The majority of skin is water-proof because of keratin, a fibrous protein.
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The human body's largest organ is the integumentary system, which includes the skin, hair, nails, glands and nerve receptors.
Skin weighs about 6 lbs. (2.7 kilograms) and sheds itself about once every 27 days, according to the Cleveland Clinic. In adults, skin covers a surface area of 18 square feet (1.7 square meters), according to the University of Pennsylvania Health System (Penn Medicine).
How the integumentary system works
The integumentary system works to waterproof, cushion and protect the body from infection, according to the National Institutes of Health. Most skin is waterproof because of keratin, a fibrous protein, and it also is made up of water, other proteins, lipids and different minerals and chemicals.
Skin 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 the 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, according to the American Academy of Dermatology (AAD). 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, according to the AAD.
Three layers of tissue
Human skin is composed of three layers of tissue: the epidermis, dermis and hypodermis, according to the Cleveland Clinic.
The epidermis is the top layer of skin and does not contain blood vessels. While it is only about one-tenth of a millimeter thick, the epidermis is made of 40 to 50 rows of stacked cells called squamous cells or keratinocytes, according to the Cleveland Clinic.
Keratinocytes produce keratin, a fibrous, waterproofing protein. The majority of the skin on the body is keratinized, meaning it's waterproof, with the exception of the lining of skin on the inside of the mouth, according to the Cleveland Clinic. Keratin is also a key component of hair and nails.
The epidermis also consists of melanocytes, which produce melanin, the dark pigment that gives skin its color; Merkel cells, which are thought to be involved in touch reception; and Langerhans cells, which help the immune system fight antigens (foreign bodies), according to Penn Medicine.
Only the deepest layer of the epidermis receives nourishment from the layer beneath it. Cells that are pushed away from this layer eventually die. When they reach the skin surface, they are sloughed off or shed. The skin sheds millions of dead keratinocytes every day.
The dermis is the middle layer of skin, and it actually has two layers, according to the AAD. The papillary layer consists of the loose connective tissue, while the reticular layer is the deep layer of the dermis and consists of dense 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, according to the AAD. The structures for hair 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 help ward off infections and other foreign bodies. The dermis, the AAD noted, 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 insulate the body and cushion internal organs, the Cleveland Clinic noted. The hypodermis is composed of connective tissue called adipose tissue, which 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 conditions such as acne and warts; chronic skin conditions such as eczema and psoriasis; and more serious diseases like skin cancer, according to the AAD. 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, the AAD noted.
There are three main types of skin cancer, the most common of which is basal cell carcinoma, said Dr. Charles E. Crutchfield III, a clinical professor of dermatology at the University of Minnesota Medical School, and medical director of Crutchfield Dermatology. More than 2 million cases of this skin cancer are diagnosed in the United States every year, according to the AAD. This type of cancer is skin colored or has a slight pearl color to it. It rarely metastasizes (i.e., it rarely spreads to other parts of the body), but it can be very problematic if it's not treated, Crutchfield cautioned. According to the AAD, basal cell carcinoma can destroy skin tissue and bone.
The second most common skin cancer is squamous cell carcinoma. This is a rough-surfaced skin-colored lesion. Squamous cell skin cancer causes death in about 10 percent of affected patients.
The most serious skin cancer is melanoma, which looks like a dark, changing, bleeding skin spot, Crutchfield said. Melanoma is fatal in as many as 35 percent of patients diagnosed with this form of skin cancer.
Warts are rough bumps caused by a viral infection. They commonly occur on the hands and feet. Sometimes, tiny black dots will be visible in a wart, Crutchfield noted. "Those are blocked blood vessels, which is a common occurrence with a papilloma viral infection." The best treatment for warts is to cause a mild irritation of the wart — usually by freezing, liquid irritation and lasers — so the immune system can recognize the viral infection and get rid of it.
Also known as dermatitis, eczema looks like red, itchy, flaky skin. It can occur anywhere. Sometimes, it happens by itself, and other times, it is caused by outside factors such as poison ivy, according to Crutchfield. Dermatitis is best treated with topical anti-inflammatory creams and ointments. For mild cases, over-the-counter medications work well, but a prescription may be required for more severe cases.
Acne, a disorder of the hair and oil glands, is among the most common conditions treated by dermatologists, Crutchfield said. "It is under the control of hormonal changes, hence the initial flare during adolescence," he said.
Acne presents itself as red bumps and pimples on the face, chest and back, Crutchfield said. Treatments include vitamin A products (retinols), salicylic acid (to unplug pores), benzoyl peroxides (to decrease bacteria) and antibiotics (to reduce inflammation).
Vitiligo is a condition in which the cells that produce skin color — melanocytes — no longer function properly. Some are attacked by the immune system. Sometimes, the cells mysteriously die or stop working, Crutchfield noted. "When this happens, the person with vitiligo can get multiple patches of white skin. One of the best treatments for vitiligo is specialized light treatment from a doctor."
Psoriasis is an inflammatory skin condition in which red, itchy plaques commonly occur on the knees and elbows. Crutchfield explained that the nails can have pits and the scalp can be red and itchy, flaky and inflamed. "About 3 percent of all people have psoriasis to some degree, and it does run in families many times," he noted. Some patients with psoriasis can also develop arthritis, called psoriatic arthritis. The best treatment for psoriasis are topical medicines, light treatments and, in severe cases, systemic prescription medicines.
Moles are normal parts of the skin. Moles can be flat or raised, and they can be red, brown, black or skin-colored. If a mole start changing — in size, color or shape, or if it bleeds and doesn't heal on its own in three weeks — it should be evaluated by a doctor to make sure it is not turning into a skin cancer, Crutchfield said.
Other systems of the body:
- Circulatory System
- Digestive System
- Endocrine System
- Immune System
- Lymphatic System
- Muscular System
- Nervous System
- Reproductive System
- Respiratory System
- Skeletal System
- Urinary System