1. Name and describe the function of each structure included on the human body.

The integumentary system, consisting of skin, hair, and nails, act as a protective barrier between the body and the outside world. It also functions to retain fluids, protect against disease, eliminate waste products, and regulate body temperature.

The skin, which is the largest organ of the human body, is made up of the epidermis and dermis. The epidermis, or outer layer of skin, is composed of about 20 layers of dead cells. These cells are constantly being shed and replaced by new cells. The cells of the epidermis are filled with a protein, called keratin, that gives the skin its rough, leathery texture. Keratin also gives the skin its waterproof quality.

Dermis, the inner layer of the skin, is composed of living cells and contains nerves, blood, and lymph vessels. The numerous nerves make it possible for the skin to receive many kinds of environmental signals. The blood vessels release heat and help the body maintain a comfortable temperature. The lymph helps the skin fight against infection.

A layer of fat cells lies below the dermis. These cells enable the skin to store food for energy. They also provide protection to the body and insulate it against heat loss.

Hair is produced by cells at the base of the hair follicles, deep pockets that extend into the dermis. Blood vessels surrounding the hair follicles nourish the hair root. The shaft of hair that extends beyond the skin consists mostly of keratin and requires no nourishment. Oil secreted by glands in the skin prevents the shaft from drying out and breaking off. Individual hairs usually grow for several years and then fall out. However hair on the scalp can grow continuously for many years. In many animals the body hair serves to increase body temperature as it once did in humans.

Nails, which protect the ends of fingers and toes, form from the cells at the end of a deep fold of epidermis. As new cells form at the base of the nail, the nail grows longer. Like hair, nails are composed primarily of keratin.

B. Discuss how each system works. Follow materials through a system, or show how signals, impulses, or substances are controlled, secreted or used.

The skin contains exocrine glands, glands that release secretions trough ducts. The main exocrine glands of the skin are the sweat glands and the oil glands.

The skin functions as an excretory organ by releasing excess water, salts, and urea through the sweat glands. By releasing excess water the skin also helps regulate the body temperature. When the body temperature rises, the circulation increases, and the skin becomes warm and flushed. The sweat glands then release sweat. As the water in the sweat evaporates the skin is cooled.

The oil glands secrete a substance known an sebum. This substance helps soften the skin and keeps it waterproof. These glands are usually associated with hair follicles. The production of sebum is controlled by hormones.

Among humans there is a great variety in skin color. A pigment called melanin, which is produced in the lower layers of the epidermis and which absorbs harmful ultraviolet light, helps determine the color of the skin. If a large amount of melanin is produced, the skin will be dark; if a small amount is produced, the skin will be light. In some individuals exposure to sunlight temporarily increases the production of melanin, thus causing the skin to become darker, called a tan. People with fair skin, which cannot produce melanin, need to minimize their exposure to the sun to protect against sunburn.

C. Discuss diseases and dysfuntions related to each system.

The SKIN is subject to many disorders. They occur either through the direct action of external agents, or as symptoms of disease in other parts of the body. Susceptibility to skin disorders is always modified and sometimes determined by genetic factors.

The appearance of abnormal changes, or lesions, in the skin is an important element in diagnosis and treatment. Various lesions can occur, not all of them disease related. A flat lesion of a color differing from surrounding skin is called a FRECKLE. A papule is a solid, elevated but superficial mass, such as a raised MOLE or a WART. A wheal is a transitory lesion resulting from an ALLERGY response, such as to an insect bite. A nodule is a solid mass that extends deeper, such as in certain TUMORS and CYSTS. A vesicle is a tiny blister filled with clear fluid, such as in early chicken pox. A bulla is a large blister, most often seen as the result of a BURN. A pustule is an elevated skin area containing pus, as in the later chicken pox lesion. PURPURA is a skin discoloration caused by deposits of blood or blood pigments within the skin.

The appearance of such lesions may be modified by secondary changes. These include scales or flakes of dead skin, as seen in PSORIASIS and DANDRUFF; crusts, typically dried masses of material that have oozed out, as in IMPETIGO; fissures, or sharp breaks, as seen in athlete's foot; ULCERS, or destruction of one or more skin layers, exposing underlying tissue; SCARS, resulting from newly formed CONNECTIVE TISSUE replacing lost tissue; and lichenification, a thickened scaly area in which normal skin creases and lines are exaggerated. Any one of these lesions may be seen in several different diseases, and any one disease may have several different lesions.

Any metastasizing tumor from elsewhere in the body may appear in the skin. Among the most common sources are breast carcinomas, LEUKEMIAS, and related diseases. Many malignant tumors also originate in the skin. Four should be mentioned. Basal cell carcinomas may originate from basal cells of the epidermis. It almost never metastasizes, although in time it may invade local tissues. It is easily cured. Squamous cell carcinoma, which may originate from the layer of flattened epidermal cells, does metastasize. Both tumors are common, especially in the elderly and in those overexposed to the Sun. Malignant melanoma derives from the same embryonic nervous tissue that produces moles. It appears to be more common in those overexposed to the Sun in childhood, and it metastasizes early and widely. Finally, the once-rare tumor called Kaposi's sarcoma has become more frequent because it is a major complication of the disease known as AIDS.

Skin diseases that result from chemical and physical agents as well as infectious organisms include contact dermatitis, sunburn, heat rash, and bunions, corns, and calluses. Contact DERMATITIS is characterized by itching and burning sensations and reddening, blistering, "weeping," and crusting of areas of the skin. It is an acute or chronic skin inflammation caused by skin contact with irritants such as chemicals or poison ivy or with substances to which an individual is allergic. Ultraviolet light causes erythema solare, or sunburn, signs of which can range from simple reddening to blistering. The symptoms can range from pain in the skin to gastrointestinal disorders, malaise, and prostration. Susceptibility is greatest in persons with very fair skin.

Miliaria (heat rash or prickly heat) is frequently seen in infants and obese adults. It is characterized by burning and itching and the aggregation of small blisters on covered areas of the skin during hot, humid weather. The blisters are caused by plugging of the ducts of sweat glands.

Bunions, corns, and calluses on the feet or toes result from pressure and friction due to body deformities that affect the distribution of weight on the feet, or from poorly fitting shoes. Some individuals have a genetic predisposition to excess callus formation at points of friction.

When the skin is exposed to heat, electricity, radiation, or chemicals, it may also suffer a burn. Depending on the severity of the burn--that is, the depth of the skin layers destroyed and the area affected--a skin graft may be required. When skin grafts from the patient (autologous grafts) are not possible in sufficient amounts, grafts from other humans or pigs or synthetic artificial skin can be used as temporary measures to reduce the loss of fluid and protein from the large burn. High-energy radiation can produce skin cancers.

ACNE, or acne vulgaris, the most common skin disease, is a skin condition thought to be secondary to hormonal changes of adolescence. Evidence exists that it is due to endocrine changes that occur at puberty in individuals who are genetically predisposed to the disease. Males are more often affected than females, and the condition is not confined to adolescence. Folliculitis, on the other hand, is caused by infection of hair follicles by staphylococci. The condition is sometimes called barber's itch because individuals with beards tend to have deeper hair follicles, that are more likely to be affected by bacteria. Furuncles (BOILS) and CARBUNCLES are deeper and more serious infections of hair follicles, resulting in abscess formation. They can cause severe pain and can give rise to serious and sometimes fatal secondary infections of internal organs.

Streptococcal pyoderma, or IMPETIGO, and erysipelas are skin infections caused by the entry of certain strains of streptococcal bacteria into breaks in the skin. Streptococcal pyoderma is characterized by the formation of localized crusty lesions. The lesions in erysipelas, a much more serious disease, are usually widely spreading zones of redness, swelling, and blistering in the skin. Erysipelas is accompanied by systemic effects, including chills and fever. Both diseases can be contagious, especially when highly virulent strains of streptococci are involved. Certain forms of erysipelas can rapidly involve large areas of the skin and can cause prostration and death from the spreading of toxic substances through the body.

Ringworm of the scalp (tinea capitis), of the body (tinea corporis), and of the groin (tinea cruris, or "jock itch") are fungal infections of the skin. The term ringworm arises from the circular form of the lesions. Dermatophytosis of the foot, or "athlete's foot," is also caused by a fungus. Fungal infections are often accompanied by itching, burning, stinging, and seepage of tissue fluids at the site of infection.

Scabies and pediculosis are common parasitic infections. Scabies is caused by mites that burrow into the skin; the mites' excretion of waste products into the skin causes intense itching, especially at night. Adults often acquire scabies by sleeping with or having sexual relations with an infected person. Pediculosis is a louse infestation of the scalp, trunk, or pubic area ("crabs"). Each of the three kinds of pediculosis is caused by a different species of louse. Pubic lice can also be acquired from infested bedding, clothing, towels, and even toilet seats.

The most common viral infections of skin are herpes simplex (cold sore or fever sore) and herpes zoster (shingles). Herpes simplex sores usually occur near the mouth and genital areas and tend to occur repeatedly in the same location, often after fever, sunburn, menstruation, fatigue, or nervous tension. Herpes zoster is a painful infection of nerves in the skin by the same virus that causes chicken pox. It is often accompanied by skin eruptions along the course of the nerve.

Warts are another form of viral infection, common in teenage children but less common in younger children and in adults. Several types of warts occur on different parts of the body, each one caused by a particular type of virus. The most common warts are small, horny lumps that can grow anywhere on the skin but most typically appear on the hands. In most cases warts disappear after a few months or years; they can also be removed by a number of techniques. Warts that appear on older persons, however, may be more serious skin conditions.