The anatomy of the integumentary system
The skin is the largest organ of the human body. In an average-size adult, it covers a total area of about 20 square feet. Forming the outer boundary of the body, the skin performs several essential functions: it acts as a barrier to protect vital internal organs from infection and injury, helps to dissipate heat and regulate body temperature, and synthesizes vitamin D when exposed to ultraviolet (UV) light. In addition, various sensory receptors present throughout the skin enable it to respond to such sensations as heat, cold, pain, and pressure.
The specialty of dermatology deals with the skin and its accessory structures: hair, nails, and sweat glands, and the subcutaneous tissue that lies beneath the skin. A physician who specializes in dermatology is called a dermatologist.
Anatomy of the integumentary system
The most important function of the skin (or integument) is protecting the body from disease. The skin provides a physical barrier that is our first line of defense against pathogens. It is the largest organ of the body, and, in addition to physically protecting the body, it also helps to do the following:
- Regulate the body temperature
- Provide information about the environment through sensory activity
- Assist in the synthesis of vitamin D
- Eliminate waste products from the body
The skin functions together with accessory structures that include the hair, nails, sebaceous glands or oil glands, and sudoriferous glands or sweat glands. Any damage or injury to the skin has the potential to lessen its ability to carry out these functions, which can lead to disease.
The skin is considered a cutaneous membrane, made up of epithelial cells that cover the entire body. The skin is composed of two layers: the epidermis, which forms the outermost layer, and the dermis, which forms the inner layer. The dermis is attached to a layer of connective tissue called the subcutaneous layer, or hypodermis, which is mainly composed of fat or adipose tissue.
Epidermis
The epidermis is the top layer of the skin and is composed of several different strata of epithelial tissue. This type of tissue covers many of the external and internal surfaces of the body and has a microscopic scaly appearance.
The epidermis is avascular, which means it contains no blood vessels. New skin cells are formed in the basal layer of the epidermis, called the stratum germinativum. This layer is also where melanin, which is a pigment, is produced by melanocytes. When the skin is exposed to ultraviolet (UV) light, the melanocytes secrete more melanin. Birthmarks, age spots, and freckles result from clumps of melanin. Individuals have different skin colors because of varying numbers of melanocytes in the stratum germinativum of the skin.
New cells produced in the stratum germinativum move upward toward the top layer, the stratum corneum. During the transition from the basal layer to the upper layer, the cell’s cytoplasm is replaced with keratin. Once this process is complete, the cells are called keratinocytes. Keratin is a hard protein material that enhances the skin by making it waterproof, abrasion resistant, and able to retain moisture in the body. These properties add to the protective nature of the skin.
A variety of microorganisms are normally found on the skin. These organisms are referred to as normal flora. Healthcare workers are taught to wash their hands or use hand sanitizer before and after each procedure. Good hand hygiene helps prevent the spread of microorganisms, reduces the likelihood of transferring possible pathogens, and decreases the risk of skin infections for our patients.
Dermal-epidermal junction
The dermal-epidermal junction is where the two layers of the skin meet. The top layer of the dermis contains dermal papillae, which are peglike projections that help fasten the dermis and epidermis together. This unique junction not only uses dermal papillae to create a bond but also uses a specialized gel that acts as a glue to keep the two layers of the skin connected.
If the dermal-epidermal junction is damaged by a burn, irritation, abrasion, or friction, a blister may occur. If the dermal-epidermal junction is destroyed, the skin will fall apart. If this happens in a small area, the body can heal the damage. If a large area of the junction is destroyed, it results in an overwhelming infection that could be serious and possibly fatal.
Dermis
The dermis is the thick, underlying layer of the skin that is composed of vascular connective tissue arranged in two layers. The dermis gives the skin strength and stretch that the epidermis does not possess. The thin upper layer of the dermis is composed of fibers made from loose connective tissue, collagen, and some elastin. The lower, thicker layer of the dermis is composed of dense collagen. Collagen gives the skin toughness, and elastin fibers help the skin stretch and have flexibility. Distributed throughout the dermis are hair follicles, sweat glands, and old glands.
There are two types of sweat glands: eccrine and apocrine. Eccrine sweat glands are dispersed throughout the body and help maintain a constant body temperature by releasing sweat through pores in the skin. Apocrine sweat glands open into hair follicles and are concentrated in the axillae (armpits), scalp, face, and the pigmented skin around the genitals. Apocrine sweat glands release a fatty sweat in response to stress. They enlarge and become more active at puberty.
The dermis contains many small blood vessels that supply the skin with nutrients and oxygen and take away waste products and carbon dioxide. Sensory receptors for the nervous system (pain, temperature, and pressure) are also located in the dermis.
Subcutaneous tissue
The subcutaneous tissue is not a layer of the skin. It lies below the dermis and creates a connection between the skin and the structures that are below the skin, such as muscle or bone. Subcutaneous tissue is made up of loose connective tissue and adipose (fat) tissue. This tissue is not rigid but rather movable and pliable. The subcutaneous tissue can slip over underlying structures. Without subcutaneous tissue, our skin would tear with movement.
The subcutaneous tissue provides insulation for the body and serves as a calorie reserve in the adipose tissue. It also contains blood vessels, nerves, and the base of hair follicles.
Subcutaneous tissue is distributed unevenly, and as the body ages, it thins. This can make administering injections or drawing blood on older adult patients more challenging. Also, the loss of subcutaneous tissue makes it more difficult for older adults to regulate body temperature; they are hotter when temperatures rise and colder when temperatures drop. Aging skin can be fragile and easily traumatized or damaged. The medical assistant must be careful to avoid injuring the skin of older patients.
The Physiology of the Integumentary System
The five functions of the integumentary system are listed and discussed next:
- Protection
- Sensory organ activity
- Temperature regulation
- Excretion
- Synthesis of vitamin D
Protection
The skin protects the internal organs by providing a flexible, waterproof barrier to the outside environment. It is part of the first line of defense from microbial pathogens, toxic chemicals, and physical tears, cuts, and abrasions. The keratin in our skin cells protects us from excessive fluid loss and dehydration. Finally, the melanin in the epidermis of the skin protects the body from damaging UV light. Skin is so amazing!
Sensory organ activity
The skin has many sensory receptors scattered throughout the tissue. Sensory receptors can feel pain, pressure, heat, and cold. These receptors send messages to the brain that allow us to respond to the environment and make appropriate changes to keep ourselves from harm.
Temperature regulation
The skin is a very good thermoregulator. When we are cold, it constricts blood vessels close to the skin’s surface. This preserves body heat and lessens the heat loss to the surrounding environment. When we are hot, it can produce sweat that evaporates and cools us off. Because of the extensive blood supply to the skin, the body can help regulate and maintain a consistent body temperature. This is all part of homeostasis.
Excretion
The body can regulate the amount of sweat produced and the chemical content of the sweat. When we sweat, we lose water, electrolytes, and small amounts of other waste products. The integumentary system can excrete substances through the skin to help maintain the necessary chemical balance in the body as a whole.
Synthesis of vitamin D
The synthesis of vitamin D is a vital function of the body, and it all starts in the skin. When skin is exposed to the sun’s ultraviolet rays, it manufactures a vitamin D precursor molecule. The molecule is carried to the liver and kidneys by way of the blood. The precursor molecule is then converted to the active form of vitamin D that the body can use. Research has uncovered many body functions affected by vitamin D, a few of which are as follows:
- Proper absorption of calcium
- Maintaining normal calcium and phosphorus levels in the blood; calcium helps maintain strong bones, proper blood clotting, muscle function, and immunity
- Protection against osteoporosis, high blood pressure, and some forms of cancer
- Preventing consistently low vitamin D levels, which can put a person at a higher risk for developing multiple sclerosis, heart disease, osteoporosis, and depression
Life span changes
Across the life span, many changes occur that affect the integumentary system. The following sections discuss those changes.
Changes in children
While a baby is in utero, a protective coating forms on the baby’s skin. This is called vernix and is a white, creamy biofilm that protects the baby’s skin from the amniotic fluid in the last trimester of the pregnancy. Lanugo, a special type of body hair, serves as an anchor to hold the vernix to the skin.
The skin of a newborn is thinner than the skin of an adult, and the skin of a preterm infant is even less than that of a full-term newborn. This can lead to increased fluid and heat loss and can result in electrolyte imbalance, reduced thermoregulation, and increased infection risk.
Children routinely appear at physician offices with skin disorders. Some of the most common are impetigo, acne, seborrheic dermatitis, cellulitis, and pediculosis. Although none of these are serious disorders, impetigo and pediculosis are highly contagious. Also seen are the different degrees of burns, usually accidental but potentially life threatening.
Changes in older adults
Older patients have a completely different set of diagnoses. The disorders categorized as those related to cornification, especially corns and calluses, are seen routinely in medical offices. These masses or thickenings of the skin are formed as a defensive response to constant friction, usually within shoes. Pressure sores (also called bedsores or decubitus ulcers) are seen most often in bedridden patients whose skin may already be atrophied and thin. Eczema, cellulitis, and fungal infections are also common complaints. The last category of skin disorders often seen in older adults are skin cancers. Although younger patients with skin cancers are often seen, older patients have had a longer time to be exposed to the sun and develop malignancies.