Your skin is your largest organ. If the skin of a typical 150-pound (68-kilogram)
adult male were stretched out flat, it would cover about 2 square yards (1.7 square
meters) and weigh about 9 pounds (4 kilograms).
Skin protects the network of muscles,
bones, nerves, blood vessels,
and everything else inside our bodies. Eyelids have the thinnest skin, the soles of
our feet the thickest.
Hair is actually a modified type of skin. Hair grows everywhere on the human body
except the palms of the hands, soles of the feet, and lips. Hair grows more quickly
in summer than winter, and more slowly at night than during the day.
Like hair, nails are a type of modified skin — and they're not just for beauty.
Nails protect the sensitive tips of our fingers and toes. Human nails are not necessary
for living, but they do provide support for the tips of the fingers and toes, protect
them from injury, and aid in picking up small objects. Without them, we'd have a hard
time scratching an itch or untying a knot. Nails can be an indicator of a person's
general health, and illness often affects their growth.
Skin is essential to a person's survival. It forms a barrier that prevents harmful
substances and microorganisms from entering the body. It protects body tissues against
injury. Our skin also controls the loss of life-sustaining fluids like blood and water,
helps us regulate body temperature through perspiration, and protects us from the
sun's damaging ultraviolet rays.
Without the nerve cells in our skin, we couldn't feel warmth, cold, or other sensations.
Our skin can also respond to situations and emotions: Muscles in the skin called erector
pili contract to make the hairs on our skin stand up straight (goosebumps)
when we are cold or frightened — for insulation and protection.
Every square inch of skin contains thousands of cells and hundreds of sweat glands,
oil glands, nerve endings, and blood vessels. Skin is made up of three layers: the
epidermis (pronounced: ep-ih-DUR-mis), dermis, and
the subcutaneous (pronounced: sub-kyoo-TAY-nee-us)
Skin Cells and Layers
The upper layer of our skin, the epidermis, is the tough, protective outer layer.
It is about as thick as a sheet of paper over most parts of the body. The epidermis
has four layers of cells that are constantly flaking off and being renewed. In these
four layers are three special types of cells:
Melanocytes (pronounced: meh-LAH-nuh-sites) produce melanin,
the pigment that gives skin its color. All people have roughly the same number of
melanocytes; the more melanin that is produced, the darker the skin. Exposure to sunlight
increases the production of melanin, which is why people get suntanned or freckled.
Keratinocytes (pronounced: ker-uh-TIH-no-sites) produce keratin,
a type of protein that is a basic component of hair and nails. Keratin is also found
in skin cells in the skin's outer layer, where it helps create a protective barrier.
Langerhans (pronounced: LAHNG-ur-hanz) cells
help protect the body against infection.
Because the cells in the epidermis are completely replaced about every 28 days,
cuts and scrapes heal quickly.
Below the epidermis is the next layer of our skin, the dermis,
which is made up of blood vessels, nerve endings, and connective tissue. The dermis
nourishes the epidermis.
Without certain molecules in the dermis, our skin wouldn't stretch when we bend
or reposition itself when we straighten up. These two types of fibers in the dermis,
collagen and elastin, help the skin stretch and
reposition itself when we move. Collagen is strong and hard to stretch and elastin,
as its name suggests, is elastic. In older people, some of the elastin-containing
fibers degenerate, which is one reason why the skin looks wrinkled (most wrinkles
are caused by sun exposure, though!).
The dermis also contains a person's sebaceous glands. These glands, which surround
and empty into our hair follicles and pores, produce an oil called sebum
(pronounced: SEE-bum) that lubricates the skin and hair. Sebaceous
glands are found mostly in the skin on the face, upper back, shoulders, and chest.
Most of the time, the sebaceous glands make the right amount of sebum. As a person's
body begins to mature and develop during the teenage years, though, hormones stimulate
the sebaceous glands to make more sebum. This can lead to acne
when pores become clogged by too much sebum and too many dead skin cells. Later in
life, these glands produce less sebum, which contributes to dry skin in older people.
The bottom layer of our skin, the subcutaneous tissue, is made
up of connective tissue, sweat glands, blood vessels, and cells that store fat. This
layer helps protect the body from blows and other injuries and helps it hold in body
There are two types of sweat-producing glands. The eccrine (pronounced:
EH-krun) glands are found everywhere in our bodies, although they
are mostly in the forehead, palms, and soles of the feet. By producing sweat, these
glands help regulate body temperature, and waste products are excreted through them.
The other type of sweat-producing gland, the apocrine glands,
develop at puberty and are
concentrated in the armpits and pubic region. The sweat from the apocrine glands is
thicker than that produced by the eccrine glands. Although this sweat doesn't smell,
when it mixes with bacteria on the skin's surface, it can cause body odor.
A normal, healthy adult secretes about 1 pint (about half a liter) of sweat daily,
but this may be increased by physical activity, fever, or a hot environment.
The hair on our heads isn't just there for looks. It keeps us warm by preserving
heat. The hair in our nose, ears, and around our eyes protects these sensitive areas
of the body from dust and other small particles. Eyebrows and eyelashes protect our
eyes by decreasing the amount of light and particles that go into them. The fine hair
that covers our bodies provides warmth and protects our skin. Hair also cushions the
body against injury.
Human hair consists of the hair shaft, which projects from the
skin's surface, and the root, a soft thickened bulb at the base of
the hair embedded in the skin. The root ends in the hair bulb. The
hair bulb sits in a sac-like pit in the skin called the follicle,
from which the hair grows.
At the bottom of the follicle is the papilla, where hair growth
actually takes place. The papilla contains an artery that nourishes the root of the
hair. As cells multiply and produce keratin to harden the structure, they are pushed
up the follicle and through the skin's surface as a shaft of hair. Each hair has three
layers: the medulla at the center, which is soft; the cortex,
which surrounds the medulla and is the main part of the hair; and the cuticle,
the hard outer layer that protects the shaft.
Hair grows by forming new cells at the base of the root. These cells multiply to
form a rod of tissue in the skin. The rods of cells move upward through the skin as
new cells form beneath them. As they move up, they are cut off from their supply of
nourishment and start to form a hard protein called keratin in a process called keratinization
(pronounced: ker-uh-tuh-nuh-ZAY-shun). As this process occurs, the hair cells die.
The dead cells and keratin form the shaft of the hair.
Each hair grows about ¼ inch (about 6 millimeters) every month and keeps
on growing for up to 6 years. The hair then falls out and another grows in its place.
The length of a person's hair depends on the length of the growing phase of the follicle.
Follicles are active for 2 to 6 years; they rest for about 3 months after that. A
person becomes bald if the scalp follicles become inactive and no longer produce
new hair. Thick hair grows out of large follicles; narrow follicles produce thin hair.
The color of a person's hair is determined by the amount and distribution of melanin
in the cortex of each hair (the same melanin that's found in the epidermis). Hair
also contains a yellow-red pigment; people who have blonde or red hair have only a
small amount of melanin in their hair. Hair becomes gray when people age because pigment
no longer forms.
All About Nails
Nails grow out of deep folds in the skin of the fingers and toes. As epidermal
cells below the nail root move up to the surface of the skin, they increase in number,
and those closest to the nail root become flattened and pressed tightly together.
Each cell is transformed into a thin plate; these plates are piled in layers to form
the nail. As with hair, nails are formed by keratinization. When
the nail cells accumulate, the nail is pushed forward.
The skin below the nail is called the matrix. The larger part
of the nail, the nail plate, looks pink because of the network of
tiny blood vessels in the underlying dermis. The whitish crescent-shaped area at the
base of the nail is called the lunula.
Fingernails grow about three or four times as quickly as toenails. Like hair, nails
grow more rapidly in summer than in winter. If a nail is torn off, it will regrow
if the matrix is not severely injured. White spots on the nail are sometimes due to
temporary changes in growth rate.
Some of the things that can affect the skin, nails, and hair are described below.
Medical experts use the term dermatitis (pronounced: dur-mah-TY-tus)
to refer to any inflammation that might be associated with swelling, itching, and
redness of the skin. There are many types of dermatitis, including:
Atopic dermatitis is also called eczema.
It's a common, hereditary dermatitis that causes an itchy rash primarily on the face,
trunk, arms, and legs. It commonly develops in infancy, but can also appear in early
childhood. It may be associated with allergic diseases such as asthma
or food, seasonal, or environmental allergies.
Contact dermatitis occurs when the skin comes into contact with
an irritating substance or a substance that a person is allergic to. The best-known
cause of contact dermatitis is poison ivy. But lots of other things cause contact
dermatitis, including chemicals found in laundry detergent, cosmetics, and perfumes,
and metals like jewelry, nickel plating on a belt buckle, or the back of the buttons
on your jeans.
Seborrheic dermatitis, an oily rash on the scalp, face, chest,
and back, is related to an overproduction of sebum from the sebaceous glands.
This condition is common in teens.
Bacterial Skin Infections
Impetigo (pronounced: im-puh-TY-go) is a bacterial infection that results in a honey-colored,
crusty rash, often on the face near the mouth and nose.
Cellulitis (pronounced: sell-yuh-LY-tus) is an infection of the skin and subcutaneous
tissue that typically occurs when bacteria are introduced through a puncture, bite,
or other break in the skin. The affected area is usually warm and tender and has some
Streptococcal and staphylococcal infections.
These two kinds of bacteria are the main causes of cellulitis and impetigo. Certain
types of these bacteria are also responsible for distinctive rashes on the skin, including
the rashes associated with scarlet fever and toxic
Fungal Infections of the Skin and Nails
Candidal dermatitis. A warm, moist environment, such as that
found in the folds of the skin in the diaper area of infants, is perfect for growth
of the yeast Candida. Yeast infections of the skin in older children, teens,
and adults are less common.
Tinea infection (ringworm).
Ringworm, which isn't a worm at all, is a fungus infection that can affect the skin,
nails, or scalp. Tinea (pronounced: TIH-nee-uh) fungi can infect the skin
and related tissues of the body. The medical name for ringworm of the scalp is tinea
capitis; ringworm of the body is called tinea corporis; and ringworm of the nails
is called tinea unguium. With tinea corporis, the fungi can cause scaly, ring-like
lesions anywhere on the body.
Tinea pedis (athlete's
foot). This infection of the feet is caused by the same types of fungi
(called dermatophytes) that cause ringworm. Athlete's foot is commonly found in adolescents
and is more likely to happen during warm weather.
Other Skin Problems
Parasitic infestations. Parasites (usually tiny insects or worms)
can feed on or burrow into the skin, often resulting in an itchy rash. Scabies
and lice are examples of parasitic infestations. Both are contagious — meaning
they can be easily caught from other people.
Viral infections. Many viruses cause characteristic rashes on
the skin, including varicella (pronounced: var-ih-SEL-uh), the virus that causes chicken
pox and shingles; herpes simplex, which causes cold sores; human papillomavirus (HPV),
the virus that causes warts;
and a host of others.
Acne(acne vulgaris). Acne is the single most
common skin condition in teens. Some degree of acne is seen in 85% of adolescents,
and nearly all teens have the occasional pimple, blackhead, or whitehead.
Skin cancer. Skin cancer is rare in children and teens, but good
sun protection habits established during these years can help prevent skin cancers
like melanoma (pronounced: mel-uh-NO-ma, a serious form of skin
cancer that can spread to other parts of the body) later in life, especially among
fair-skinned people who sunburn easily.
In addition to these diseases and conditions, the skin can be injured in a number
of ways. Minor scrapes, cuts, and bruises heal quickly on their own, but other injuries
— severe cuts and burns, for example — require medical treatment.
Disorders of the Scalp and Hair
Tinea capitis, a type of ringworm, is a fungal infection that
forms a scaly, ring-like lesion in the scalp. It's contagious and common among school-age
Alopecia (pronounced: ah-luh-PEE-sha) is an
area of hair loss. Ringworm is a common cause of temporary alopecia in children. Alopecia
can also be caused by tight braiding that pulls on the hair roots (this condition
is called traction alopecia). Alopecia areata (where a person's hair
falls out in round or oval patches on the scalp) is a less common condition that
can sometimes affect teens.