Skin is our 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). Our skin protects the
network of muscles, bones, nerves, blood vessels, and everything else inside our bodies.
Our 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
Like hair, nails are a type of modified skin. Nails protect the
sensitive tips of fingers and toes. Human nails aren't 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 in many ways. It forms a barrier that prevents harmful substances
and microorganisms from entering the body. It protects body tissues against injury.
It also controls the loss of life-sustaining fluids like blood and water, helps regulate
body temperature through perspiration (sweating), and protects from the sun's damaging
Without the nerve cells in skin, people couldn't feel warmth, cold, or other sensations.
For instance, goosebumps form when the erector pili muscles contract to make hairs
on the skin stand up straight when someone is cold, excited, or frightened —
the blood vessels keep the body from losing heat by narrowing as much as possible
and keeping the warm blood away from the skin's surface, offering 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, dermis, and the subcutaneous
The upper layer of our skin, the epidermis, is the tough, protective outer layer.
It's 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 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 produce keratin, a type of protein that is a basic
component of hair, skin, nails, and helps create an intact barrier.
Langerhans 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.
Two types of fibers in the dermis — collagen and elastin — help the skin
stretch when we bend and reposition itself when we straighten up. 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
The dermis also contains a person's sebaceous glands. These glands,
which surround and empty into hair follicles and pores, produce the oil sebum 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. When pores become clogged by too much sebum
and too many dead skin cells, this contributes to acne. Later in life, these glands
produce less sebum, which contributes to dry skin as people age.
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 glands:
The eccrine glands are found everywhere, although they're 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 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 the nose, ears, and around the eyes protects these sensitive areas
from dust and other small particles. Eyebrows and eyelashes protect eyes by decreasing
the amount of light and particles that go into them. The fine hair that covers the
body provides warmth and protects the 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, which 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're 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're cut off from their supply of
nourishment and start to form a hard protein called keratin in a process called keratinization.
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.
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 isn't severely injured. White spots on the nail are sometimes due to
temporary changes in growth rate.
Things That Can Affect Skin, Hair, and Nails
The term dermatitis refers to any inflammation (swelling, itching, and redness)
possibly associated with the skin. There are many types of dermatitis, including:
dermatitis (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 and seasonal, environmental, and food allergies.
Contact dermatitis. This occurs when the skin comes into contact
with an irritating substance or one that the person is allergic or sensitive to. The
best-known cause of contact dermatitis is poison ivy, but there are many others, including
chemicals found in laundry detergent, cosmetics, and perfumes, and metals like nickel
plating on jewelry, belt buckles, and the back of a snap.
Seborrheic dermatitis. This oily rash, which appears on the scalp,
face, chest, and back, is related to an overproduction of sebum from the sebaceous
glands. This condition is common in infants and adolescents.
Bacterial Skin Infections
Impetigo is a bacterial infection that results in a honey-colored, crusty rash, often
on the face near the mouth and nose.
Cellulitis 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 area with cellulitis is usually warm, tender, and has some redness.
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 shock
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.
Ringworm, which isn't a worm at all, is a fungus infection that can affect the skin,
nails, or scalp. Fungi called dermatophytes 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
This infection of the feet is caused by the same types of fungi that cause ringworm.
Athlete's foot is commonly found in adolescents and is more likely to occur during
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 and can be easily caught from other people.
Viral infections. Many viruses cause characteristic rashes on
the skin, including varicella, the virus that causes chickenpox
and shingles; herpes simplex, which causes cold sores; human papillomavirus, the virus
that causes warts; and a host
Acne(acne vulgaris). Acne is most common 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 such as melanoma
(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.
Besides 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
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 (called
tension alopecia). Alopecia areata (when hair falls out in round or oval patches on
the scalp) is a less common condition that can affect children and teens.