Functions of the Integumentary SystemIN THIS CHAPTER:
Functions of the Integumentary System
Structure of the Skin
Associated Structures of the Skin
Physiology of the Skin
Solved Problems
The integumentary system is composed of the skin, or integument, and associated structures (hair, glands, and nails). This system accounts for approximately 7 percent of the body weight and is a dynamic interface between the body and the external environment.
The functions of the integumentary system include physical protection, hydroregulation, thermoregulation, cutaneous absorption, synthesis, sensory reception, and communication. The skin is a physical barrier to most microorganisms, water, and most UV light. The acidic surface (pH 4.0-6.8) retards the growth of most pathogens. The skin protects the body from desiccation (dehydration) when on dry land and from water absorption when immersed in water. A normal body temperature of 37°C (98.6 °F) is maintained by the antagonistic effects of shivering and sweating, as well as by vasodilation and vasoconstriction of the blood vessels to the skin. The skin permits the absorption of small amounts of UV light necessary for synthesis of vitamin D. It is important to note that certain toxins and pesticides also may enter the body through cutaneous absorption. The skin synthesizes melanin (a protective pigment) and keratin (a protective protein). Numerous sensory receptors are located in the skin, especially in parts of the face, palms, and fingers of the hands, soles of the feet, and genitalia. The skin interacts with numerous body systems in accomplishing these various functions including the circulatory system, the immune system, and the nervous system.
A diagram of the skin is shown in Figure 5-1.
The layers of the skin are, from superficial to deep, the:
• Epidermis
• Dermis and Hypodermis
The outer epidermis is composed of stratified squamous epithelium that is 30 to 50 cells thick. The layered cells are avascular; the outer cells are dead, keratinized, and cornified. The epidermis is stratified into five structural and functional layers, from superficial to deep, stratum corneum, stratum lucidum, stratum granulosum, stratum spinosum, and stratum basale. The stratum basale lies on the basement membrane of this epithelial tissue in close proximity to the underlying blood supply. Mitosis occurs primarily in the deep stratum basale and to a slight extent in the stratum spinosum. As the cells divide, only half of them remain in contact with the dermis. The other cells are pushed away from the underlying blood supply and cell death occurs. As cells move toward the surface, specialized cells, keratinocytes, fill with keratin (keratinization), a protein that toughens and waterproofs the skin, and all cells become flattened and scalelike (cornification). The dead cells of the epidermis buffer the body from the external environment.
Also found within the stratum basale and stratum spinosum are pigment forming cells, melanocytes. Melanin is a brown-black pigment produced by melanocytes. The amount of melanin produced varies among different ethnic groups. Other pigments that contribute to skin coloration are carotene, a yellow pigment found in epidermal cells, and hemoglobin, an oxygen binding pigment found in red blood cells.
The thick and deeper dermis is composed of highly vascularized connective tissue and consists of a variety of living cells, and numerous collagenous, elastic, and reticular fibers. The dermis also has numerous sweat and oil glands and hair follicles, as well as sensory receptors for heat, cold, touch, pressure, and pain. There are two layers of the dermis, the papillary layer is in contact with the epidermis and the deeper, thicker reticular layer is in contact with the hypodermis. Not considered a separate layer, the hypodermis (subcutaneous tissue) contains loose (areolar) connective tissue, adipose tissue, and blood and lymph vessels. Collagenous and elastic fibers reinforce the hypodermis. The hypodermis binds the dermis to underlying organs, stores lipids, insulates and cushions the body, and regulates temperature via autonomic vasoconstriction or vasodilation.
Hair, nails, and three kinds of exocrine glands form from the epidermal skin layer. These structures develop as down-growths of germinal epidermal cells into the vascular dermis, where they receive sustenance and mechanical support.
The hair follicle is the germinal epithelial layer that has grown down into the dermis (Figure 5-2). Mitotic activity of the hair follicle accounts for growth of the hair. The shaft of the hair is the dead, visible, projecting portion; the root of the hair is the living portion within the hair follicle; and the bulb of the hair is the enlarged base of the root of the hair that receives nutrients and is surrounded by sensory receptors. The outer keratinized cuticle layer appears scaly under a dissecting microscope. Variation in the amount of melanin accounts for different hair colors. Each hair follicle has an associated arrector pili muscle (smooth muscle) that responds involuntarily to thermal or psychological stimuli, causing the hair to be pulled into a more vertical position. Hair on the scalp and eyebrows protects against the sunlight, hair in the nostrils and the eyelashes protect against airborne particles. A secondary function of hair is as a means of individual recognition and sexual attraction.
Figure 5-2. A hair within a hair follicle.
Nails are formed from the hardened, transparent stratum corneum of the epidermis. Nails serve to protect the digits and aid in grasping small objects. All lizards, birds, and mammals have some sort of hardened sheath (claw, talon, hoof, or nail) protecting their terminal phalanges.
The three types of exocrine glands are formed from the epidermal layer of skin.
• Sebaceous glands: Oil glands, secrete acidic sebum, lubricate and waterproof the skin.
• Sudoriferous glands are sweat glands. Eccrine glands, abundant on the forehead, back, palms, and soles, function in evaporative cooling. Apocrine glands, in the axillary and pubic regions, function at puberty as a sexual attractant.
• Mammary glands are specialized sudiferous glands within the breasts of females. Secrete milk under hormonal influence.
Table 5.1 Summary of the Physiology of the Skin
True or False
1. Skin is the largest tissue in the body, accounting for about 7 percent of body weight. (True)
2. Mitotic activity is characteristic of all layers of the epidermis except the dead stratum disjunction, which is constantly being shed. (False)
3. Mammary glands are modified sebaceous glands that are hormonally prepared to lactate in association with the birth of a baby. (False)
Completion
1. The dermis of the skin consists of an upper _____________ layer and a deeper _____________ layer. (papillary layer, reticular layer)
2. The eipidermis of the skin consists of _____________ _______________epithelial tissue. (stratified, squamous)
