Skin: Facts, Diseases & Conditions

While it may not immediately come to mind when asked to name the body’s major organs, the integumentary system, or skin, is its largest organ. It comprises the skin as well as hair and nails, which are appendages of the skin. In humans, this system accounts for about 15 percent of total body weight.

The majority of skin is water-proof because of keratin, a fibrous protein.
Credit: PanicAttack | Shutterstock

Description of the integumentary system

The integumentary system works to waterproof, cushion, and protect the body from infection. It also excretes wastes, regulates temperature and prevents dehydration by controlling the level of perspiration. It also houses sensory receptors that detect pain, sensation and pressure.

The skin is also body's initial defense against bacteria, viruses and other microbes. Skin and hair provide protection from harmful ultraviolet radiation and the skin guards against sunburn by secreting melanin. Human skin color is determined by the interaction of melanin, carotene, and hemoglobin.

Storage of water, fat, glucose and vitamin D is also a function of the integumentary system.

layers of skin
Skin is composed of several layers of tissue.
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Three layers of tissue

The human skin is composed of three layers of tissue: the epidermis; dermis; and hypodermis.

The epidermis is the top layer of skin and does not contain blood vessels. This layer consists mostly of keratinocytes, or basal cells, as well as melanocytes, Merkel cells, and Langerhans' cells. While it is only about one-tenth of a millimeter thick, the epidermis is made of 40 to 50 rows of stacked squamous cells.

Keratinocytes produce keratin, a fibrous, water-proofing protein. The majority of the skin on the body is keratinized, meaning waterproofed, with the exception of the lining of skin on the inside of the mouth. Keratin is also a key component of hair and nails. The skin sheds millions of dead keratinocytes every day.

The dermis is the middle layer of skin and it actually has two layers. The Papillary layer, which consists of the areolar connective tissue, and the Reticular layer, which is the deep layer of the dermis and consists of the dense irregular connective tissue. These layers provide elasticity, allowing for stretching while also working to fight wrinkling and sagging.

The dermal layer provides a site for the endings of blood vessels and nerves. The structures for hair in humans and feathers in birds are in this layer of skin.

Lymph vessels, which supply the clear fluid containing white blood cells of the immune system, are also housed in this layer to skin tissue to help ward off infections and other foreign bodies. The dermis is also home to the sweat glands and oil glands, which are attached to hair follicles.

The hypodermis — also called subcutaneous tissue — is the deepest layer of the skin. It helps to insulate the body and cushion internal organs. The hypodermis is composed of a connective tissue called adipose tissue that stores excess energy as fat. Blood vessels, lymph vessels, nerves and hair follicles also run through this layer of skin.

Diseases of the integumentary system

Dermatologists specialize in treating diseases, disorders and injuries of the skin, hair, and nails. They treat common diseases that many people get, such as acne and warts, as well as skin cancer and chronic skin conditions A residency in dermatology involves one year as an intern in either surgery or internal medicine, followed by a three-year residency.

After this, many dermatologists pursue further training through one- or two-year fellowships in specialized fields such as cosmetic surgery, laser medicine or immunodermatology.

Acne is a skin disease marked by pimples on the face, chest, and back and is the result of increased levels of androgens (male hormones) that cause oil glands to oversecrete.

Alopecia areata is an autoimmune skin disease that causes the body to attack the hair follicles, resulting in baldness.

Athlete’s foot is a common fungus infection in which the skin between the toes becomes itchy and cracked. It is caused by an infection that is the result of fungi that grow well in warm, damp areas such showers and locker rooms

Burns cans be caused by intense heat, electricity, UV radiation or certain chemicals such as acids).

Dermatitis is any inflammation of the skin. The two common types are contact dermatitis, which is often a quick and severe reaction to something touching the skin such as poison ivy, and seborrheic dermatitis, which comes on more gradually and typically affects the scalp.

Psoriasis is a chronic skin disease characterized by inflamed lesions with silvery-white scabs. Psoriasis causes skin cells to mature much more quickly than normal, causing a backup of cells on the skin surface.

Skin cancer, which is the growth of abnormal skin cells, is the most common type of cancer in humans. In basal cell carcinoma — the most common form of skin cancer — basal cells in the epidermis are damaged and stop producing keratin. Squamous cell carcinoma affects the cells of the second deepest layer of the epidermis. Like basal cell carcinoma, this type of skin cancer also involves skin exposed to the sun: face, ears, hands, and arms.

Malignant melanoma, the most serious type of skin cancer, affects the melanocytes cells in the lower epidermis that produce melanin.

Infographic: Details of the integumentary system, incuding skin, hair and nails


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