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:''For alternate meanings see [[skin (disambiguation)]].'' |
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In [[zootomy]] and [[dermatology]], '''skin''' is an [[organ (anatomy)|organ]] of the [[integumentary system]] made up of multiple layers of [[epithelial]] [[biological tissue|tissues]] that guard underlying [[muscle]]s and [[organ (anatomy)|organ]]s. As the interface with the surroundings, it plays the most important role in protecting against [[pathogen]]s. Its other main functions are [[Thermal insulation|insulation]] and [[temperature]] regulation, sensation and [[vitamin D]] and [[Vitamin B|B]] synthesis. Skin is considered one of the most important parts of the body. |
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Skin has [[pigment]]ation, or [[melanin]], provided by [[melanocyte]]s, which absorb some of the potentially dangerous [[ultraviolet radiation]] in [[sunlight]]. It also contains [[DNA]] repair [[enzyme]]s which help to reverse UV damage, and people who lack the [[gene]]s for these enzymes suffer high rates of [[skin cancer]]. One form predominantly produced by UV light, [[malignant]] [[melanoma]], is particularly invasive, causing it to [[metastasize|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 [[human skin color|skin color]]. |
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[[Mammal]]ian 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 sex characteristic|secondary sexual characteristic]] or as [[camouflage]]. On some animals the skin is very hard and thick, and can be processed to create [[leather]]. [[Reptile]]s and [[fish]] have hard protective scales on their skin for protection, and [[bird]]s have hard feathers, all made of tough β-[[keratin]]s. [[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. |
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Damaged skin will try to heal by forming [[scar|scar tissue]], often giving rise to discoloration and depigmentation of the skin. |
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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. |
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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. |
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==Layers== |
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[[Image:skin.jpg|thumb|right|400px|Diagram of the layers of human skin]] |
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'''Skin''' is composed of three primary layers: the ''[[epidermis (skin)|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]]. |
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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 vessel]]s, 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 [[keratinocyte]]s, with [[melanocyte]]s and [[Langerhans cell]]s 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. |
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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. |
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The dermis lies below the epidermis and contains a number of structures including blood vessels, nerves, [[hair]] follicles, [[smooth muscle]], [[gland]]s and [[lymphatic tissue]]. It consists of loose connective tissue otherwise called areolar connective tissue - [[collagen]], [[elastin]] and [[reticular fiber]]s are present. Erector muscles, or [[arrectores pilorum]] attached between the hair papilla and epidermis, can contract, resulting in the hair fibre pulled upright and consequentially the formation of [[goose bumps]]. [[Sebaceous gland]]s 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 gland]]s open up via a duct onto the skin by a pore. |
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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 [[fingerprint]]s 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. |
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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. |
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Microorganisms like ''Staphylococcus epidermidis'' colonize the skin surface. These microorganisms serve as [[ecoorgan]]. The density of skin flora depends on region of the skin. The disinfected skin surface gets recolonized from bacteria residing in the deeper areas of the hair follicle, gut and urogenital openings. |
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==Functions== |
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#Protection: an anatomical barrier between the internal and external environment in bodily defense; [[Langerhans cell]]s in the skin are part of the [[adaptive immune system]] |
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#[[Sensation]]: contains a variety of nerve endings that react to heat, cold, touch, pressure, vibration, and tissue injury; see [[somatosensory system]] and [[touch]]. |
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#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 muscle]]s are significant in animals. |
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#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 [[burn (injury)|burns]]. |
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#Aesthetics and communication: others see our skin and can assess our mood, physical state and attractiveness. |
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#Storage and synthesis: acts as a storage centre for lipids and water, as well as a means of synthesis of [[vitamin D]] and [[Vitamin B|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. |
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#Excretion: The concentration of [[urea]] is 1/130th that of urine. Excretion by sweating is at most a secondary function to temperature regulation. |
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#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 [[transdermal patch|patch]], such as the [[nicotine patch]] or [[iontophoresis]]. The skin is an important site of transport in many other organisms |
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==Hygiene== |
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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 dead cells that continually slough off 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. |
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The skin supports its own ecosystems of [[microorganism]]s, including [[yeast]]s 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 [[antibiotic]]s 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. |
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== Aging and disease == |
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{{details|ageing}} |
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[[Image:Rash.jpg|thumb|right|250px|A typical rash]] |
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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. |
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In [[medicine]], the branch concerned with the skin is called [[dermatology]]. |
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The skin is subject to constant attack from without, and so can be afflicted by numerous ailments, such as these: |
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'''Tumors''': |
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* Benign tumors of the skin: [[Squamous cell papilloma]] |
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* [[Skin cancer]] |
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'''Others''': |
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* [[Rash]]es |
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* [[Blister]]s |
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* [[Acne vulgaris|Acne]] |
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* [[Keratosis pilaris]] |
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* [[fungus|Fungal]] infections such as [[athlete's foot]] |
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* [[microbe|microbial]] infections. |
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* [[calcinosis cutis]] |
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* [[ulcer]] |
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* [[sun burn]] |
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* [[list of skin diseases]] |
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==See also== |
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* [[Hyperpigmentation]] |
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* [[Dermatology]] |
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* [[Human skin color|Skin color]] |
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* [[Cosmetics]] and [[cosmetic surgery]] |
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* [[Hair]] |
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* [[Meissner's corpuscle]] |
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* [[Nail (anatomy)|Nails]] |
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* [[Pacinian corpuscle]] |
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* [[Polyphenol antioxidant]] |
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* [[Sweat]] |
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* [[Anthropodermic bibliopegy]] |
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* [[Callus]] |
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{{wiktionary}} |
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{{integumentary_system}} |
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{{human anatomical features}} |
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<!----> |
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[[Category:Sensory organs]] |
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[[Category:Integumentary system]] |
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[[Category:Dermatology]] |
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[[af:Vel]] |
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[[ar:جلد]] |
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[[bg:Кожа]] |
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[[ca:Pell]] |
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[[cs:Kůže]] |
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[[cy:Croen]] |
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[[da:Hud]] |
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[[de:Haut (Säugetiere)]] |
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[[et:Nahk]] |
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[[es:Piel]] |
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[[eo:Haŭto]] |
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[[fa:پوست]] |
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[[fr:Peau]] |
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[[gl:Pel]] |
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[[hr:Koža]] |
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[[id:Kulit]] |
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[[it:Pelle]] |
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[[he:עור]] |
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[[lt:Oda]] |
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[[nl:Huid]] |
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[[ja:皮膚]] |
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[[no:Hud]] |
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[[nrm:Pé (la)]] |
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[[pam:Balat]] |
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[[pl:Skóra (histologia)]] |
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[[pt:Pele]] |
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[[scn:Peddi]] |
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[[simple:Skin]] |
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[[sk:Koža]] |
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[[sl:Koža]] |
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[[sr:Кожа]] |
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[[su:Kulit]] |
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[[fi:Iho]] |
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[[sv:Hud]] |
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[[vi:Da]] |
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[[tr:Cilt (Organ)]] |
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[[zh:皮膚]] |
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