The skin is our largest organ. If the skin of a typical 150-pound (68-kilogram) adult male were stretched out flat, it would cover about 2 square yards (1.7 square meters) and weigh about 9 pounds (4 kilograms). Our skin protects the network of muscles, bones, nerves, blood vessels, and everything else inside our bodies. Our eyelids have the thinnest skin, the soles of our feet the thickest.

Hair is actually a modified type of skin. Hair grows everywhere on the human body except the palms of the hands, soles of the feet, eyelids, and lips. Hair grows more quickly in summer than winter, and more slowly at night than during the day.

Like hair, nails are a type of modified skin - and they're not just for beauty. Nails protect the sensitive tips of our fingers and toes. Human nails are not necessary for living, but they do provide support for the tips of the fingers and toes, protect them from injury, and aid in picking up small objects. Without them, we'd have a hard time scratching an itch or untying a knot. Nails can be an indicator of a person's general health, and illness often affects their growth.

What Is the Skin and What Does It Do?
Skin is essential to a person's survival. It forms a barrier that prevents harmful substances and microorganisms from entering the body. It protects body tissues against injury. Our skin also controls the loss of life-sustaining fluids like blood and water, helps us regulate body temperature through perspiration, and protects us from the sun's damaging ultraviolet rays. Without the nerve cells in our skin, we couldn't feel warmth, cold, or other sensations. Our skin can also respond to situations and emotions: Muscles in the skin called erector pili contract to make the hairs on our skin stand up straight when we are cold or frightened.

Every square inch of skin contains thousands of cells and hundreds of sweat glands, oil glands, nerve endings, and blood vessels. Skin is made up of three layers: the epidermis (pronounced: eh-puh-dur-mis), dermis, and the subcutaneous (pronounced: sub-kyoo-tay-nee-us) tissue.

The upper layer of our skin, the epidermis, is the tough, protective outer layer. It is about as thick as a sheet of paper over most parts of the body. The epidermis has four layers of cells that are constantly flaking off and being renewed. In these four layers are three special types of cells:

• Melanocytes (pronounced: meh-lah-nuh-sites) produce melanin, the pigment that gives skin its color. All people have roughly the same number of melanocytes; those of dark-skinned people produce more melanin. Exposure to sunlight increases the production of melanin, which is why people get suntanned or freckled.
• Keratinocytes (pronounced: ker-uh-tih-no-sites) produce keratin, a type of protein that is a basic component of hair and nails.
• Langerhans (pronounced: lahng-ur-hanz) cells help protect the body against infection.

Because the cells in the epidermis are completely replaced about every 28 days, cuts and bruises heal quickly.

Below the epidermis is the next layer of our skin, the dermis, which is made up of blood vessels, nerve endings, and connective tissue. The dermis nourishes the epidermis.

Without certain molecules in the dermis, our skin wouldn't stretch when we bend or reposition itself when we straighten up. These two types of molecules, collagen (pronounced: kah-luh-jen) and elastin (pronounced: ih-las-tin), combine in fibers in the dermis to give us our ease of movement. Collagen is strong and hard to stretch and elastin, as its name suggests, is elastic. In older people, some of the elastin-containing fibers disappear, which is one reason why the skin looks wrinkled.

The dermis also contains a person's sebaceous glands. These glands, which surround and empty into our hair follicles and pores, produce an oil called sebum (pronounced: see-bum) that lubricates the skin and hair. Sebaceous glands are found mostly in the skin on the face, upper back, shoulders, and chest.

Most of the time, the sebaceous glands make the right amount of sebum. As a person's body begins to mature and develop during the teenage years, though, hormones stimulate the sebaceous glands to make more sebum. This causes acne when pores become clogged by too much sebum and too many dead skin cells. Later in life, these glands produce less sebum, which contributes to dry skin.

The bottom layer of our skin, the subcutaneous tissue, is made up of connective tissue, sweat glands, blood vessels, and cells that store fat. This layer helps protect the body from blows and other injuries and helps it hold in body heat.

There are two types of sweat-producing glands. The eccrine (pronounced: eh-krun) glands are found everywhere in our bodies, although they are mostly in the forehead, palms, and soles of the feet. By producing sweat, these glands help regulate body temperature, and waste products are excreted through them.

The other type of sweat-producing gland, the apocrine glands, develop at puberty and are concentrated in the armpits and pubic region. The sweat from the apocrine glands is thicker than that produced by the eccrine glands. Although this sweat doesn't smell, when it mixes with bacteria on the skin's surface, it can cause body odor. A normal, healthy adult secretes about 1 pint (about half a liter) of sweat daily, but this may be increased by physical activity, fever, or a hot environment.

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