Integumentary System Help
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.
Functions of the Integumentary System
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. Anormal 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.
Structure of the Skin
A diagram of the skin is shown in Figure 5-1.
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.