The
skin is an organ of the integumentary system; which is composed of a layer of tissues that protect underlying muscles and organs. Skin is used for insulation, vitamin D production, sensation, and excretion (through sweat).
Skin on creatures regularly subjected to sunlight have pigmentation which absorbs the potentially dangerous radiation contained therein. Human skin pigmentation varies across populations in a visually dramatic manner.
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 in 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. For the average adult homo sapiens, it has a surface area of between 1.5-2.0 square
meters.
Layers
Skin is composed of the epidermis and the dermis. Below these layers lies the hypodermis, which is not usually classified as a layer of skin.
The outermost epidermis is made up of stratified squamous epithelium with an underlying basement membrane. It contains no blood vessels, and is nourished by diffusion from 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 in to the following strata (beginning with the outermost layer): corneum, lucidum, granulosum, spinosum, basale. Cells are formed through mitosis at the innermost layers. They move up the strata changing shape and composition as they differentiate and become filled with keratin. They eventually reach the corneum and become sloughed off. This process is called keratinization and takes place within about 30 days.
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 is made up of dense connective tissue - collagen, elastin and reticular fibres are present. Erector muscles, attached between the hair papilla and epidermis, can contract, resulting in the hair fibre pulled upright and consequentially goose bumps. The main cell types are fibroblasts, adipocytes (fat storage) and macrophages. Sebaceous glands are exocrine glands which produce sebum, a mixture of lipids and waxy substances: lubrication, water-proofing, softening and antibatericidal 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 in to the papillary and reticular layers. The papillary layer is outermost and extends in to the dermis to supply it with vessels. It is composed of loosely arranged fibres. Papillary ridges make up the lines of the hands. 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 is made up 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.
Types
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.
Functions
Protection: a barrier between the internal and external environment in bodily
defense
Sensation: enables irritability
Storage: acts as a storage center for lipids and water, as well as a means of synthesis of vitamin D by action of UV on certain parts of the skin.
Absorption: Oxygen, nitrogen and carbon dioxide can diffuse into the epidermis in small amounts.
Excretion: The concentration of urea is 1/130th than that of urine. Excretion by sweating is at most a secondary function to temperature regulation.
Aging and disease
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:
Skin cancer
Rashes
Blisters
Acne
Fungal infections such as athlete's foot
microbial infections.
calcinosis cutis
ulcer
Skin cancer is an increasingly common condition, in part attributed to increased exposure to ultraviolet radiation, against which no level of sun screens offer any decisive protection. The increased exposure is mainly due to the recent popularity of sun tanning (sun bathing), but in part also due to ozone depletion and the consequently increased levels of ultraviolet radiation.
Minor surface skin cancers are readily treatable by simple surgery, but if the cancer is allowed to grow then it will penetrate through the layers of skin and affect the lymphatic system. It may also metastasize and spread to other parts of the body.
A rash is a change in the skin which affects its appearance or texture. A rash may be localised to one part of the body, or affect all the skin. Rashes may cause the skin to change colour, itch, become warm, bumpy, dry, cracked or blistered, swell and may be painful. The causes, and therefore treatments for, rashes vary widely. Diagnosis must take into account such things as the appearance of the rash, other symptoms, what the patient may have been exposed to, occupation, and occurrence in family members.
A blister is a defense mechanism of the human body. It consists of a pool of lymph and other bodily fluids beneath the upper layers of the skin. It may be formed in response to burns or friction, and helps to repair damage to the skin. If a blister is punctured, it forms an open wound and must be bandaged.
If a blister is associated with sub-dermal bleeding, it may partially fill with blood, forming an unpleasant blood blister.
Certain autoimmune diseases feature extensive blistering. These include pemphigus and pemphigoid. Blistering also occurs as part of
food borne illness with Vibrio vulnificus (seafood).
The class of chemical weapons known as vesicants acts by causing blisters (often within in the respiratory tract). Mustard gas and lewisite are examples of such agents.
Acne is an inflammatory disease of the skin, caused by changes in the pilosebaseous units (skin structures consisting of a hair follicle and its associated sebaceous gland). The most common form of acne is known as "acne vulgaris", which means common acne. Excessive secretion of oils from the glands combine with naturally occurring dead skin cells to block the hair follicles. Oil secretions build up beneath the blocked pore, providing a perfect environment for the skin bacteria Propionibacterium acnes to multiply uncontrolled. In response, the skin inflames, producing the visible lesion. The face, chest, back and upper arms are especially affected.
The typical lesions of acne are: small whitish or skin-colored bumps (whiteheads) or blackish elevations (blackheads). More
inflamed rashes take the form of pus-filled, or reddish bumps, even boil-like tender swellings. After resolution of the lesions, prominent unsightly scars may remain.
Calcinosis cutis (or cutaneous calcification) is a type of calcinosis wherein calcium deposits form in the skin. A variety of factors can result in this condition. The most common source is dystrophic calcification, which occurs in soft tissue as a response to injury.
|
