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Covering the entire outside of the body, the skin is the body’s largest organ. It also grows faster than any other organ in the body. People continuously renew their skin throughout their lifetime.

In a 150 pound (68 kilogram [kg]) person, the skin measures approximately 2 square yards (1.7 square meters) and weighs about 9 pounds (4 kg). The skin’s average thickness is around a tenth of an inch (0.3 centimeters). The thickest skin is located on the upper back, the palms of the hands and the soles of the feet. The thinnest sections of skin are on the eyelids and the inner folds of the elbows. 

Hair and nails are modified versions of skin. Hair maintains warmth in the body by preserving heat. Hair located around the eyes and in the nose and ears protects these areas from small particles such as dust. It also cushions the body against injury.

Hair is composed of a hair shaft, which projects from the skin’s surface, and a root. Embedded in the skin, the root is a soft thickened bulb located at the base of the hair. Each root ends in a hair bulb, which sits in a tubular sac known as a hair follicle. There are more than 100,000 hair follicles located on the head alone. At the bottom of each hair follicle is a papilla, the area from which the hair sprouts. It contains an artery that nourishes the root of the hair.

The hair grows in response to new cells forming at the base of the root. As the cells multiply they form a rod of tissue in the skin. The rod of cells then moves upward through the skin as new cells form under it.  As the cells move up, they are cut off from their supply of nourishment. This causes the cells to die. The dead cells and keratin (a hard protein formed from the cells) form the shaft of the hair. When a hair grows and falls out, another hair takes its place.

Hairs grow approximately half an inch (about 12.7 millimeters) each month, and continue to grow for about six years. The length of a hair is related to the growing phase of its follicle. Hair follicles are active for two to six years, followed by a rest period of about three months. Typically, hair grows more rapidly in the summer than in the winter. It also grows more quickly during the day than at night.

The size of a person’s hair follicles determines whether they have thick or thin hair. Large follicles produce thick hair, and narrow follicles generate thin hair. When these follicles become damaged or die they no longer produce hair, resulting in baldness.

Hair color varies from person to person. It is determined by the amount of melanin (dark pigment) in each hair. People with light hair, such as blondes and redheads, have only a small amount of melanin in their hair while people with black or dark brown hair have a larger amount. As people age, their hair becomes grey because pigment no longer forms in the hair.

Nails sprout from deep folds in the fingers and toes. They provide protection and support for the sensitive fingertips and toes. They also provide insight into a person’s general health, as illness can impact the appearance and growth rate of the nails.

As cells below the nail root move up to the skin’s surface, the cells located closest to the nail root flatten out and become tightly pressed together. The cells transform into thin plates, which stack into layers to form the nail. As these cells accumulate, the nail is gradually pushed forward.

The skin located beneath the nail is known as the matrix. Nails grow from the matrix, and if the matrix is damaged, the nail will grow ridged. The larger pink part of the nail is known as the nail plate. Its pink appearance is due to the network of blood vessels in the underlying layer of skin. The white, crescent shaped area at the base of the nail is known as the lunula. A nail that is torn will regrow, as long as its matrix is not badly injured. On average, it takes nine to 12 months for fingernails to grow and 12 to 18 months for toenails to grow. As with hair, nails typically grow more rapidly in the summer than in the winter.

The skin is the body’s natural protective covering. Its external location, however, makes it vulnerable to a variety of diseases, disorders, discolorations and growths. The majority of skin problems can be treated by a dermatologist with a minimal amount of pain and risk. Dermatologists are physicians that specialize in diagnosing and treating conditions of the skin, mouth, hair, nails and external genitalia. People with skin problems such as rashes, itchiness (pruritus) or growths should seek medical treatment from a dermatologist soon after the condition develops.

The skin is susceptible to damage from the environment. To minimize external damage to the skin, people should take steps to protect their skin. This involves wearing hats, sunscreen, long-sleeved shirts and pants when spending time in the sun.  

In addition, the natural aging process of the skin is damaging because the dermis loses moisture, elasticity and thickness as individuals grow older. It is impossible to prevent all of the changes that occur to the skin as a person ages. However, older individuals can take steps to slow the pace of these changes, or to treat symptoms that result from these changes. Avoiding sun exposure, moisturizing the skin, maintaining a healthy diet and consuming adequate fluids can be very beneficial.