
Sebaceous glands, aka oil glands, are located along the hair follicle and produce a waxy
oil called sebum that lubricates the hair and skin.
These glands are found in the greatest concentration on the face and scalp and are the culprits
behind acne.
Sweat glands, unsurprisingly, produce sweat.
Sweat is mostly water but it does contain salt and some other waste products.
Each sweat gland has a duct that reaches up to the surface of the skin which allows sweat
to escape.
Moving on, the hypodermis or subcutaneous tissue is the innermost layer of skin and
is used mostly for fat storage.
Your skin plays a critical role in keeping you alive.
It acts as a barrier between your body and the environment around you, preventing bacteria
or other microorganisms from getting in, and preventing too much water from getting out.
Skin helps regulate your temperature.
Evaporating sweat can help cool you down.
Blood vessels in the skin can also dilate or constrict as needed to increase or decrease
the blood flow to the surface of your body, depending on whether you need to vent or conserve
heat.
Let's not forget about hair and nails!
Both hair and nails are made primarily of keratin, making them strong and waterproof.
Hair plays a role in maintaining homeostasis by protecting your head from the sun and insulating
your body from heat loss.
Eyelashes and eyebrows help you avoid getting dust and sweat in your eyes, and hair in your
nose helps filter out dust and microorganisms before they can enter your body.
Finally, finger and toe nails act as shields for the delicate ends of your fingers and
toes.
You may also be surprised to learn that the nails increase sensitivity of your digits
by giving the pad a surface to press against when you touch something.
So let's review: the integumentary system of the body consists mainly of the hair, skin,
and nails, and it provides an external covering for the body.
You may take it for granted, but your integumentary system performs multiple complex functions
and helps maintain the conditions inside your body that are necessary for life.