10.2 Integumentary Basic Concepts
Open Resources for Nursing (Open RN)
Skin
Skin is made up of three layers: epidermis, dermis, and hypodermis. See Figure 10.1[1] for an illustration of skin layers. The epidermis is the thin, topmost layer of the skin. It contains sweat gland duct openings and the visible part of hair known as the hair shaft. Underneath the epidermis lies the dermis where many essential components of skin function are located. The dermis contains hair follicles (the roots of hair shafts), sebaceous oil glands, blood vessels, endocrine sweat glands, and nerve endings. The bottommost layer of skin is the hypodermis (also referred to as the subcutaneous layer). It mostly consists of adipose tissue (fat), along with some blood vessels and nerve endings. Beneath the hypodermis layer lies bone, muscle, ligaments, and tendons.
Hair
Hair is a filament that grows from a hair follicle in the dermis of the skin. See Figure 10.2[2] for an illustration of a hair follicle. It consists mainly of tightly packed, keratin-filled cells called keratinocytes. The human body is covered with hair follicles except for the mucous membranes, lips, palms of the hands, and soles of the feet. The part of the hair that is located within the follicle is called the hair root, the only living part of the hair. The part of the hair that is visible above the surface of the skin is the hair shaft. The shaft of the hair has no biochemical activity and is considered dead.
Functions of Hair
The functions of head hair are to provide insulation to retain heat and to protect the skin from damage by UV light. The function of hair in other locations on the body is debated. One idea is that body hair helps to keep us warm in cold weather. When the body is cold, the arrector pili muscles contract, causing hairs to stand up and trapping a layer of warm air above the epidermis. However, this action is more effective in mammals that have thick hair than it is in relatively hairless human beings.
Human hair has an important sensory function as well. Sensory receptors in the hair follicles can sense when the hair moves, whether it is because of a breeze or the touch of a physical object. Some hairs, such as the eyelashes, are especially sensitive to the presence of potentially harmful matter. The eyebrows protect the eyes from dirt, sweat, and rain. In addition, the eyebrows play a key role in nonverbal communication by expressing emotions such as sadness, anger, surprise, and excitement.[3]
Nails
Nails are accessory organs of the skin. They are made of sheets of dead keratinocytes and are found on the distal ends of the fingers and toes. The keratin in nails makes them hard but flexible. Nails serve a number of purposes, including protecting the fingers, enhancing sensations, and acting like tools. A nail has three main parts: root, plate, and free margin. Other structures around or under the nail include the nail bed, cuticle, and nail fold. See Figure 10.3 for an illustration