Skin
For alternate meanings see skin (disambiguation). |
A close-up of human skin. |
In
zootomy and
dermatology,
skin is an
organ of the
integumentary system made up of multiple layers of
epithelial tissues that guard underlying
muscles and
organs. As the interface with the surroundings, it plays the most important role in protecting against
pathogens. Its other main functions are
insulation and
temperature regulation, sensation and
vitamin D and
B synthesis. Skin is considered one of the most important parts of the body.
Skin has
pigmentation, or
melanin, provided by
melanocytes, which absorb some of the potentially dangerous
ultraviolet radiation in
sunlight. It also contains
DNA repair
enzymes which help to reverse UV damage, and people who lack the
genes for these enzymes suffer high rates of
skin cancer. One form predominantly produced by UV light,
malignant melanoma, is particularly invasive, causing it to
spread quickly, and can often be deadly. Human skin pigmentation varies among populations in a striking manner. This has sometimes led to the classification of people(s) on the basis of
skin color.
Mammalian skin often contains hairs, which in sufficient density is called
fur. The
hair mainly serves to augment the insulation the skin provides, but can also serve as a
secondary sexual characteristic or as
camouflage. On some animals the skin is very hard and thick, and can be processed to create
leather.
Reptiles and
fish have hard protective scales on their skin for protection, and
birds have hard feathers, all made of tough β-
keratins.
Amphibian skin is not a strong barrier to passage of chemicals and is often subject to
osmosis. A
frog sitting in an
anesthetic solution will quickly go to sleep.
Damaged skin will try to heal by forming
scar tissue, often giving rise to discoloration and depigmentation of the skin.
The skin is often known as "the largest organ of the human body". This applies to exterior surface, as it covers the body,
appearing to have the largest surface area of all the organs. Moreover, it applies to weight, as it weighs more than any single internal organ, accounting for about 15 percent of body weight. For the average adult human, the skin has a surface area of between 1.5-2.0 square meters, most of it is between 2-3 mm thick. The average square inch of skin holds 650 sweat glands, 20 blood vessels, 60,000 melanocytes, and more than a thousand nerve endings.
The use of natural or synthetic
cosmetics to treat the appearance of the face and condition of the skin (such as
pore control and
black head cleansing) is common among many cultures.
|
Diagram of the layers of human skin |
Skin is composed of three primary layers: the
epidermis, which provides waterproofing and serves as a barrier to infection; the
dermis, which serves as a location for the
appendages of skin; and the
hypodermis (subcutaneous adipose layer), which is called the
basement membrane.
The outermost epidermis consists of
stratified squamous epithelium with an underlying connective tissue section, or dermis, and a hypodermis, or
basement membrane. The epidermis contains no
blood vessels, and cells in the deepest layers are nourished by diffusion from blood capillaries extending to the upper layers of the dermis. The main type of cells which make up the epidermis are
keratinocytes, with
melanocytes and
Langerhans cells also present. The epidermis can be further subdivided into the following
strata (beginning with the outermost layer): corneum, lucidum, granulosum, spinosum, basale. Cells are formed through
mitosis at the basale layer. The daughter cells, (see
cell division) move up the strata changing shape and composition as they die due to isolation from their blood source. The cytoplasm is released and the protein
keratin is inserted. They eventually reach the corneum and slough off (
desquamation). This process is called
keratinization and takes place within about 30 days. This keratinized layer of skin is responsible for keeping water in the body and keeping other harmful chemicals and
pathogens out, making skin a natural barrier to infection.
Blood capillaries are found beneath the epidermis, and are linked to an arteriole and a venule. Arterial shunt vessels may bypass the network in ears, the nose and fingertips.
The dermis lies below the epidermis and contains a number of structures including blood vessels, nerves,
hair follicles,
smooth muscle,
glands and
lymphatic tissue. It consists of loose connective tissue otherwise called areolar connective tissue -
collagen,
elastin and
reticular fibers are present. Erector muscles, or
arrector pili muscles attached between the hair papilla and epidermis, can contract, resulting in the hair fibre pulled upright and consequentially the formation of
goose bumps.
Sebaceous glands are
exocrine glands which produce
sebum, a mixture of lipids and waxy substances: lubrication, water-proofing, softening and antibactericidal actions are among the many functions of sebum.
Sweat glands open up via a duct onto the skin by a pore.
The dermis can be split into the
papillary and
reticular layers. The papillary layer is outermost and extends into the epidermis to supply it with nutrients. It is composed of loosely arranged fibres. Papillary ridges make up the lines of the hands and feet, producing individually unique
finger prints and foot prints. The reticular layer is more dense and is continuous with the hypodermis. It contains the bulk of the structures (such as sweat glands). The reticular layer is composed of irregularly arranged fibres and resists stretching.
The hypodermis is not part of the skin, and lies below the dermis. Its purpose is to attach the skin to underlying
bone and
muscle as well as supplying it with blood vessels and nerves. It consists of loose connective tissue and elastin. The main cell types are fibroblasts, macrophages and adipocytes (the hypodermis contains 50% of body fat). Fat serves as padding and insulation for the body.
Skin can be divided into
thick and
thin types. Thick skin is present on the soles of the feet and the palms of the hands. It has a larger stratum corneum with a higher keratin content. Thick skin does not grow hair; its purpose is to help grip. Thin skin is present on the bulk of the body and has a smaller
stratum corneum and fewer papillae ridges. It has hair and is softer and more elastic. The characteristics of the skin, including sensory nerve density and the type of hair, vary with location on the body.
#Protection: an anatomical barrier between the internal and external environment in bodily defense;
Langerhans cells in the skin are part of the
adaptive immune system#
Sensation: contains a variety of nerve endings that react to heat, cold, touch, pressure, vibration, and tissue injury; see
somatosensory system and
touch.#Heat regulation: the skin contains a blood supply far greater than its requirements which allows precise control of energy loss by radiation, convection and conduction. Dilated blood vessels increase perfusion and heat loss while constricted vessels greatly reduce cutaneous blood flow and conserve heat.
Erector pili muscles are significant in animals.#Control of evaporation: the skin provides a relatively dry and impermeable barrier to fluid loss. Loss of this function contributes to the massive fluid loss in
burns.#Aesthetics and communication: others see our skin and can assess or mood, physical state and attractiveness.#Storage and synthesis: acts as a storage centre for lipids and water, as well as a means of synthesis of
vitamin D and
B by action of
UV on certain parts of the skin. This synthesis is linked to pigmentation, with darker skin producing more vitamin B than D, and vice versa.#Excretion: The concentration of
urea is 1/130th that of urine. Excretion by sweating is at most a secondary function to temperature regulation.#Absorption: Oxygen, nitrogen and carbon dioxide can diffuse into the epidermis in small amounts, some animals using their skin for their sole
respiration organ. In addition, medicine can be administered through the skin, by ointments or by means of adhesive
patch, such as the
nicotine patch or
iontophoresis. The skin is an important site of transport in many other organisms.
The skin must be regularly cleaned. Unless enough care is taken it will become cracked or inflamed. Unclean skin favors the development of pathogenic organisms. The constantly peeling off dead cells of the epidermis mix with the secretions of the sweat and sebaceous glands and the dust found on the skin to form a filthy layer on its surface. If not washed away, the dirt and dead skin slurry begins to decompose emitting a foul smell. Functions of the skin are disturbed when it is dirty and it becomes more easily damaged. The release of antibacterial compounds decreases. Dirty skin is more prone to develop infections. Cosmetics should be used carefully because these may cause allergic reactions. Each season requires suitable clothing in order to facilitate the evaporation of the sweat. Sunlight, water and air play an important role in keeping the skin healthy.
The skin supports its own ecosystems of
microorganisms, including
yeasts and
bacteria, which cannot be removed by any amount of cleaning. In general these organisms keep one another in check and are part of a healthy skin. When the balance is disturbed, e.g., by
antibiotics which kill bacteria, there may be an overgrowth and infection by yeasts. The skin is continuous with the inner
epithelial lining of the body at the orifices, each of which supports its own complement of flora.
 |
A typical rash |
As skin ages, it becomes thinner and more easily damaged. Intensifying this effect is the decreasing ability of skin to heal itself. Skin sagging is caused by the fall in elasticity. Skin also receives less blood flow and lower gland activity.
In
medicine, the branch concerned with the skin is called
dermatology.
The skin is subject to constant attack from without, and so can be afflicted by numerous ailments, such as these:
Tumors:
* Benign tumors of the skin:
Squamous cell papilloma *
Skin cancerOthers:
*
Rashes
*
Blisters
*
Acne*
Keratosis pilaris*
Fungal infections such as
athlete's foot*
microbial infections.
*
calcinosis cutis*
ulcer*
sun burn*
list of skin diseases*
Hyperpigmentation*
Dermatology*
Skin color*
Cosmetics and
cosmetic surgery*
Hair*
Nails*
Polyphenol antioxidant*
Sweat*
Anthropodermic bibliopegy*
Callus
