The integumentary system includes the skin and related structures that cover and protect the body. The human integumentary system is composed of the skin, and includes glands, hair, and nails. The largest organ in the body, the skin protects the body, prevents water loss, regulates body temperature, and senses the external environment.
The integumentary system serves many protective functions for the body. It acts as a mechanical barrier, preventing water from entering the body, as well as excessive water loss. It also limits access of microorganisms that could cause illness, and protects underlying tissues from mechanical damage. Pigments in the skin, called melanin, give skin its color and absorb and reflect the sun's harmful ultraviolet (UV) radiation.
In an adult, the skin covers about 21.5 sq. ft. (2 sq. m), and weighs about 11 lb. (5 kg). Depending on location, the skin thickness ranges from 0.02-0.16 in. (0.5-4.0 mm). Skin is composed of an outer layer, or epidermis, and a thicker inner layer, the dermis. A subcutaneous layer of fatty or adipose tissue is immediately below the dermis. Fibers from the dermis attach the skin to the subcutaneous layer, and the underlying tissues and organs also connect to the subcutaneous layer.
Ninety percent of the epidermis, including the outer layers, contains keratinocytes, cells that produce keratin, a protein that helps waterproof and protect the skin. Melanocytes are pigment cells that produce melanin, a brown-black pigment that adds to skin color and absorbs UV light, thereby shielding the genetic material in skin cells from damage. Merkel disks are touch-sensitive receptors found in the deepest layer of the epidermis of hairless skin.
In most areas of the body, the epidermis consists of four layers. On the soles of the feet and palms of the hands where there is considerable friction, the epidermis has five layers. Calluses, abnormal thickenings of the epidermis, occur on skin subject to constant friction. At the skin surface, the outer layer of the epidermis constantly sheds the dead cells containing keratin. The uppermost layer consists of about 25 rows of flat dead cells that contain keratin.
The dermis is made up of connective tissue that contains protein, collagen, and elastic fibers. It also contains blood and lymph vessels, sensory receptors, related nerves, and glands. The outer part of the dermis has fingerlike projections, called dermal papillae, that indent the lower layer of the epidermis. Dermal papillae cause ridges in the epidermis above it, which give rise to fingerprints. The ridge pattern of fingerprints is inherited, and is unique to each individual. The dermis is thick in the palms and soles, but very thin in other places, such as the eyelids.
The blood vessels in the dermis contain a volume of blood. When a part of the body, such as a working muscle, needs more blood, blood vessels in the dermis constrict, shifting blood from the skin to supply muscles and other body parts. Sweat glands with ducts that pass through the epidermis and open on the skin surface through pores are embedded in the deep layers of the dermis. Hair follicles and hair roots originate in the dermis, and the hair shafts extend from the hair root through the skin layers to the surface. The dermis also contains sebaceous glands associated with hair follicles. Sebaceous glands produce an oily substance called sebum. Sebum softens the hair and prevents it from drying, but if sebum blocks a sebaceous gland, a whitehead appears on the skin. A blackhead results if the material oxidizes and dries. Acne pimples are caused by infections of the sebaceous glands.
The skin is an important sense organ and includes several types of nerves, which are mainly in the dermis, with a few reaching the epidermis. Nerves carry impulses to and from hair muscles, sweat glands, and blood vessels, and receive messages from touch, temperature, and pain receptors. Some nerve endings are specialized, such as sensory receptors that detect external stimuli. The nerve endings in the dermal papillae, known as Meissner corpuscles, detect light, touch, and the feel of clothing on the skin. Pacinian corpuscles, located in the deeper dermis, are stimulated by stronger pressure on the skin. Receptors near hair roots detect displacement of the skin hairs by stimuli such as touch or wind. Bare nerve endings throughout the skin supply information to the brain about temperature change (both heat and cold), texture, pressure, and trauma.
The skin thins with aging and changes occur at the cellular level. The network of elastic and collagen fibers decrease over time, making individuals prone to injury and skin infections, and also to sagging and wrinkles. It is important then, to understand how staying fit and eating right benefit the integumentary system.
The cells also need energy nutrients such as carbohydrates, proteins, and fats transported through the body for healthy skin, nails, and hair. Omega-3 essential fatty acids found in walnuts, flax seeds, wild-caught salmon, herring, and mackerel help promote smooth skin, strong nails, and shiny hair.
Foods rich in antioxidants also promote healthy skin. Antioxidants are molecules found in “superfoods” that can prevent or repair damage to cells in the body. Blackberries, red grapes, cranberries, leafy greens, and broccoli are all rich in antioxidants.
As an individual ages, a decrease in the melanin pigment occurs, making the skin more sensitive to effects of the sun. Avoiding long periods of time in the sun is recommended; however, less time in the sun causes the production of vitamin D to decrease by as much as 75%. A decrease in vitamin D results in muscle weakness and reduced bone strength (osteoporosis). It is important that individuals have levels of vitamin D checked periodically and take supplemental vitamin D as directed.
Suggestions to help benefit skin and to minimize skin damage include:
Exercise improves blood flow and helps in the production of new cell growth. Regular exercise is a key component in maintaining good skin health. While exercising, individuals should be sure not to wear constrictive clothing as this may interfere with circulation. Both weight-bearing and aerobic exercise help fuel the circulation, bringing needed oxygen and nutrients to cells throughout the body, speeding wound healing, and dispersing moisture to the skin through sweat. It is important that individuals adhere to adequate fluid replacement when exercising in order to replenish fluid lost when sweating.
Many forms of exercise have a beneficial effect on the integumentary system. Stretching can help tone the skin, lifting weights can strengthen and build muscle, and engaging in an aerobic workout aids in good circulation and minimizing inflammation.
Along with its vital role as shield against microorganisms and regulating body temperature, skin provides information about overall health and a variety of medical conditions. The color, texture, temperature, and elasticity of skin can aid in diagnosing a variety of disorders. For example, patients with hepatitis may have a characteristic yellow tinge to their skin. Similarly, cold sores and fever blisters are indications of infection with herpes simplex virus, and warts (intraepidermal skin tumors) result from infection with human papilloma virus (HPV).
Acne, caused by clogged pores and bacterial infection, is commonly diagnosed in teenagers and young adults. Acne may be mild, moderate, or severe and is characterized by blackheads, whiteheads, papules, pustules, and cysts on the face, shoulders, chest, and back. Mild acne may be treated with topical antimicrobial agents to kill the bacteria on the skin and topical retinoids to open the pores. Moderate and severe acne often respond to treatment with systemic antibiotics such as tetracycline or doxycycline.
Common viral skin disorders include infection with herpes simplex or herpes zoster. Herpes simplex is responsible for cold sores, fever blisters, and lesions on the genitals and buttocks. Herpes zoster produces a painful rash characterized by vesicles. Both conditions are treated with acyclovir or other orally administered anti-viral agents.
Skin reactions include eczema; allergic contact dermatitis (rashes), such as those resulting from contact with poison ivy, sumac, or oak; and hives. Contact dermatitis, an eruption of itchy skin vesicles, is an allergic skin reaction. Patients are advised to avoid contact with the suspected allergen, and mild cases may be treated with warm soaks and topical ointments to reduce inflammation and soothe inflamed skin.
Skin testing is an important diagnostic tool in the evaluation of allergies. It involves a series of superficial injections of one or more suspected allergens. A positive response such as redness or inflammation at the site of the skin test helps to pinpoint the culprit.
Cosmetic damage and potentially fatal skin disorders may result from overexposure to the UV rays in sunlight. At first, overexposure to sunlight results in injury known as sunburn. UV rays damage skin cells, blood vessels, and other dermal structures. Continual overexposure produces leathery skin, wrinkles, and discoloration and may lead to skin cancer. Anyone excessively exposed to UV rays runs a risk of skin cancer, regardless of the amount of pigmentation normally in the skin. Seventy-five percent of all skin cancers are basal cell carcinomas that arise in the epidermis and rarely metastasize (spread) to other parts of the body. Physicians can surgically remove basal cell cancers. Squamous cell carcinomas also occur in the epidermis, and these may metastasize. Malignant melanomas are life-threatening skin cancers that metastasize rapidly. There can be a 10-to 20-year delay between exposure to sunlight and skin cancer development.
Dermatology is the medical specialty devoted to the diagnosis and treatment of skin disorders. Primary care physicians and dermatologists are frequently called upon to diagnose and treat the following conditions:
Many dermatologists also provide a range of cosmetic services to reduce the signs of aging, such as wrinkles, sagging skin, and discoloration, and reverse some of the effects of sun damage to skin. Microdermabrasion, laser skin resurfacing, and injections of collagen are among the techniques used to improve the appearance of skin.
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Habif, Thomas P., et al. Skin Disease: Diagnosis and Treatment, 3rd ed. New York: Saunders, 2011.
James, William D., et al. Andrew's Diseases of the Skin, 12th ed. New York: Saunders, 2015.
Rizzo, Donald C. Introduction to Anatomy and Physiology. Clifton Park: Delmar, 2011.
Sanchez, Nestor P., ed. Atlas of Dermatology in Internal Medicine. New York: Springer, 2011.
Tannous, Zeina, et al. Color Atlas of Cosmetic Dermatology, 2nd ed. New York: McGraw-Hill Professional, 2011.
American Academy of Dermatology, PO Box 4014, Schaumburg, IL, 60168, (847) 240-1280, (888) 462-3376, Fax: (847) 240-1859, http://www.aad.org .
National Institutes of Health, 9000 Rockville Pike, Bethesda, MD, 20892, (301) 496-4000, NIHinfo@ od.nih.gov, http://www.nih.gov/index.html .
National Cancer Institute, BG 9609 MSC 9760, 9609 Medical Center Dr., Bethesda, MD, 20892, (800) 422-6237, http://www.cancer.gov .
Revised by Laura Jean Cataldo, RN, EdD