The Skin

Before we address the aetiology of pressure ulcers/ injuries it is important to understand the structure and functions of healthy skin.

Skin Anatomy

The skin is the largest organ in the body covering an area of almost two square meters and is made up of three layers, the epidermis, dermis and the hypodermis or subcutaneous tissue. Its thickness is between≤ 0,1mm at its thinnest part such as the eyelids to 1.5 mm on its thickest part palms and soles of the feet (Kolarsick et al, 2011)


The main functions of the skin are:

– Protection

– Thermoregulation

– Sensation

– Excretion

– Metabolism/Absorption

– The epidermis is the outermost layer consists of four sublayers and five in the thickest areas such as the palms, soles of feet and digits. – The main function of the epidermis is protection providing a physical and biological barrier to the external environment protecting underlying tissues and organs from mechanical damage and ultraviolet light. It also prevents irritants and allergens entering, maintains water retention and has an important role in thermoregulation. – The layers correspond to the progressive stages of differentiation in the keratinocytes the most common cell within the epidermis (95%) (Lawton S, 2019). The keratinocytes continually generate migrating from the stratum basale to the stratum corneum where they are flattened, dead and shed, this process takes approximately 28 days. The Keratinocytes produce the protein keratin which provides the strength to the epidermis. – Other cells include melanocytes which give the skin and hair colour, langerhans cells which play a part in the body’s immune system engulfing antigens and merkel cells found in areas such as the fingertips, palms, soles, lips and genitalia have a role in touch and sensation.
– The dermis is the deeper layer of the skin lying directly beneath the stratum basale layer of the epidermis, thicker than the epidermis being 1-5mm (White and Butcher 2005) – The main functions of the dermis are protection, cushioning the deeper structures from mechanical injury. It also provides nourishment to the epidermis and plays an important role in wound healing (Lawton S, 2019) – It is made up of two layers: Superficial papillary dermis is the thinner layer of connective tissue composed predominantly of a fine network of collagen fibres and elastic fibres. The capillary network within the dermis provides nourishment to the epidermis. The Reticular dermis is the deeper layer forming the bulk of the dermis. It contains collagen and elastic fibres, motor and sensory nerves which translate mechanical forces into nerve impulses serving the senses of touch and pressure. When pressure is unrelieved signals are sent to the brain to reposition the body.Individuals with neurological diseases such as multiple sclerosis may have a loss of sensation making them unaware of discomfort and the need to reposition themselves to relieve the mechanical force of pressure. Blood and lymphatic vessels are found here, vital for healthy skin providing it with oxygen and nutrients as well as enabling carbon dioxide and waste products to be exchanged. Hair follicles sweat and sebaceous glands are also found here. – Collagen gives the dermis its tensile strength, resisting stretch whilst elastin provides elasticity enabling the tissues to return to their original position after being subjected to stretch. Elastin also provides compliancy allowing for bend. All these properties enable the skin to move and stretch in response to body movements, protecting from shear and tearing. As we age elastin fibres degenerate resulting in sagging and stretching of the skin increasing the risk of tissue damage

Cells within the dermis

– Cells within the dermis include mast cells,fibroblasts and macrophages. The mast cells initiate the inflammatory stage of wound healing, fibroblasts are responsible for the manufacture of collagen and elastin fibres and also play a major role in wound healing. The macrophages engulf bacteria and other debris playing a central role in the inflammatory process of wound healing.

Other Functions of the skin

Thermoregulation. The skin protects the body from heat and cold. Elevations in temperature cause the blood vessels to dilate as well as sweat being produced from the sweat glands. Sweating will deliver fluid to the skin evaporating on its surface causing the body to cool down. Alternatively if the body is exposed to cold there will be vasoconstriction in the blood vessels reducing the amount of heat loss, also the hair follicles will raise trapping the warm air next to the skin. – Vitamin D is produced when the skin is exposed to UV light essential for the absorption of calcium to maintain healthy bones and teeth. – Excretion the sebaceous glands excrete sebum an oily substance which help to maintain the skins suppleness and integrity.

– Below the dermis is the hypodermis or subcutaneous layer consisting of fat. It provides the main structural support of the skin as well as insulating the body from cold and shock absorption.