Ure 614

Figure

(a) If normal skin is (b) injured deeply, (c) blood escapes from dermal blood vessels, and (d) a blood clot soon forms. (e) The blood clot and dried tissue fluid form a scab that protects the damaged region. (f) Later, blood vessels send out branches, and fibroblasts migrate into the area. (g) The fibroblasts produce new connective tissue fibers, and when the skin is mostly repaired, the scab sloughs off.

mild edema, and, in time, the surface layer of skin may be shed. A burn injuring only the epidermis is called a superficial partial-thickness (first degree) burn. Healing usually occurs within a few days to two weeks, with no scarring.

A burn that destroys some epidermis as well as some underlying dermis is a deep partial-thickness (second degree) burn. Fluid escapes from damaged dermal capillaries, and as it accumulates beneath the outer layer of epidermal cells, blisters appear. The injured region becomes moist and firm and may vary in color from dark red to waxy white. Such a burn most commonly occurs as a result of exposure to hot objects, hot liquids, flames, or burning clothing.

The healing of a deep partial-thickness burn depends upon accessory organs of the skin that survive the injury because they are located deep in the dermis. These organs, which include hair follicles, sweat glands, and sebaceous glands, contain epithelial cells. During healing, these cells grow out onto the surface of the dermis, spread over it, and form a new epidermis. In time, the skin usually completely recovers, and scar tissue does not develop unless an infection occurs.

A burn that destroys the epidermis, dermis, and the accessory organs of the skin is called a full-thickness (third degree) burn. The injured skin becomes dry and leathery, and it may vary in color from red to black to white.

A full-thickness burn usually occurs as a result of immersion in hot liquids or prolonged exposure to hot objects, flames, or corrosive chemicals. Since most of the epithelial cells in the affected region are likely to be destroyed, spontaneous healing can occur only by growth of epithelial cells inward from the margin of the burn. If the injury is extensive, treatment may involve removing a thin layer of skin from an unburned region of the body and transplanting it to the injured area. This procedure is called an autograft.

If the burn is too extensive to replace with skin from other parts of the body, cadaveric skin from a skin bank may be used to cover the injury. In this case, the transplant, a homograft, is a temporary covering that decreases the size of the wound, helps prevent infection, and helps preserve deeper tissues. In time, after healing has begun, the temporary covering may be removed and replaced with an autograft, as skin becomes available in areas that have healed. However, skin grafts can leave extensive scars.

Various skin substitutes also may be used to temporarily cover extensive burns. These include amniotic membrane that surrounded a human fetus, and artificial membranes composed of silicone, polyurethane, or nylon together with a network of collagenous fibers. Another type of skin substitute comes from cultured human epithelial cells. In a laboratory, a bit of human skin the size of a postage stamp can grow to the size of a bathmat in about three weeks. Skin substitutes are a major focus of tissue engineering, discussed in Clinical Application 5.2 (page 162).

The treatment of a burn patient requires estimating the extent of the body's surface that is affected. Physicians use the "rule of nines," subdividing the skin's surface into regions, each accounting for 9% (or some multiple of 9%) of the total surface area (fig. 6.15). This estimate is important in planning to replace body fluids and electrolytes lost from injured tissues and for covering the burned area with skin or skin substitutes.

H What is the tissue response to inflammation?

What occurs within a healing wound to cause the sloughing of the scab?

Which type of burn is most likely to leave a scar? Why?

Life-Span Changes

We are more aware of aging-related changes in skin than in other organ systems, simply because we can easily see them. Aging skin affects appearance, temperature regulation, and vitamin D activation.

The epidermis maintains its thickness as the decades pass, but as the cell cycle slows, cells tend to grow larger and more irregular in shape. Skin may appear scaly because, at the microscopic level, more sulfur-sulfur bonds form within keratin molecules. Patches of pigment commonly called "age spots" or "liver spots" appear and grow (fig. 6.16). These are sites of oxidation of fats in the secretory cells of apocrine and eccrine glands and reflect formation of oxygen free radicals.

The dermis becomes reduced as synthesis of the connective tissue proteins collagen and elastin slows. The combination of a shrinking dermis and loss of some fat from the subcutaneous layer results in wrinkling and sagging of the skin. Fewer lymphocytes delay wound healing. Some of the changes in the skin's appearance result from specific deficits. Less oil from sebaceous glands means that the skin becomes considerably drier.

The skin's accessory structures also show signs of aging. Slowed melanin production causes hair to become gray or white as the follicle becomes increasingly transparent. Hair growth slows, the hairs thin, and the number of follicles decreases. Males may develop pattern baldness, which is hereditary but not often expressed in females. A diminished blood supply to the nail beds impairs their growth, dulling and hardening them. Sensitivity to pain and pressure diminishes with age as the number of receptors falls. A ninety-year-old's skin has only one-third the number of such receptors as the skin of a young adult.

The ability to control temperature falters as the number of sweat glands in the skin falls, as the capillary beds that surround sweat glands and hair follicles shrink, and as the ability to shiver declines. In addition, the number of blood vessels in the deeper layers decreases,

Blood Vessels

Figure

As an aid for estimating the extent of damage burns cause, the body is subdivided into regions, each representing 9% (or some multiple of 9%) of the total skin surface area.

Figure

As an aid for estimating the extent of damage burns cause, the body is subdivided into regions, each representing 9% (or some multiple of 9%) of the total skin surface area.

as does the ability to shunt blood towards the body's interior to conserve heat. As a result, an older person is less able to tolerate the cold and cannot regulate heat. An older person might set the thermostat ten to fifteen degrees higher than a younger person in the winter. Fewer blood vessels in and underlying the skin account for the pale complexions of some older individuals. Changes in the distribution of blood vessels also contribute to development of pressure sores in a bedridden person whose skin does not receive adequate stimulation.

Aging of the skin is also related to skeletal health. The skin is the site of activation of vitamin D, which requires exposure to the sun. Vitamin D is necessary for the absorption of calcium by bone tissue. Many older people do not get outdoors much, and the wavelengths of light that are important for vitamin D activation do not readily penetrate glass windows. In addition, older skin has a diminished ability to activate the vitamin. Therefore, homebound seniors often take vitamin D supplements to help maintain bone structure.

Figure 6.16

Aging-associated changes are very obvious in the skin.

Figure 6.16

Aging-associated changes are very obvious in the skin.

What changes occur in the epidermis and dermis with age?

How do the skin's accessory structures change over time?

Why do older people have more difficulty controlling body temperature than do younger people?

Common Skin Disorders athlete's foot (ath -ietz foot) Fungus infection (Tinea pedis)

usually in the skin of the toes and soles. birthmark (berth mark) Congenital blemish or spot on the skin, visible at birth or soon after. boil (boil) Bacterial infection (furuncle) of the skin, produced when bacteria enter a hair follicle. carbuncle (kar bnng-kl) Bacterial infection, similar to a boil, that spreads into the subcutaneous tissues. cyst (sist) Liquid-filled sac or capsule. eczema (ek zi-mah) Noncontagious skin rash often accompanied by itching, blistering, and scaling. erythema (er iwthe mah) Reddening of the skin due to dilation of dermal blood vessels in response to injury or inflammation.

herpes (her pez) Infectious disease of the skin usually caused by the herpes simplex virus and characterized by recurring formations of small clusters of vesicles. keloid (ke luid) Elevated, enlarging fibrous scar usually initiated by an injury. mole (mol) Fleshy skin tumor (nevus) that is usually pigmented; colors range from brown to black. pediculosis (pe-dik uw-lo sis) Disease produced by an infestation of lice. pruritus (proo-ri tus) Itching of the skin. pustule (pus tul) Elevated, pus-filled area. scabies (ska buz) Disease resulting from an infestation of mites.

seborrhea (seb ow-re auh) Hyperactivity of the sebaceous glands, accompanied by greasy skin and dandruff. ulcer (ul sur) Open sore.

urticaria (ur tw-ka ru-ah) Allergic reaction of the skin that produces reddish, elevated patches (hives). wart (wort) Flesh-colored, raised area caused by a viral infection.

Integumentary System

integumentary System

Skeletal System

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