The integumentary system includes the skin and the 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, simultaneously preventing water from entering the body and 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 radiation.
In addition to serving as a protective barrier, the skin helps to regulate the body temperature by several mechanisms. If heat builds up in the body, sweat glands in the skin produce more sweat that evaporates and cools the skin. When the body overheats, blood vessels in the skin dilate (expand), bringing more blood to the surface, and allowing body heat to dissipate. When the body is too cold, the blood vessels in the skin constrict, shunting blood away from the body surface, thus conserving heat. Along with temperature regulation, the skin serves as a minor excretory organ, since sweat removes small, clinically insignificant amounts of nitrogenous wastes produced by the body. The skin also functions as a sense organ since it contains millions of nerve endings that detect touch, heat, cold, pain and pressure. Finally, the skin produces vitamin D in the presence of sunlight, and renews and repairs damage to itself.
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 ultraviolet light, thereby shielding the genetic material in skin cells from damage. Merkel's cells disks are touchsensitive cells 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 in the digits 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 also 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 as such includes a number of types of nerves, which are mainly
Role in human health
Along with its vital roles as shield against microorganisms and regulating body temperature, skin often 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).
Skin testing is an important diagnostic tool in the evaluation of allergies. Skin testing 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.
Common diseases and disorders
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 bacterial skin infections are impetigo, folliculitis, and cellulitis. Impetigo is a contagious skin infection caused by streptococci or staphylococcus. It produces crusty patches on the skin. Local outbreaks may be treated with antibacterial ointment, and patients with widespread infections are given oral antibiotics. Infectious folliculitis produces erythema (redness) and pustules. It is caused by staphylococcus and treated with oral antibiotics. Cellulitis is swelling, erythema, warmth, and pain caused by infection of the dermis and subcutaneous tissue often near a wound site. Cellulitis is usually caused by Group A streptococci or staph aureus and is treated with a course of anti-strep or anti-staph antibiotics.
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 agent.
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.
Cosmetic damage as well as potentially fatal skin disorders may result from overexposure to the ultraviolet (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 also 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 the development of skin cancers.
Dermatology is the medical specialty devoted to the diagnosis and treatment of skin disorders. In addition to the disorders previously described, primary care physicians and dermatologists are frequently called upon to diagnose and treat the following conditions:
- alopecia (hair loss)
- athlete's foot (fungus infection)
- psoriasis (scaly skin on the scalp, trunk, arms and legs)
- rosacea (symmetrically distributed papules and pustules on the nose and cheeks)
- scabies (skin infestation with mites that produces inflammatory papules in the wrists, web spaces, and sides of feet)
- seborrheic dermatitis (facial redness and scaling)
- spider veins, varicose veins
- vitiligo (loss of skin color on patches of skin, usually affects the face and extremities)
Today, 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.
Chitin—Polysaccharide that forms the exoskeleton of insects, crustaceans, and other invertebrates.
Dermis—Thicker layer of skin lying below the epidermis.
Epidermis—Thinner outermost layer of the skin.
Keratin—Insoluble protein found in hair, nails, and skin.
Melanin—Brown-black pigment found in skin and hair.
Freedberg, Irwin, et al. Fitzpatrick's Dermatology In General Medicine, Fifth Edition. Vols. I, II, and III. New York, NY: McGraw-Hill, 1999.
Odom, Richard B., et al. Andrew's Diseases of the Skin. Philadelphia, PA: W.B. Saunders Company, 2000.
Skin Deep. Video and videodisc. Princeton, NJ: Films for the Humanities and Sciences, 1995.