Integumentary System Anatomy, Physiology, and Function of Skin (Nursing Standard PDF)

Art & science If you would like to contribute to the Art & science section, email [email protected] The synthesis of art and science is lived by the nurse in the nursing act Josephine G Paterson The integumentary system: anatomy, physiology and function of skin McLafferty E et al (2012...

Art & science If you would like to contribute to the Art & science section, email [email protected] The synthesis of art and science is lived by the nurse in the nursing act Josephine G Paterson The integumentary system: anatomy, physiology and function of skin McLafferty E et al (2012) The integumentary system: anatomy, physiology and function of skin. Nursing Standard. 27, 3, 35-42. Date of acceptance: April 13 2010. Abstract This article, which forms part of the life sciences series, examines the anatomy and physiology of skin, also termed the integumentary system. Skin is composed of two main layers, the epidermis and dermis. The structure of the epidermis and dermis are described and their functions are discussed. Accessory structures, such as nails and hair are also considered. Although many diseases of the skin exist, two common conditions – psoriasis and decubitus ulcers – are described in this article. Authors Ella McLafferty Retired, was senior lecturer, School of Nursing and Midwifery, University of Dundee. Charles Hendry Retired, was senior lecturer, School of Nursing and Midwifery, University of Dundee. Alistair Farley Lecturer in nursing, School of Nursing and Midwifery, University of Dundee. Correspondence to: [email protected] Keywords Anatomy and physiology, body systems, integumentary system, skin and skin disorders Review All articles are subject to external double-blind peer review and checked for plagiarism using automated software. Online Guidelines on writing for publication are available at www.nursing-standard.co.uk. For related articles visit the archive and search using the keywords above. © NURSING STANDARD / RCN PUBLISHING p35-42w3.indd 35 The skin is a complex arrangement of structures with a range of different, but important, functions. The skin is composed of two main layers, the epidermis and dermis (Waugh and Grant 2010, Tortora and Derrickson 2009a). The subcutaneous layer is found beneath the dermis and is not considered part of the skin (Tortora and Derrickson 2009a). Nurses need to be knowledgeable about what is considered to be healthy skin (Pringle and Penzer 2002). This is important, as the condition of the skin may often be a sign of underlying disease (Casey 2002). Changes in the skin may be one of the first indicators of an underlying health problem. Many nurses will be familiar with cyanosis where, because of poor delivery of oxygen to the tissues, the patient’s skin appears blue. Structure of the skin The skin is the largest organ in the body, accounting for approximately 16% of the total body weight of an adult (Tortora and Derrickson 2009a). The skin weighs twice as much as the brain, approximately 3-5kg (Turkington and Dover 2007). Skin varies in thickness according to function and area of the body. On the eyelids, the skin is only 0.5mm thick, whereas it can be as much as 3-4mm thick on the soles of the feet (Brooker 1998). Skin is generally 1-2mm thick (Tortora and Derrickson 2009a). The skin consists of thick outer layers, a widespread system of sweat glands sensitive to temperature changes and an extensive layer of fatty tissue under the surface of the skin. The skin also contains many cells that are sensitive to touch, pain, pressure, itching and temperature (Turkington and Dover 2007). september 19 :: vol 27 no 3 :: 2012 35 14/09/2012 13:02 Art & science life sciences: 6 Epidermis The epidermis is composed of stratified keratinised squamous epithelium (Tortora and Derrickson 2009a) and is made up of many layers near the surface of the skin (Turkington and Dover 2007). The epidermis contains four main types of cells, most of which are keratinocytes, which make up 90% of the cells found in this layer. Melanocytes make up 8% of epidermal cells and are responsible for producing the pigment, melanin. Langerhans cells and Merkel cells are also found within the epidermis. Langerhans cells are involved in the immune response and Merkel cells function in the sensation of touch (Tortora and Derrickson 2009a). The epidermis is avascular (without blood vessels) and is dependent on blood vessels of the dermis for oxygenation, metabolite provision and removal of metabolic waste products. The epidermis is made up of a number of layers, including (Figure 1): Stratum  basale, the deepest layer (also called the stratum germinativum). Stratum  spinosum or prickle cell layer. Stratum  granulosum or granular layer. Stratum  lucidum (finger tips, palms and soles). Stratum  corneum, which is the top layer. These layers represent the different stages of maturation of the cells and their movement from the stratum basale up to the stratum corneum, where they are shed. The epidermis renews itself through cell division in its deepest layer (Burr and Penzer 2005). Stratum basale The stratum basale is made up of a single row of columnar keratinocytes. Other types of cells found within this layer include melanocytes and Merkel cells (Tortora and Derrickson 2009a). The stratum basale is the only layer within the epidermis that consists of cells capable of division. Keratinocytes in the stratum basale undergo mitosis and two daughter cells are produced; one remains in the stratum basale, while the other migrates up through the other layers to the surface of the epidermis. This process takes approximately 28 days in an average epidermis of 0.1mm thickness (Tortora and Derrickson 2009a). The stratum basale is the nearest layer to the dermis and is located under the epidermis. The dermis contains a blood supply and provides nourishment to the stratum basale. However, as the daughter cells move further away from the stratum basale, they receive less nutrition. As a result the cells die. In addition, the cells become more keratinised – they accumulate more keratin, which is a fibrous protein involved in protecting the skin from heat, chemicals and microorganisms. In healthy skin, a balance between the formation of new keratinocytes in the stratum basale and the shedding of dead keratinocytes from the stratum corneum is maintained. The stratum basale also contains melanocytes, which produce melanin. Melanocytes account for one in every six cells in the stratum basale (Turkington and Dover 2007). Melanin is a pigment that protects the skin from the harmful effects of ultraviolet (UV) light. Melanocytes have FIGURE 1 The skin showing the layers of the epidermis and main structures of the dermis Hair shaft Stratum corneum Opening of sweat ducts Epidermis Germinative layer Sebaceous gland Sweat gland Dermis Stratum corneum Stratum lucidum Stratum granulosum Epidermis Germinative layer Hair root 36 september 19 :: vol 27 no 3 :: 2012 p35-42w3.indd 36 Dermis Subcutaneous tissue peter lamb Hair follicle © NURSING STANDARD / RCN PUBLISHING 14/09/2012 13:02 long, slender projections that contain melanin granules. The projections extend between the keratinocytes and transfer the granules to them. Once inside the keratinocytes, the granules gather to form a protective covering around the nucleus, protecting the keratinocytes from damage caused by UV light. The colour of the skin is genetically determined and environmentally modified by UV light exposure. Skin colour is related to the amount of melanin in epidermal cells. The more melanin produced, the darker the skin. Nurses should be able to advise patients of the importance of avoiding too much UV light, whether from natural sunlight or sunbeds. They should also inform patients of the need to cover their skin when outdoors and to use appropriate UV filter creams, commonly known as sun blocks. Tortora and Derrickson (2009a) recommended a sun protection factor of no less than 15 to protect against UV light. Merkel cells are also found in the stratum basale scattered among the keratinocytes. Merkel cells make contact with the flattened process of a sensory neuron called a Merkel disc. Merkel cells and their discs detect the sensation of touch. Stratum spinosum As the daughter cells of the keratinocytes move into the next layer, the stratum spinosum, they lose their ability to divide. They also become rounder and ‘spikier’ in shape (hence this layer’s name). The stratum spinosum is five to 12 cells thick. While in this layer, the single daughter cells join together via intracellular bridges called desmosomes. As the cells move through this layer, the daughter cells continuously break and reform desmosomes. These intracellular bridges have thorn-like projections that draw adjacent cells close together (Thibodeau and Patton 2007). This arrangement contributes to the tensile strength and flexibility of the skin. Langerhans cells, developed from specialised dendritic cells of the immune system, are found in the stratum spinosum. They are also found in the dermis, lymph nodes and thymus (Turkington and Dover 2007). These cells are produced in the red bone marrow and then migrate to the stratum spinosum, where they participate in immune responses against microorganisms. They function by attracting and phagocytosing microbes and presenting their antigens to T lymphocytes, thereby activating the lymphocytes to destroy the appropriate cells (Pringle and Penzer 2002). Langerhans cells are crucial in helping other cells of the immune system to recognise invading microorganisms and destroy them. © NURSING STANDARD / RCN PUBLISHING p35-42w3.indd 37 Stratum granulosum As the cells move through the epidermal layers towards the surface of the skin they become longer and flatten horizontally to form the stratum granulosum. The stratum granulosum is composed of three to five layers of flattened keratinocytes. In this layer, the cells go through a process known as apoptosis, which is an orderly, genetically programmed cell death during which the nucleus breaks up and the cell dies. The cells are no longer able to perform any metabolic functions (Tortora and Derrickson 2009a). By this stage, the cells lose their nucleus and become keratinised and are comprised entirely of a tough pliable protein called keratin (Pringle and Penzer 2002). Keratohyalin is also present in this layer. It consists of darkly staining proteins that convert tonofilaments to keratin. Odland’s bodies may also be seen. These are membrane-coating, lamellar granules that produce lipid, which extrudes into the spaces between the cells and helps them stick together. Stratum lucidum The stratum lucidum is only found in areas where the skin is thick, such as the palms of the hands and soles of the feet (Tortora and Derrickson 2009a). This layer contains three to five layers of clear, dead keratinocytes that are flattened and made up of large amounts of keratin and thickened plasma membranes. The stratum lucidum lies between the stratum granulosum and the stratum corneum and provides some degree of waterproofing to the skin. Stratum corneum The stratum corneum is the uppermost layer. It consists of 25-30 layers of flattened, dead keratinocytes (Tortora and Derrickson 2009a). The cells are arranged in orderly, vertical stacks and appear to be composed of cell membranes that are firmly attached to each other. The cells contain the protein keratin, which helps protect the skin and underlying tissues from heat, microorganisms and chemicals. The intracellular lipid from the lamellar granules in the stratum granulosum cements the cells together and is vital in preventing the cells from drying out. As cells move through the stratum corneum they lose their stickiness and are shed singly or in clumps, known as squamae. Most people who have ever seen or had a plaster cast removed will recognise the accumulation of dead skin cells on the surface of the skin. Dandruff and the majority of house dust are dead, shed skin cells – the favoured food of house dust mites that trigger september 19 :: vol 27 no 3 :: 2012 37 14/09/2012 13:02 Art & science life sciences: 6 asthma in susceptible individuals (Kumar and Clark 2009). Each of the layers represents a stage in the life of an epidermal cell. Since the stratum corneum is the final layer between the body and the external environment, it is at risk of considerable wear and tear. Cells are lost from this layer on a continous basis. If a part of the skin is exposed to constant friction a hard callus can form, which is an abnormal thickening of the stratum corneum (Tortora and Derrickson 2009a). Dermis The dermis lies below the epidermis and above the subcutaneous layer, and is responsible for providing nutrients and physical support to the epidermis (Burr and Penzer 2005). The dermis contains lymph vessels, nerve endings, hair follicles and glands (Figure 1) (Pringle and Penzer 2002). It is anchored to the epidermis by rete ridges, which are furrows. These furrows stabilise and allow the exchange of nutrients between the dermis and epidermis (Turkington and Dover 2007). However, the two layers may become separated as a result of shearing forces or friction, allowing fluid to collect between the epidermis and dermis, forming what is commonly known as a blister. The dermis is composed of two layers: the reticular and papillary layers. The papillary layer contains the nerves and capillaries that nourish the epidermis, whereas the reticular layer is made up of strong connective tissue containing collagen and elastic fibres (Pringle and Penzer 2002). Collagen and elastin Collagen and elastin in the dermis are arranged in a woven network of fibres that have significant tensile strength, providing the dermis with the ability to stretch and contract (Tortora and Derrickson 2009a). Collagen is a protein that contributes to approximately 70% of the dry weight of the dermis. When the skin is stretched the collagen fibres prevent tearing as a result of their high tensile strength (Pringle and Penzer 2002). Elastin fibres are synthesised by fibroblasts. These fibres are finer than collagen and are found, interwoven, among the collagen bundles. Elastin also has elastic properties that allow the skin to return to its normal position after stretching. With increasing age, there is a reduction in the number of collagen fibres, which stiffen and break up. This results in the collagen fibres losing their shape and becoming tangled. Meanwhile, the elastic fibres lose some of their elasticity, thicken into bundles and fray. These changes result in the appearance of wrinkled skin (Tortora and Derrickson 2009a). 38 september 19 :: vol 27 no 3 :: 2012 p35-42w3.indd 38 Glands in the dermis The skin contains three to four million sweat glands. The function of these glands is to release sweat into hair follicles or on to the skin surface through pores. There are two types of sweat glands – eccrine glands and apocrine glands – based on their structure, location and type of secretion (Tortora and Derrickson 2009a). Eccrine glands Eccrine glands are simple, coiled glands that are distributed in many areas of the skin, but especially in the skin of the forehead, palms of the hands and soles of the feet. The eccrine glands produce sweat, which is predominantly composed of water, but includes sodium and chlorine ions, urea, uric acid, ammonia, amino acids, glucose and lactic acid. A total of 600ml of sweat is produced daily. Sweat glands have an important role in thermoregulation through evaporation (Tortora and Derrickson 2009a). Apocrine glands Apocrine glands are not active during childhood, but are activated by sex hormones during puberty (Pringle and Penzer 2002). The apocrine glands are also simple coiled tubular glands and are mainly found in the axillae, groin, areolae of the breasts and bearded regions of the face in adult males (Tortora and Derrickson 2009a). Unlike the eccrine glands, the sweat produced by the apocrine glands is slightly viscous and has a milky or yellowish appearance. This sweat has no smell when it leaves the gland. However, when it meets bacteria on the surface of the skin, the bacteria metabolise sweat components to produce a musky odour, commonly described as body odour (Tortora and Derrickson 2009a). Sebaceous glands Sebaceous glands are simple, branched acinar glands. An acinar gland is a gland that has a sac-like secretory unit and an obvious lumen. Most sebaceous glands, but not all, are connected to hair follicles. They are most commonly found on the face, neck and back (Pringle and Penzer 2002). Sebaceous glands secrete sebum, which is an oily substance composed of a combination of triglycerides, cholesterol, proteins and organic salts. Sebum covers the surface of the hairs and protects them from drying and becoming brittle. Sebum also inhibits excessive evaporation of water from the skin so that the skin remains soft and supple (Tortora and Derrickson 2009a). As well as acting as a lubricant, sebum also has antifungal and antibacterial properties (Pringle and Penzer 2002). Hormonal activity during puberty may cause overactivity of sebaceous glands leading to overproduction of sebum, which can sometimes lead to the presence of open © NURSING STANDARD / RCN PUBLISHING 14/09/2012 13:02 comedones (blackheads) and closed comedones (whiteheads). Ceruminous glands Ceruminous glands, which are found in the external ear, are modified sweat glands. These glands produce a waxy lubricating secretion, which combines with secretions from the sebaceous glands to produce a yellowish substance called cerumen. The function of cerumen is to provide a sticky barrier, which together with the hairs in the external auditory canal inhibits the entrance of foreign bodies and insects into the ear. Cerumen also prevents bacteria and fungi from entering the cells because of its waterproofing ability (Tortora and Derrickson 2009a). Blood vessels There are two main networks of cutaneous arteries. The deep plexus (a network of blood vessels) is found where the dermis and the subcutaneous fat layer join. This network supplies the dermis and subcutaneous layers of tissue with blood. Small tributaries run from this plexus, supplying hair follicles, sweat glands and other structures within the dermis. At the uppermost level of the dermis, the superficial plexus branches off and carries blood vessels to the epidermis and dermis boundary (Pringle and Penzer 2002). Hair Each hair is made up of columns of dead, keratinised epidermal cells connected together with extracellular proteins. The shaft of hair is the segment that projects above the surface of the skin. The root of the hair is the segment of hair that lies deep in the shaft and is anchored in the dermis or subcutaneous layer. The root of the hair is surrounded by a hair follicle. This follicle is made up of an external and internal root sheath, which together make up the epithelial root sheath. The dermis that encircles the hair follicle is called the dermal root sheath. The base of each hair follicle and the surrounding dermal root sheath is called the bulb and has a similar shape to an onion. The bulb contains the layer of cells called the hair matrix. The hair matrix cells arise from the stratum basale, which is the layer where cell division occurs. Therefore the hair matrix cells are responsible for the growth of existing hairs. They also produce new hairs when old hairs are shed (Tortora and Derrickson 2009a). The growth cycle for each hair follicle has three stages: Growth.  Regression.  Resting.  © NURSING STANDARD / RCN PUBLISHING p35-42w3.indd 39 The cells of the hair matrix divide during the growth stage. New cells are added at the base of the hair so that existing cells are pushed up through the shaft and the hair grows longer. During this process the hair becomes keratinised and dies. The hair stops dividing during the regression stage and the hair follicle atrophies and the hair no longer grows. Finally, the hair follicle enters a resting stage. Scalp hair remains in the growth stage for two to six years, while the regression stage lasts for two to three weeks and the resting stage lasts for about three months. Normal hair loss in the adult scalp is 70-100 hairs per day (Tortora and Derrickson 2009a). Nurses should note that, for most patients, care of the hair is important. Nurses should assist patients to wash and style their hair because poor hair care causes oils from sebaceous glands to collect on the hair and scalp, making the hair look and feel greasy. Patients may find it physically and psychologically distressing not to have their hair washed. The arrector pili muscle is connected to the follicle and is responsible for the appearance of goose bumps, which are experienced when in a cold environment or when frightened or excited. Nails The finger and toe nails are made of sheets of keratin and are tough. Their function is to protect the ends of the digits and to allow the performance of intricate movements. Nails grow from germinal cells called the nail root. The tip of the finger, lying beneath the distal end of the nail, is known as the hyponychium, which is an area of thickened epidermis that allows for greater protection of the digit ends. Nail beds are usually pink in colour because there is an extensive network of capillaries beneath the nail (Pringle and Penzer 2002). Functions of the skin The skin has several important functions, including sensation, thermoregulation, protection and synthesis of vitamin D. Sensory function The skin is able to react to external stimuli such as cold, heat, pain, touch and pressure. It is supplied with approximately one million nerve fibres, most of which end in the face and extremities (hands and feet). Thermoregulatory function Receptors in the skin monitor temperature and transmit impulses to central control mechanisms september 19 :: vol 27 no 3 :: 2012 39 14/09/2012 13:02 Art & science life sciences: 6 in the hypothalamus. The hypothalamus is a region of the forebrain that co-ordinates the autonomic nervous system, including the control of body temperature, thirst, hunger and other homeostatic systems. Thermoregulatory mechanisms occurring in the skin include insulation, sweating and control of blood flow. The body is insulated by subcutaneous adipose tissue, which is found under the dermis. Eccrine glands are stimulated to produce sweat when the core temperature rises above 37˚C. Sweat, in turn, cools the body through the process of evaporation (McLafferty et al 2009). The skin is also provided with an abundant blood supply, which aids thermoregulation. The body’s core temperature has to remain constant to maintain homeostasis. When the core temperature rises, the body cools itself by increasing blood flow to the skin. Heat is removed from the body by the process of radiation. Heat is also lost through the skin by conduction and convection (Tortora and Derrickson 2009a). If the body is becoming too cold, heat loss is reduced by the process of vasoconstriction, which reduces the flow of warm blood to the extremities from the body’s core (McLafferty et al 2009). Thermoregulatory mechanisms are immature in children and their large body surface area increases the risk of hypothermia (Hockenberry and Wilson 2011). As people age, their thermoregulatory mechanisms become less efficient, which makes it more difficult for older people to detect and respond to temperature variations (Farley et al 2011). Protective function The skin is a physical, protective barrier for the internal organs. It prevents loss of fluids so that the internal organs do not dry out. Acidic secretions from the skin prevent colonisation by harmful microorganisms (Pringle and Penzer 2002). The epidermis is efficient at holding water, which helps to maintain the elasticity of the skin and has a role in the body’s fluid and electrolyte balance (Turkington and Dover 2007). Vitamin D synthesis Vitamin D is synthesised by the skin as a consequence of the exposure of the skin to UV light. Vitamin D is necessary for controlling the amount of calcium and phosphorus that is absorbed through the small intestine and mobilised from the bone. A deficiency of vitamin D can lead to rickets in children and osteomalacia in adults (Tortora and Derrickson 2009b). Subcutaneous adipose tissue acts as a fat reserve, which is useful 40 september 19 :: vol 27 no 3 :: 2012 p35-42w3.indd 40 in thermoregulation and as an energy source to meet the energy needs of the body. Psychological function The skin has an important role in psychological wellbeing. The skin is highly visible and has high cosmetic, aesthetic and cultural significance when it functions normally (Pringle and Penzer 2002). Skin disorders There are several common disorders associated with the skin. Only psoriasis and decubitus ulcers are considered in this article, as both conditions affect one or more layers of the skin, and knowledge of physiology of the skin and its functions are important to provide appropriate nursing management. Psoriasis Psoriasis occurs as a result of increased turnover of keratinocytes, so that cells take only three to five days to move from the stratum basale to the stratum corneum to be shed, a process that usually takes 28 days. The exact cause of psoriasis has not been identified, although there is a variety of factors that can trigger and/or exacerbate the condition (Alexander et al 2006). Psoriasis commonly presents with plaques on the elbows, knees and scalp. According to Paige (2005), a plaque is a ‘large flat-topped, elevated, palpable lesion’. Scalp scaling can affect 50% of patients and can be very thick, especially around the hairline, but it can cover the whole scalp (Buxton and Morris-Jones 2009). The nails show pits and also thickening with separation of the nail from the nail bed. The development of a particular type of psoriasis called guttate psoriasis, where the plaques are shaped as drops, is associated with a history of a streptococcal throat infection (Alexander et al 2006). Pustular psoriasis commonly appears on the hands and feet and features sterile pustules. Erythrodermic psoriasis is a generalised form of inflammatory psoriasis affecting all sites of the body, including the face, hands, feet, nails, trunk and extremities, and is a dermatological emergency. Two of the functions of the skin are to maintain thermoregulation and prevent colonisation by harmful microorganisms. Erythrodermic psoriasis can be life-threatening if psoriasis affects a large skin surface area and disrupts the two functions mentioned above. Increased blood flow to the skin results in heat and water loss, and dehydration and hypothermia can cause death (Buxton and © NURSING STANDARD / RCN PUBLISHING 14/09/2012 13:02 Morris-Jones 2009). In addition, there is a risk of overwhelming infection. Treatments for psoriasis include: Phototherapy,  which can be administered using UV light. UVB is a wavelength of UV light that occurs in natural sunlight. It penetrates the epidermis and is responsible for causing sunburn. UVA also occurs in natural sunlight and may be administered, but it has to be preceded by the administration of the drug psoralen two hours before exposure to UVA. Psoralen increases the sensitivity of the skin in preparation for the treatment. UVA is the wavelength that will penetrate through to the dermis and is responsible for the ageing effects of sunlight. The therapeutic, controlled administration of psoralen and UVA slows down production of skin cells (NHS Choices 2007). Topical  ointments, including emollients such as emulsifying ointment added to baths, soap substitutes, for example aqueous cream, and coal tar ointments. Other topical therapies include the use of dithranol (an aqueous cream), vitamin D analogues – topical applications that reduce dermal proliferation (McCance and Huether 2006) – and scalp therapy, including warmed olive oil to soften and remove psoriatic scales on the scalp. Emollients soothe and hydrate the skin and can be prescribed for all dry or scaling disorders. They may also have an anti-proliferative effect in psoriasis (British National Formulary 2012). Coal-tar-based ointments suppress cell proliferation (Buxton and Morris-Jones 2009). Systemic  medication, including methotrexate. Vitamin A derivatives are used to slow the epidermal turnover of cells and retinoids, which reduce scaling of the skin and the thickness of plaques associated with psoriasis (Alexander et al 2006). Decubitus ulcers Decubitus ulcers are also known as pressure ulcers (Waugh and Grant 2010). They are caused by shearing forces and prolonged or repeated pressure over skin, soft tissue, muscle and/or bone, occluding the capillaries within the compressed tissue and causing ischaemia (Alexander et al 2006). Ischaemia causes hypoxia and malnourishment in the compressed soft tissues and a build-up of toxic metabolites locally, increasing the rate of cell death. These processes lead to necrosis of the skin and underlying tissue and the formation of pressure ulcers. Pressure ulcers commonly develop over bony prominences, although they can develop © NURSING STANDARD / RCN PUBLISHING p35-42w3.indd 41 on any area of the body where there is prolonged pressure (Bansal et al 2005). Decubitus ulcers are classified according to the depth of visible tissue damage. Therefore, the use of any classification for such ulcers needs to be interpreted with caution, as further damage may be occurring at a deeper level, which is not necessarily visible at the time of assessment. The National Pressure Ulcer Advisory Panel and European Pressure Ulcer Advisory Panel (2009) pressure ulcer classification system is shown in Box 1. Conclusion In this article, the structure and functions of the skin have been examined and the layers of the skin and its accessory structures have been described. The skin protects underlying tissue from microbial invasion. It provides a barrier against most chemicals and also protects the underlying tissues from mechanical injury. It is involved in BOX 1 International pressure ulcer classification system Category/stage 1: non-blanchable redness of intact skin usually over a bony prominence. Discolouration of the skin, warmth, oedema, hardness or pain may be present. Category/stage 2: partial-thickness skin loss or blister presenting as a shallow ulcer that may include the epidermis and dermis with pigmentation changes. It may present as an abrasion, blister or superficial ulcer. Category/stage 3: full-thickness skin loss. Subcutaneous fat may be visible. Bone, tendon or muscle are not exposed. Some slough may be present. Category/stage 4: full thickness tissue loss with exposed bone, tendon or muscle. Slough or eschar may be present. The depth of the pressure ulcer depends on the anatomical location. (Adapted from European Pressure Ulcer Advisory Panel and National Pressure Ulcer Advisory Panel 2009) POINTS FOR PRACTICE Use your knowledge of the skin to identify the changes that occur as a result of the ageing process. Reflect on what advice you would give to the mother of a young child in relation to sun exposure and protection. Consider the physical and psychological effects on patients of using emollients every day to manage psoriasis. Consider the advantages and disadvantages of using assessment tools in the prevention and early detection of pressure ulcers. september 19 :: vol 27 no 3 :: 2012 41 14/09/2012 13:02 Art & science life sciences: 6 thermoregulation through convection, conduction, radiation and evaporation. It also synthesises vitamin D and functions as a sophisticated sense organ. Intact, healthy skin is essential for patient wellbeing. Understanding the normal structure and function of the skin is a prerequisite to understanding and managing skin disorders such as psoriasis and decubitus ulcers NS GLOSSARY Conduction Heat lost to, or gained from, objects in direct contact with the body. Convection When air makes contact with exposed parts of the body it is warmed. It then rises away from the body surface. Cooler air replaces the rising air and convection currents are initiated. Evaporation The conversion of a liquid to a vapour, such as when heat is used to convert water in sweat to water vapour and in the process cools the body. Hypodermis Sometimes referred to as the subcutaneous tissue. It is not part of the skin, but the skin rests on this layer, which attaches it to underlying bone and muscle. Keratinocyte This is the primary cell found in the epidermal layer. Langerhans cells These cells are found in the stratum spinosum and function in the immune response. Melanocyte This is a pigment-producing cell found in the epidermal layer. Merkel cells Merkel cells function in the sensation of touch. Mitosis A form of cell division where the parent cell replicates to produce two daughter cells that are genetically identical to the parent. Radiation The transfer of heat between a warmer object and a cooler one without any direct contact. Tonofilament A tonofilament is a proteinaceous fibre found in epithelial cells. Bundles of tonofilaments form a tonofibril, which has a supporting function. References Alexander MF, Fawcett JN, Runciman PJ (Eds) (2006) Nursing Practice: Hospital and Home: The Adult. Third edition. Churchill Livingstone Elsevier, Edinburgh. Buxton PK, Morris-Jones R (2009) ABC of Dermatology. Fifth edition. Wiley-Blackwell, Chichester. Bansal C, Scott R, Stewart D, Cockerell CJ (2005) Decubitus ulcers: a review of the literature. International Journal of Dermatology. 44, 10, 805-810. European Pressure Ulcer Advisory Panel and National Pressure Ulcer Advisory Panel (2009) Treatment of Pressure Ulcers: Quick Reference Guide. www.epuap.org/guidelines/ Final_Quick_Treatment.pdf (Last accessed: August 30 2012.) British National Formulary (2012) British National Formulary No. 63. BMJ Group and Royal Pharmaceutical Society of Great Britain, London. Brooker C (1998) Human Structure and Function: Nursing Applications in Clinical Practice. Second edition. Mosby Elsevier, London. 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