Anatomy & Physiology – The Integumentary System
The Integumentary System
The integumentary system is essentially your skin and it includes hair, nails, and glands. Skin is the body’s largest organ, making up over fifteen percent of a person’s total body weight. It performs a wide array of functions, including:
- Protecting the body’s internal organs and tissues.
- Aiding vitamin D production.
- Thwarting disease.
- Eliminating waste products.
- Regulating body temperature.
- Maintaining hydration.
- Receiving and relaying sensory information.
Skin cells also synthesize melanin and carotenes, which give skin its color, play a part in wound healing, and perform a vital role in regulating body temperature by increasing or decreasing heat loss through one of three ways:
- Hairs on the skin retain more warmth when standing up, and less when lying flat. Goose bumps are a vestige from when early humans had more hair.
- If the body is too hot, glands under the skin secrete sweat onto the surface of the skin, which is cooled when the sweat is evaporated.
- Capillaries near the surface of the skin can open when the body needs to cool off and close when it needs to conserve heat.
The skin is comprised of two layers: the epidermis and dermis. A third layer, the subcutaneous tissue or hypodermis, which is beneath the dermis, is generally not considered to be part of the skin. Each layer has a unique function that enables skin to carry out its varied roles.
Epidermis
The epidermis is the outer most layer of skin. The thickness of the epidermis depends on where it is located on the body. It is the thinnest on the eyelids and thickest on the palms and soles of the feet.
The epidermal layer is comprised mostly of keratinocytes, which produce the fibrous protein keratin that protect the epidermis. There are four specialized cell types found in the epidermis:
- Keratinocytes which produce the protein keratin that hardens and waterproofs the skin.
- Melanocytes which produce the brown pigment melanin.
- Langerhans cells which are involved in immune response.
- Merkel cells which are involved in nerve responses but their exact function is unclear.
- Stratum corneum (top), made of dead, flat skin cells that shed every 14 days.
- Stratum lucidum, found only in the fingertips, palms, and soles of the feet.
- Stratum granulosum, site of keratin formation.
- Stratum spinosum, provides strength and flexibility to the skin.
- Stratum basale (bottom), forms new cells, basal cells.
As new cells are formed in the basale they push previously produced cells upward. As the cells make their way up to the top layer, they become flattened and die. The production of new cells is a continuous process.
Dermis
Beneath the epidermis is the dermis, or dermal layer, of skin. The primary cells in the dermis are fibroblasts. The primary function of the dermis is to sustain and support the epidermis. To that end the dermis, which is 10 to 40 times thicker than the epidermis, contains several specialized structures.
- Hair follicles, formed by the epidermis and dermis.
- Sebaceous glands, which produce the oily substance sebum.
- Scent glands.
- Eccrine, or sweat, glands.
- Apocrine glands, similar in structure to eccrine glands; they are another type of sweat gland that only functions after puberty.
- Blood vessels, which bring oxygen and nutrients to the skin and then remove wastes.
- Nerves, which transmit sensations such as pain and temperature.
The dermis is made up of three types of tissue that are evenly distributed: collagen, elastic tissues, and reticular fibers.
Collagen is a type of fibrous protein that connects and supports other body tissues. It is one of the most common proteins in mammals and one of more than 25 types of collagens which occur naturally in humans. Collagen is sometimes referred to as the glue that holds the body together.
Elastic tissue is a type of connective tissue that gives skin the ability to stretch and then return to its original shape.
Collagen type III.
The dermis has two layers. The upper, closest to the epithelial layer, is called the papillary, which contains a thin coat of collagen fibers. The lower layer is the reticular, composed of a thick arrangement of collagen fibers positioned perpendicular to the skin’s surface.
The dermis contains many specialized cells and structures. The sebaceous glands produce a natural oil called sebum. It travels to the surface of the epidermis to keep your skin lubricated and protected. It is the substance that makes skin waterproof. When sweat, which is produced continually, emerges from the pores in your skin, it combines with sebum to form a protective film.
The hair follicles are lined with protein synthesizing cells that form hair, which is coated with oil from a sebaceous gland. Attached to the base of each follicle is a small muscle, the arrector pili, that contracts when the skin is chilled to make the hair stand up.
Nerve cells called Meissner’s and Vater-Pacini corpuscles transmit the sensations of touch and pressure.
Subcutaneous Tissue
The bottom layer of skin is the subcutaneous (or hypodermis), comprised primarily of fat and connective tissue that contains larger blood vessels and nerves. The size of this layer depends on the area of the body and also varies from person to person.
The subcutaneous assists in the body’s temperature regulation, acts as a shock absorber, and helps hold skin to all the tissues underneath it.
Skin Accessory Organs
Hair, nails, glands, horns, and even feathers are animal structures derived from the skin.
Hair
The hair shaft is the part visible above the skin surface while the hair root is located beneath the surface, extending down to the base in the subcutaneous. There are hair follicles everywhere on the external body except on the lips, palms of the hands, and soles of the feet.
Nails
Nails are made of highly keratinized, modified epidermal cells. The nail root is the portion beneath the skin.
Glands in the integumentary system primarily assist in regulating body temperature and preventing growth of bacteria. There are two main types of glands associated with the integumentary system:
- Sebaceous, or oil glands, are located in the dermis and secrete sebum. They help to prevent bacterial growth and keep the skin and hair moist.
- Sudoriferous, or sweat glands. There are four main types of sweat glands: Eccrine, regulates body temperature through evaporation, Apocrine, remnant of the mammalian sexual scent gland, no current function; Ceruminous, located in subcutaneous tissue, secretes cerume, (ear wax) into the ear canal; and Mammary glands which produce milk.
Conclusion
The integumentary system serves many purposes, from protecting the internal organs to helping prevent disease and regulating metabolism.