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The
body's largest organ
The
skin is the body's largest organ, and it has an area of nearly
two square meters. The skin has many different tasks.
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Protects
against including bacteria, viruses, corrosive substances and
abrasion.
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Helps control body temperature at the right level.
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Avoid losing too much fluid.
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Excrete water and salts through sweating and evaporation.
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Stores the liquid and fat.
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Protects the body against harmful ultraviolet radiation from
the sun.
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Produces vitamin D with sun's help, as needed, for the body
to be able to absorb calcium into bone.
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Serves as major sensory organs that can sense cold, heat,
pressure, touch and pain.
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Signals to the surroundings, for example, blush when you get
embarrassed.
Skin
structure
Although
the skin seems very thin, it consists of several layers of different
types of tissues. Skin components are
The
Normal Skin
The skin, or integument, consists of two layers, the epidermis
and the underlying dermis, along with their associated appendages,
such as hair follicles and glands. The epidermis consists of
five layers, which vary in thickness at different areas of the
body. For example, facial skin is relatively thin, but the soles
are protected by a thick layer of skin (primarily stratum corneum).
There are no blood vessels or
nerves in the epidermis.
Nutrients
and fluid diffuse into it from blood vessels in the dermis.
The epidermis
is the stratum basale, located on the basement membrane. New
squamous epithelial cells form by mitosis in the stratum basale,
and one of each pair of cells then moves upward, forming, in
turn, the stratum spinosum, the stratum granulosum, and the
stratum lucidum (which is present primarily in thick skin),
eventually being shed from the outer layer, the stratum corneum.
While these cells are in the stratum granulosum, keratin, a
protein found in skin, hair, and nails, is deposited in them.
Keratin prevents both loss of body fluid through the skin and
entry of excessive water into the body, as when swimming. The
epithelial cells become flatter as they progress upward away
from the dermis, and they eventually die from lack of nutrients.
Thus, the stratum corneum, the top or outer layer of the epidermis,
consists of many layers of dead, flat, keratinized cells that
are constantly sloughed from the surface a few weeks after being
formed in the basal layer.
The epidermis
also contains melanocytes, specialized pigment-producing cells.
The amount of melanin, or dark pigment, produced by these cells
determines skin color. Melanin production depends on genetic
as well as environmental factors such as sun exposure (ultraviolet
light). African Americans rarely develop skin cancer because
of increased melanin in the skin, a protection from ultraviolet
rays.
Albinism
results from a recessive trait leading to a lack of melanin
production. A person with this trait has white skin and hair
and lacks pigment in the iris of the eye. This individual must
avoid exposure to the sun. Vitiligo refers to small areas of
hypo-pigmentation. Melasma, or chloasma, refers to patches of
darker skin, often on the face, that may develop during pregnancy.
An additional pigment, carotene, gives a yellow color to the
skin. Pink tones in the skin are increased with additional vascularity
or blood flow in the dermis.
The dermis
is a thick layer of connective tissue that includes elastic
and collagen fibers and varies in thickness over the body. These
constituents provide both flexibility and strength, in the skin
and support for the nerves and blood vessels passing through
the dermis. Many sensory receptors for pressure or texture,
pain, heat, or cold are found in the dermis. The junction of
the dermis with the epidermis is marked {by papillae, irregular
projections of dermis into the epidermal region. More capillaries
are located in the papillae to facilitate diffusion of {nutrients
into the epidermis. Blood flow is controlled by the sympathetic
nervous system.
Embedded
in the skin are the appendages, or accessory structures; the
hair follicles, sweat and sebaceous glands, and nails.
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The hair
follicles are lined by epidermis that is continuous with the
surface, the stratum basale producing the hair. Each hair follicle
has smooth muscle attached to it, the arrector pili, controlled
by sympathetic nerves. These may be stimulated by emotion or
exposure to cold, causing the hairs to stand upright (on end)
or creating small elevations on the skin (goose bumps).
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Sebaceous glands may
be associated with hair follicles or may open directly onto the
skin. These glands produce an oily secretion, sebum, which keeps
the hair and skin soft and retards fluid loss from the skin.
Secretions of sebum increase at puberty under the influence of
the sex hormones.
Sweat glands are of two types:
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Eccrine,
or merocrine, glands are located all over the body and secrete
sweat through pores onto the skin in response to increased heat
or emotional stress (SNS control).
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Apocrine
sweat glands are located in the axillae, scalp, face, and external
genitalia, and the ducts of these glands open into the hair
follicles.
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The secretion,
sweat or perspiration, is odorless when formed, but bacterial
action by normal flora on the constituents of sweat often causes
odor to develop.
The subcutaneous
tissue or hypodermis, which consists of connective tissue, fat
cells, macrophages, fibroblasts, blood vessels, nerves, and
the base of many of the appendages.
A complex
mix of resident (normal) flora is present on the skin, and the
components differ in various body areas. Microbes residing under
the fingernails may infect inflammatory lesions or breaks in
the skin, particularly when one scratches the skin. Microbes,
primarily bacteria and fungi, are also present deep in the hair
follicles and glands of the skin and may be a source of opportunistic
infections when there is injury such as burns or other inflammatory
lesion. Infection may spread systemically from skin lesions.
Skin
has many functions:
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When unbroken,
it provides the first line of defense against invasion by microorganisms
and other foreign material. Sebum is acidic and inhibits bacterial
growth. The resident flora of the skin is a deterrent to invading
organisms.
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Skin prevents
excessive fluid loss.
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It is
important in controlling body temperature, using two mechanisms: cutaneous vasodilation, which increases peripheral blood flow,
and increased secretion and evaporation of sweat both have a
cooling effect on the body.
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Sensory
perception provided by the skin is important as a defense against
environmental hazards, as a learning tool, and as a means of
communicating emotions.
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Another
important function of the skin is the synthesis and activation
of vitamin-D on exposure to small amounts of ultraviolet light.
The skin
is easily damaged because of its exposure to many types of irritants,
trauma, or insect bites. Systemic disorders additionally may
affect the skin. Also the skin changes with aging; showing loss
of elasticity, thinning, and loss of subcutaneous tissue.
Minor abrasions
or cuts of the skin heal quickly with mitosis of the epithelial
cells. When large areas of the skin are damaged, appendages
may be lost, function impaired, and fibrous scar tissue forms,
perhaps restricting mobility of joints.
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