Functions of the Skin

FUNCTIONS OF THE SKIN

Protection

The skin covering most of the body is no more than 1 mm thick, but it provides very effective protection against invasion by bac-teria and other foreign matter. The thickened skin of the palms and soles protects against the effects of the constant trauma that occurs in these areas.

The epidermis is the outermost layer of the skin and is com-posed of several layers of keratinocytes that change character as they migrate to the surface. The stratum corneum, the outer layer of the epidermis, provides the most effective barrier to epidermal water loss and penetration of environmental factors such as chem-icals, microbes, and insect bites.

Various lipids are synthesized in the stratum corneum and are the basis for the barrier function of this layer. These are long-chain lipids that are better suited than phospholipids for water re-sistance. The presence of these lipids in the stratum corneum creates a relatively impermeable barrier for water egress and for the entry of toxins, microbes, and other substances that come in contact with the surface of the skin.

Some substances do penetrate the skin but meet resistance in trying to move through the channels between the cell layers of the stratum corneum. Microbes and fungi, which are part of the body’s normal flora, cannot penetrate unless there is a break in the skin barrier.

The dermis–epidermis junction is the basal layer, which is composed of collagen. The basal layer serves four functions. It acts as a scaffold for tissue organization and a template for regen-eration; it provides selective permeability for filtration of serum; it is a physical barrier between different types of cells; and it ad-heres the epithelium to underlying cell layers.

Sensation

The receptor endings of nerves in the skin allow the body to con-stantly monitor the conditions of the immediate environment. The primary functions of the receptors in the skin are to sense temperature, pain, light touch, and pressure (or heavy touch). Different nerve endings respond to each of the different stimuli. Although the nerve endings are distributed over the entire body, they are more concentrated in some areas than in others. For ex-ample, the fingertips are more densely innervated than the skin on the back.

Fluid Balance

The stratum corneum (ie, outermost layer of the epidermis) has the capacity to absorb water, thereby preventing an excessive loss of water and electrolytes from the internal body and retaining moisture in the subcutaneous tissues. When skin is damaged, as occurs with a severe burn, large quantities of fluids and electro-lytes may be lost rapidly, possibly leading to circulatory collapse, shock, and death.

The skin is not completely impermeable to water. Small amounts of water continuously evaporate from the skin surface. This evap-oration, called insensible perspiration, amounts to approximately 600 mL daily in a normal adult. Insensible water loss varies with the body and ambient temperature. In a person with a fever, the loss can increase. During immersion in water, the skin can accu-mulate water up to three or four times its normal weight, such as swelling of the skin that occurs after prolonged bathing.

Temperature Regulation

The body continuously produces heat as a result of the metabo-lism of food, which produces energy. This heat is dissipated pri-marily through the skin. Three major physical processes are involved in loss of heat from the body to the environment. The first process, radiation, is the transfer of heat to another object of lower temperature situated at a distance. The second process, conduction, is the transfer of heat from the body to a cooler object in contact with it. Heat transferred by conduction to the air surrounding the body is removed by the third process, con-vection, which consists of movement of warm air molecules away from the body.

Evaporation from the skin aids heat loss by conduction. Heat is conducted through the skin into water molecules on its surface, causing the water to evaporate. The water on the skin surface may be from insensible perspiration, sweat, or the environment.

Normally, all of these mechanisms for heat loss are used. When the ambient temperature is very high, however, radiation and convection are ineffective, and evaporation becomes the only means for heat loss.

Under normal conditions, metabolic heat production is bal-anced by heat loss, and the internal temperature of the body is maintained constant at approximately 37°C (98.6°F). The rate of heat loss depends primarily on the surface temperature of the skin, which is a function of the skin blood flow. Under normal conditions, the total blood circulated through the skin is approx-imately 450 mL per minute, or 10 to 20 times the amount of blood required to provide necessary metabolites and oxygen. Blood flow through these skin vessels is controlled primarily by the sympathetic nervous system. Increased blood flow to the skin results in more heat delivered to the skin and a greater rate of heat loss from the body. In contrast, decreased skin blood flow de-creases the skin temperature and helps conserve heat for the body. When the temperature of the body begins to fall, as occurs on a cold day, the blood vessels of the skin constrict, thereby reducing heat loss from the body.

Sweating is another process by which the body can regulate the rate of heat loss. Sweating does not occur until the core body temperature exceeds 37°C, regardless of skin temperature. In ex-tremely hot environments, the rate of sweat production may be as high as 1 L per hour. Under some circumstances (eg, emotional stress), sweating may occur as a reflex and may be unrelated to the need to lose heat from the body.

Vitamin Production

Skin exposed to ultraviolet light can convert substances necessary for synthesizing vitamin D (cholecalciferol). Vitamin D is essen-tial for preventing rickets, a condition that causes bone deformi-ties and results from a deficiency of vitamin D, calcium, and phosphorus.

Immune Response Function

Research findings (Demis, 1998) indicate that several dermal cells (ie, Langerhans cells, interleukin-1–producing keratinocytes, and subsets of T lymphocytes) and three varieties of human leukocyte antigen (ie, protein marker on white blood cells indicating the type of cell) are important components of the immune system. Ongoing research is expected to more clearly define the role of these dermal cells in immune function.

Gerontologic Considerations

The skin undergoes many physiologic changes associated with normal aging. A lifetime of excessive sun exposure, systemic dis-eases, poor nutrition, and certain medications (eg, antihista-mines, diuretics) can enhance the range of skin problems and the rapidity with which they appear. The outcome is an increasing vulnerability to injury and to certain diseases. Skin problems are common among older people.

Before conducting a skin assessment, the nurse needs to be aware of significant changes that occur with aging. The major changes in the skin of older people include dryness, wrinkling, uneven pigmentation, and various proliferative lesions. Cellular changes associated with aging include a thinning at the junction of the dermis and epidermis. This results in fewer anchoring sites between the two skin layers, so that even minor injury or stress to the epidermis can cause it to shear away from the dermis. This phenomenon of aging may account for the increased vulnerabil-ity of aged skin to trauma. With increasing age, the epidermis and dermis thin and flatten, causing wrinkles, sags, and overlapping skin folds (Fig. 55-2).

Loss of the subcutaneous tissue substances of elastin, collagen, and subcutaneous fat diminishes the protection and cushioning of underlying tissues and organs, decreases muscle tone, and re-sults in the loss of the insulating properties of fat.

Cellular replacement slows as a result of aging. As the dermal layers thin, the skin becomes fragile and transparent. The blood supply to the skin also changes with age. Vessels, especially the cap-illary loops, decrease in number and size. These vascular changes contribute to the delayed wound healing commonly seen in the elderly patient. Sweat and sebaceous glands decrease in number and functional capacity, leading to dry and scaly skin. Reduced hormonal levels of androgens are thought to contribute to declin-ing sebaceous gland function.

Hair growth gradually diminishes, especially over the lower legs and dorsum of the feet. Thinning is common in the scalp, ax-illa, and pubic areas. Other functions affected with normal aging include the barrier function of skin, sensory perception, and ther-moregulation.

Photoaging, or damage from excessive sun exposure, has detri-mental effects on the normal aging of skin. A lifetime of outdoor work or outdoor activities (eg, construction work, lifeguarding, sunbathing) without prudent use of sunscreens can lead to pro-found wrinkling; increased loss of elasticity; mottled, pigmented areas; cutaneous atrophy; and benign or malignant lesions.

Many skin lesions are part of normal aging. Recognizing these lesions enables the examiner to assist the patient to feel less anx ious about changes in skin. Chart 55-1 summarizes some skin le-sions that are expected to appear as the skin ages. These are normal and require no special attention unless the skin becomes infected or irritated.