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The Skin

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VOL: 98, ISSUE: 25, PAGE NO: 43

MAUREEN BENBOW, MSc, BA, Tissue Viability Nurse, Mid Cheshire Hospital Trust, Leighton Hospital, Cheshire

Sponsored by Pfizer

This is the first of three articles examining aspects of skin care and wound management. In this Part the structure of the skin and its functions are discussed, followed by a description of the effects of ageing. An understanding of these aspects is an important basis for wound management.

This is the first of three articles examining aspects of skin care and wound management. In this Part the structure of the skin and its functions are discussed, followed by a description of the effects of ageing. An understanding of these aspects is an important basis for wound management.

The skin
As the skin is the largest and most visible organ of the body, it becomes immediately apparent when something is wrong with it. In turn, the skin may provide visible signs of other system dysfunction through changes in its colour or sensitivity. For example, flushed skin indicates pyrexia.

The skin accounts for 15-20% of a human's total body weight, or around 3kg. It has a surface area of about 1.7 square metres, and contains about one-third of the total circulating blood volume. It is capable of self-regeneration, and is able to withstand mechanical and chemical assaults (Wysocki, 2000). There are wide variations in the thickness of skin on different parts of the body.

Structure of the skin
The skin has two distinct layers: the outer or surface layer, the epidermis, and the inner layer, the dermis, which is anchored to the subcutaneous layer (Herlihy and Maebius, 2000).

The epidermis
The outer layer of skin or epidermis is avascular, waterproof and consists of superficial, stratified, squamous epithelial cells or keratinocytes. Oxygen and nutrients diffuse into the lower epidermis from the underlying dermis. The epidermis is divided into five layers: the outer stratum corneum, the stratum lucidum, the stratum granulosum, the stratum spinosum, and the stratum germinativum or basal layer.

The stratum corneum

The stratum corneum is composed of dead keratinised cells, which are abraded daily when a person washes, scratches or exercises (Wysocki, 2000). The human epidermis is renewed every 1,530 days depending, among many factors, where it is on the body, and its age. Cells in the stratum corneum are constantly sloughing off and being replaced by other cells moving up from the underlying layers of the skin (Herlihy and Maebius, 2000).

Keratin

Keratin is an insoluble protein found in hair and nails. It is resistant to changes in pH, temperature and chemical digestion by trypsin and pepsin but is capable of absorbing large quantities of water. This can lead to maceration (a softening or sogginess of the tissue owing to retention of excessive moisture) (Cutting, 1996) and skin breakdown (Lloyd and Moody, 1999).

Stratum lucidum

The second layer of epidermis, the stratum lucidum, varies in thickness.

Stratum granulosum

The stratum granulosum is one to three cells thick and contains flattened and irregularly shaped cells. It is at this level of the epidermis that the keratinocytes (which originate in the stratum germinativum) become non-viable.

Stratum spinosum

The stratum spinosum, or prickly layer, contains keratinocytes that are polyhedral in shape and are differentiated from the basal cells described below.

Stratum germinativum

The stratum germinativum, or basal layer, consists of mitotically active basal keratinocytes that can be regarded as the stem cells of the epidermis. This layer contains rete ridges to anchor the epidermis, and melanocytes (cells that synthesise melanin to give skin its colour).

Sunlight stimulates increased production of melanocytes. Genetic differences control the amount of melanocytes that are incorporated in the epidermis and therefore how dark the skin becomes. Albinism is caused by an absence of pigment-forming enzyme in the skin.

A breach in the integrity of the skin that extends to the stratum germinativum exposes the body to infection.

Think Point: Which layer of the epidermis forms the cuticle?

The dermis
The basal membrane, or dermo-epidermal junction, separates the epidermis from the dermis and contains fibronectin and collagen. The capillary network lies directly under the basal membrane and supplies oxygen and nutrients to the overlying epidermal cells.

The dermis, also called the 'true skin', is the thickest layer (2-5mm) of skin. It is composed of dense fibrous connective tissue and contains numerous collagenous and elastic fibres (Wysocki, 2000), surrounded by a gelatinous fluid matrix which makes the dermis strong and enables it to stretch.

Collagen is the body's main structural protein which, in the skin, is secreted as tropocollagen (Wysocki, 2000). It is collagen that gives skin its tensile strength and acts as a buffer to external pressure. Elastin gives skin its property of elastic recoil (Wysocki, 2000).

If the skin is overstretched, for example during pregnancy, the dermis may be damaged and form pink lines that gradually turn white (otherwise known as stretch marks).

Accessory structures
Accessory structures found in the skin include hair follicles, sebaceous glands, sweat glands, nervous tissue and some muscle tissue. Many of the nerves have endings - called sensory receptors - that detect pain, temperature, pressure and touch.

The skin also contains about three million eccrine glands. These produce water, waste salts and urea. There are different types of eccrine glands: for example, those in the ear produce wax. Apocrine glands in the genital and axillary areas produce a thicker fluid (containing sebum). Sebaceous glands adjacent to the hair follicles produce sebum to oil the hair, and lubricate the skin by keeping sweat on the epidermis.

Macrophages and fibroblasts, key cells for healing, are found in the gelatinous fluid matrix of the dermis. Pacini's corpuscles in the dermis are the nerve endings responsible for detecting pressure: they direct the brain to initiate movement by reflex action when a person becomes uncomfortable.

Think Point: What is the difference between apocrine and eccrine glands?

The subcutaneous layer
Beneath the dermis lies the subcutaneous layer, or hypodermis, which consists of adipose (fatty) tissue, connective tissue that attaches the skin to deeper structures such as muscles and bones, and a network of blood vessels (Wysocki, 2000).

The layer of fat cells protects infants and small children from injury, insulates from cold and provides an energy reserve. As we mature, the fat distribution changes: in men fat accumulates around the neck, upper arms, lower back and buttocks, while in women it accumulates in the breasts, buttocks, hips and thighs. Both men and women may accumulate excessive amounts in the abdomen (Herlihy and Maebius, 2000).

The functions of skin
As the largest body organ, skin has many important functions that are closely inter-linked to its structure.

Protection

The outer layer of skin covers and protects the underlying tissues from trauma, chemicals, water, ultraviolet radiation and infection. It prevents loss of body fluids and electrolytes, which is important in maintaining homoeostasis (Wysocki, 2000). The protective function of skin is key to preventing or ameliorating many dry skin conditions including some types of eczema, contact dermatitis and psoriasis which can cause dry, cracking skin allowing ingress of irritants and allergens (Lookingbill and Marks, 1993).

Infection

Many harmless bacteria and fungi inhabit the skin, but it is not until the opportunity arises for penetrating it that they become pathogenic.

If the skin is breached and the bacteria are transferred to another area of the body, cellulitis (a diffuse inflammation of connective tissue) or wound infection may occur (Parker, 2000). The horny keratin layer acts as a physical barrier to infection when the skin is intact.

The skin also produces bactericidal secretions and, in addition, is protected by resident flora. The skin's immune system comprises the Langerhans' cells, tissue macrophages (which ingest and digest bacteria and other substances), antigen-presenting cells found in the epidermis, and mast cells, which contain histamine.

Protection against ultraviolet light

Melanocytes produce melanin, the pigment that darkens skin. When the skin is exposed to sunlight, secretion of melanin increases (the summer tan) in an attempt to protect deeper layers of tissue from ultraviolet light.

Freckles and moles are examples of melanin that is concentrated in local areas.

Maintenance of body temperature

Heat can be lost or dissipated through radiation, conduction, convection and evaporation.

It is lost through radiation when heat from a warm body is lost to the cooler air of a cold room, and by conduction when a warm body is in contact with a cold surface, such as the use of a cooling blanket to reduce very high temperatures in clinical practice. Heat is lost by convection or via air currents - for example, using a fan to cool a patient - and by evaporation when liquid becomes a gas. The latter will happen when alcohol is rubbed on the skin: as it evaporates it cools the skin.

In conditions of extreme humidity, sweat cannot evaporate from the surface of the skin, so the body will not cool by evaporation and radiation. However, it can still cool by convection and conduction (Herlihy and Maebius, 2000).

Skin regulates heat loss to the environment through the thermoregulatory mechanisms of sweating and vasoconstriction/vasodilation. Body temperature is usually maintained at 37°C by the hypothalamus, which acts as the body's thermostat. When body temperature deviates from the normal, temperature regulation is assisted by three structures in the skin: the blood vessels, the sweat glands and the arrector pili muscles (Herlihy and Maebius, 2000).

Response to an increase in temperature

The normal response of the blood vessels, arrector pili muscles and sweat glands to an increase in temperature is to allow more blood to flow to the skin surface, thereby transferring heat from deeper tissues. At the same time, the sweat glands are stimulated to produce sweat to aid evaporation and cooling.

Response to a fall in temperature

A fall in temperature results in the opposite effect to an increase: the blood vessels constrict, the sweat glands become less active, and there is activation of peripheral nerves and contraction of the arrector pili muscles (gooseflesh), causing an increase in the production of heat.

THINK POINT: What are the four methods of heat loss from the skin?

The storage of nutrients

Deep-seated areas of the dermis contain adipose tissue, where vitamin D2 is synthesised in the presence of sunlight. Vitamin D is needed for absorption of calcium and phosphorus (Herlihy and Maebius, 2000).

Sensory reception
Nerve receptors in the skin are sensitive to pain, touch, temperature and pressure. They serve to protect the skin by causing the body to react to painful or unpleasant stimuli. Differences in pressure are detected and transmitted to the brain, where a reflex indicating discomfort stimulates movement, encouraging blood to flow into the affected area (Herlihy and Maebius, 2000).

Communication

Glands in the skin are involved in the production of sexual stimulants (pheromones) that produce a change in the sexual or social behaviour of an individual.

Excretion

The skin is able to excrete small amounts of salt, water and waste such as urea (Herlihy and Maebius, 2000).

Think Point: List six functions of the skin.

Factors that influence skin characteristics
Adequate skin hydration is provided by sebum production and the presence of intact skin. Factors that can affect skin hydration include relative humidity, removal of sebum and age (Herlihy and Maebius, 2000).

Normal skin pH ranges from 4.0 to 5.5. Excessive use of alkaline soap, alcohol and acetone can have deleterious effects on the skin. Alkaline soap reduces the thickness and number of cell layers in the stratum corneum (White et al., 1987), reduces the water-holding capacity of the skin and increases skin pH, all of which heighten the risk of bacterial entry and resistance. The application of alcohol and acetone to the skin can dehydrate and/or reduce sebum production (Leyden, 1986).

A well-balanced diet, consisting of protein, carbohydrate, fat, vitamins A, B, C, D, and the minerals iron, zinc and copper, is needed to maintain healthy skin (Wysocki, 1992). An increase in any of these will not normally improve the condition of skin, except when trauma has occurred (Wysocki, 2000).

Vitamin C is useful for collagen formation during healing, fats are needed by cells to form the fatty layer, and carbohydrates are needed for extra energy requirements during cell division and for cell metabolism.

Certain groups of drugs can affect the condition of the skin: for example, steroid medications cause thinning of skin. Some medications can cause phototoxicity and photosensitivity: for example, antibiotics and antipsychotic drugs, while others interfere with the inflammatory reaction: for example, steroids and non-steroidal anti-inflammatory drugs (Wysocki, 2000).

Ageing
Skin care is particularly important in older people, as their resistance to normal wear and tear is much reduced because their skin is drier, more easily damaged, slower to heal and they have an altered early inflammatory response (Tanj and Phillips, 2001). The speed and quality of wound healing is affected by the quantity and distribution of growth factors in ageing skin (Ashcroft et al., 1997a). Oestrogen is believed to be involved in increasing the rate of wound healing, possibly by stimulating the production of certain growth factors (Ashcroft et al., 1997b).

As humans age, all components of the skin are affected:

- The sensitivity of the immune system is reduced owing to the loss of macrophages and other cells of the immune system which, in turn, predispose the person to infection and skin damage. The reduced number of Langerhans' cells affects the immunocompetence of the skin and is responsible for an increased risk of skin cancer and infection. A reduced inflammatory response may alter allergic reactions and impede the rate of healing. However, in spite of poor healing, the quality of scar tissue is improved in older people (Tanj and Phillips, 2001)

- Epidermal cells reproduce more slowly in older people, and are larger and more irregular in shape. These changes result in thinner, more translucent skin (Herlihy and Maebius, 2000), and it often becomes dry and scaly. Older people have reduced sebum production and perspiration which leads to dry, coarse, itchy skin. They also experience a marked increase in the incidence of skin problems such as pruritis, dermatitis, squamous-cell carcinoma, basal-cell carcinoma, blistering diseases, venous leg ulcers and pressure ulcers (Tanj and Phillips, 2001)

- Sensitivity to the effects of the sun increases in older people: less melanin is produced and the skin becomes paler. Some melanocytes produce extra melanin, resulting in age spots in areas exposed to the sun

- The incidence of injuries and infections increases, as the epidermis thins and sensory receptors diminish in capacity. Older people are therefore more likely to be accidentally burned or injured without being aware of the damage (Herlihy and Maebius, 2000)

- Sagging and wrinkling appear, as the skin weakens - the dermis thins by about 20% (Wysocki, 1992), collagen is lost and elasticity is reduced, particularly in areas exposed to the sun. As the hypodermis grows thinner, people become more prone to pressure damage, bruising and small haemorrhages (Herlihy and Maebius, 2000)

- The sweat glands become less effective and dermal blood supply diminishes in older people, reducing their ability to lose heat, which can lead to increased sensitivity to temperature changes (Herlihy and Maebius, 2000)

- Intrinsic ageing is associated with delayed wound healing (Tanj and Phillips, 2001). There is slower healing in the older person, as the frequency of skin cell replacement is reduced by the ageing process (Wysocki, 1992)

- Vascularity is reduced in subcutaneous tissue in the elderly which means that drugs administered subcutaneously are absorbed more slowly; healing time is also reduced. The reduced vascularity of the sacrum in older people means that less external pressure is needed to stop blood flow than in younger people (Shubert and Heraud, 1994), increasing the risk of pressure damage.

Think Point: How does ageing affect the skin?

Types of skin damage
The skin can withstand considerable damage through its ability to regenerate and replace lost epidermal and dermal cells. However, this can be a slow process when large areas are damaged and there is the constant potential for fluid loss and infection. Deliberate incisional wounds heal relatively quickly, with little loss of function compared with a deep abrasion or ulcer with accompanying tissue loss.

Skin can be damaged by several factors: mechanical, chemical, vascular, infectious, allergic and thermal, and by radiation and extravasation. Each will result in a different initial response, for example, erythema, maculae, papulae, vesicles, erosion or ulcers, and later develop into a more serious secondary lesion (Bryant, 1992). Correct recognition of the type of lesion and its cause is essential before treatment is initiated, as different lesions will require different treatment plans.

Pressure damage

Mechanical damage to the skin can be caused by pressure, shearing or friction. The aetiology of pressure ulcer development is multidimensional and complex but it is believed that the degree of damage is related to the ability of the skin and underlying structures to tolerate the effects of pressure (Braden and Bergstrom, 1987).

The application of localised pressure, its duration and intensity, complicated by shear forces, the contact area, underlying bone, bone depth, tissue distortion, contact surface and pressure distribution are all significant in the development of tissue damage (Bennett and Lee, 1985; Sacks, 1989; Bliss, 1993). Other extrinsic and intrinsic factors implicated in pressure damage include friction, moisture and skin irritants, age-related changes to collagen in skin (Hall et al., 1974), nutritional status (Bergstrom and Braden, 1992), tissue perfusion and skin temperature (Nixon et al., 2000).

Chemical damage

Chemical damage can occur from exposure to urine and faeces, use of inappropriate cleansing solutions, improper use of substances such as skin sealants, and drainage from fistulae, which erode the surface of the skin (Benbow and Iosson, 2002).

Vascular damage

Vascular damage may manifest as ulcers on the legs or feet owing to venous or arterial insufficiency, neuropathy, or a combination of factors. Injuries resulting from minor knocks are also slow to heal (Herlihy and Maebius, 2000).

Infection

Infectious lesions of the skin may be precipitated by Candida albicans, impetigo, Herpes simplex or Herpes zoster. In all cases the underlying cause must be identified and treated.

Allergic reactions

Allergic reactions may develop after exposure to a local skin irritant or allergens such as wound-dressing adhesives, local skin treatments, cosmetics, soaps and other everyday substances.

Ultraviolet light

Ultraviolet radiation can have a range of harmful effects on skin, from making it tough and leathery to damaging the DNA of skin cells, which increases the risk of squamous-cell carcinoma and malignant melanoma (Herlihy and Maebius, 2000). Short-term exposure of lightly pigmented skin to ultraviolet radiation can lead to blistering - a second-degree burn (Wysocki, 1992).

Excess exposure to ultraviolet radiation accelerates ageing of the skin. One obvious solution is to avoid exposure to natural sunlight, but the social pressures associated with a tan that imply good health and attractiveness prevail in spite of health education to the contrary.

Think Point: Examine your own skin care practices and compare them with what is best practice.

- Next week: Skin and wound assessment

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