One of the body’s most important organs is the skin. This protective covering is part of the integumentary system, which consists of the skin, hair, nails, sweat glands, and oil glands. Also known as the integument or the cutaneous membrane, the skin has five basic functions:
1. The skin protects the body against invasion by microorganisms and protects underlying body structures and delicate tissues from injury. The pigment melanin, which provides color to the skin, further protects the skin from the harmful effects of the ultraviolet rays of the sun. 2. The skin regulates body temperature by protecting the body from excessive loss of heat and fluids from underlying tissues. Sweat glands, which are located under the skin, secrete a watery fluid that cools the body as it evapo-rates from the surface of the skin.
3. The skin serves as a sensory receptor for sensations such as touch, pressure, pain, and temperature. These sensations are detected by the nerve endings within the skin and relayed to the brain. The appearance of the skin, in the form of facial expressions (e.g., grimaces shivering, frowns, or smiles), is sometimes visible evidence of the sensations felt by the skin.
4. The skin provides for elimination of body wastes in the form of perspiration. Substances such as water, salts, and some fatty substances are excreted through the pores (openings) of the skin.
5. The skin is responsible for the first step in the synthesis of vitamin D, which is essential for bone growth and development. When exposed to the ultraviolet rays of the sun, molecules within the skin are converted to a chemical that is transported in the blood to the liver and kidneys—where it is converted into vitamin D.
The study of the skin is known as dermatology. The physician who specializes in the treatment of diseases and disorders of the skin is known as a dermatologist. Epidermis The (1) epidermis, the outer layer of the skin, contains no blood or nerve supply. It consists of squamous epithelial cells, which are flat, scalelike, and arranged in layers (strata). The epidermis actually has about five different layers of stratified epithelium cells, each carrying on specific functions. The two layers of the epidermis that will be mentioned here are the stratum basale and the stratum corneum. The basal layer (stratum basale) is where new cells are continually being reproduced, pushing older cells toward the outermost surface of the skin. It is the innermost, or deepest, layer of the epidermis. The basal layer also contains melanocytes, which provide color to the skin and some protection from the harmful effects of the ultraviolet rays of the sun. The outermost layer of the epidermis is the stratum corneum, where the dead skin cells are constantly being shed and replaced. When the cells reach the outermost layer of the epidermis and die, they become filled with a hard water-repellant protein called keratin. This characteristic of keratin (waterproofing the body) creates a barrier, or a first line of defense for the body, by not allowing water to penetrate the skin or to be lost from the body—and by not allowing microorganisms to penetrate the unbroken skin. If the skin is injured and the barrier layer is damaged, microorganisms and other contaminants can easily pass through the epidermis to the lower layers of the skin—and fluids can escape the body (as occurs with burns). Sweat pore Hair shaft (3) Dermal papilla
Sensory nerve Stratum ending for touch corneum
(1) Epidermis
Stratum basale (2) Dermis Sebaceous (oil) gland (corium)
Hair follicle
Papilla of hair (4) Subcutaneous layer (fatty tissue) Nerve fiber
Vein
Artery Nerve Sweat gland
Layers and structures of the skin Dermis The (2) dermis is the inner thicker layer of skin lying directly beneath the epidermis. It is also known as the corium. It protects the body against mechanical injury and compression and serves as a reservoir (storage area) for water and electrolytes. Composed of living tissue, the dermis contains capillaries, lymphatic channels, and nerve endings. The hair follicles, sweat glands, and sebaceous (oil) glands are also embedded in the dermis. The dermis contains both connective tissue and elastic fibers to give it strength and elasticity. If the elastic fibers of the dermis are overstretched as a result of rapid increase in size of the abdomen (for example, due to obesity or during pregnancy), the fibers will weaken and tear. These linear tears in the dermis are known as stretch marks or stria. They begin as pinkish-blue streaks with jagged edges and may be accompanied by itching. As they heal and lose their color, the stria remain as silvery-white scar lines. The thickness of the dermis varies from the very thin delicate layers of the eyelids to the thicker layers of the palms of the hands and soles of the feet. Look at your hands and notice the distinct pattern of ridges on your fingertips. These ridges provide friction for grasping objects and are a result of the papillae (projections) of the superficial layer of the dermis that extend into the epidermis. The thin layer of the epidermis conforms to the ridges of the (3) dermal papillae, forming the characteristic ridges you are observing on your fingertips. In each of us, these ridges form a unique pattern that is genetically determined. These patterns are the basis of fingerprints and footprints. Subcutaneous Layer The (4) subcutaneous tissue, which lies just beneath the dermis, consists largely of loose connective tissue and adipose (fatty) tissue that connects the skin to the surface muscles. It is sometimes called the superficial fascia or subcutaneous fascia. The subcu-taneous, or fatty, tissue serves as insulation for the body and protects the deeper tissues. It is rich in nerves and nerve endings, including those that supply the dermis and epider-mis. The major blood vessels that supply the skin pass through the subcutaneous layer, and sweat glands and hair roots extend from the dermis down into the subcutaneous layer. The thickness of the subcutaneous layer varies, from the thinnest layer over the eyelids to the thickest layer over the abdomen. Accessory Structures
Hair A strand of hair is a long slender filament of keratin that consists of a (1) hair root, which is embedded in the (2) hair follicle, and a (3) hair shaft (which is the visible part of the hair). Each hair develops within the hair follicle, with any new hair forming from the keratin cells located at the bottom of the follicles.
Hair covers most of the human body, with the exception of the palms of the hands, the soles of the feet, the lips, the nipples, and some areas of the genitalia. Toward the end of the second trimester of pregnancy (about the fifth month), the developing fetus is almost completely covered with a soft downy (very fine) hair known as lanugo. This hairy coating is almost completely gone by birth, with any remaining lanugo disappearing shortly after birth. When present at birth, lanugo appears as a very fine velvety coating of hair over the baby’s skin.
Hair gets its color from the melanocytes (darkly pigmented cells) that surround the core of the hair shaft. These cells produce melanin, which gives hair a black or brown color depending on the amount produced. A unique type of melanin containing iron is responsible for red hair. When hair turns gray or white, usually due to the aging process, the amount of melanin has decreased significantly in the hair.
(3) Shaft of hair
Epidermis
Sebaceous gland
(1) Root of hair (2) Hair follicle Dermis Arrector pili muscle
Bulb Papilla of the hair Structure of the hair Nails The fingernails and toenails are protective coverings for the tips of the fingers and toes. These hard keratinized nail beds cover the dorsal surface of the last bone of each finger or toe. See Figure 5-3. The visible part of the nail is called the (1) nail body. The fold of skin at the base of the nail body is known as the (2) cuticle. Beneath the cuticle is the extension of the nail body known as the root of the nail. It lies in a groove hidden by the cuticle. At the base of the nail body nearest the root is a crescent-shaped white area known as the (3) lunula. The (4) free edge of the nail extends beyond the tip of the finger tip or toe. Nails grow approximately 0.5 mm per week. The nail body is nourished by the (5) nail bed, which is an epithelial layer located directly beneath it. The rich supply of blood vessels contained in the nail bed generate the pink color you can see through the translucent nail bodies.
(4) Free edge (1) Nail body
(3) Lunula (5) Nail bed (located beneath (2) Cuticle the nail body)
Structure of the nail Glands The glands of the skin complete the accessory structures of the skin.
The (1) sweat, or sudoriferous, gland is a small structure that originates deep within the dermis and ends at the surface of the skin with a tiny opening called a (2) pore. The sweat glands are found on almost all body surfaces, particularly the palms of the hands, soles of the feet, forehead, and armpits (axillae). Two main functions of the sweat glands are to cool the body by evaporation and to eliminate waste products through their pores.
Hair shaft
(2) Pore
Papilla Epidermis Capillary
Duct of sweat gland Dermis (3) Sebaceous gland
Nerve fiber Subcutaneous layer (1) Sweat gland
Blood vessel
Adipose cells
Glands of the skin
The sweat glands produce a clear watery fluid known as sweat (or perspiration), which travels from the gland to the surface of the skin (where it is excreted through the pores). As the sweat evaporates from the surface of the skin into the air, it creates a cooling effect on the body. In addition to being clear or colorless, sweat is odorless. It is made up of mostly water containing a small amount of dissolved substances such as salts, ammonia, uric acid, urea, and other waste products. These waste products are eliminated from the body through the pores of the sweat glands. As the sweat comes in contact with the bacteria present on the surface of the skin, it becomes contaminated and decomposes. This inter-action of the sweat with the bacteria found on the surface of the skin creates the odor we often associate with sweating.
The (3) sebaceous gland, also known as the oil gland, secretes a substance necessary for lubricating the hair and keeping the skin soft and waterproof. This substance, known as sebum, is secreted along the shaft of the hair follicles and directly onto the skin through ducts that open directly onto the epidermis. Secretion of sebum is influenced by the sex hormones and increases during adoles-cence. As a result of this increased secretion of sebum, the sebaceous gland ducts often become blocked and a pimple or blackhead may develop. The sebaceous glands (pres-ent throughout most of the body) are more numerous on the scalp, forehead, face, and chin. They are absent on the palms of the hands and the soles of the feet.
The ceruminous gland is actually classified as a modified sweat gland. Opening onto the free surface of the external ear canal, the ceruminous glands lubricate the skin of the ear canal with a yellowish-brown waxy substance called cerumen (also known as ear wax).