What Does Skin Do?

Skin, our largest organ, has many jobs. It:

• protects the network of muscles, bones, nerves, blood vessels, and everything else inside our bodies
• forms a barrier that prevents harmful substances and germs from entering the body
• protects body tissues against injury
• helps control body temperature through sweating when we're hot and by helping keep heat in the body when we're cold

Without the nerve cells in skin, people couldn't feel warmth, cold, or other sensations.

Every square inch of skin contains thousands of cells and hundreds of sweat glands, oil glands, nerve endings, and blood vessels.

What Are the Parts of Skin?

Skin has three layers: the epidermis (pronounced: ep-ih-DUR-mis), dermis (pronounced: DUR-mis), and the subcutaneous (pronounced: sub-kyoo-TAY-nee-us) tissue.

The epidermis is the upper layer of skin. This tough, protective outer layer is thin in some areas and thick in others. The epidermis has layers of cells that constantly flake off and are renewed. In these layers are three special types of cells:

• Melanocytes (pronounced: meh-LAH-nuh-sites) make melanin, the pigment that gives skin its color. All people have roughly the same number of melanocytes; the more melanin made, the darker the skin. Exposure to sunlight increases the production of melanin, which is why people get suntanned or freckled.
• Keratinocytes (pronounced: ker-uh-TIH-no-sites) make keratin, a type of protein that's a basic component of hair, skin, and nails. Keratin in the skin's outer layer helps create a protective barrier.
• 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 scrapes heal quickly.

Below the epidermis is the dermis. This is where our blood vessels, nerve endings, sweat glands, and hair follicles are. The dermis nourishes the epidermis. Two types of fibers in the dermis — collagen and elastin — help skin stretch and stay firm.

The dermis also contains a person's sebaceous (pronounced: sih-BAY-shiss) glands. These glands make the oil sebum (pronounced: SEE-bum), which softens the skin and makes it waterproof.

The bottom layer of skin is the subcutaneous (pronounced: sub-kyuh-TAY-nee-iss) tissue. It's made of , blood vessels, and cells that store fat. This layer helps protect the body from blows and other injuries and helps hold in body heat.

What Does Hair Do?

The hair on our heads doesn't just look nice. It keeps us warm by preserving heat.

Hair in the nose, ears, and around the eyes protects these sensitive areas from dust and other small particles. Eyebrows and eyelashes protect eyes by decreasing the amount of light and particles that go into them.

The fine hair that covers the body provides warmth and protects the skin.

What Are the Parts of Hair?

Human hair consists of:

• the hair shaft, the part that sticks out from the skin's surface
• the root, a soft thickened bulb at the base of the hair
• the follicle (pronounced: FAHL-ih-kul), a sac-like pit in the skin from which the hair grows

At the bottom of the follicle is the papilla (pronounced: puh-PILL-uh), where the actual hair growth happens. The papilla contains an artery that nourishes the root of the hair. As cells multiply and make keratin to harden the structure, they're pushed up the follicle and through the skin's surface as a shaft of hair.

Each hair has three layers:

1. the medulla (pronounced: meh-DULL-uh) at the center, which is soft
2. the cortex, which surrounds the medulla and is the main part of the hair
3. the cuticle (pronounced: KYOO-tuh-kull), the hard outer layer that protects the shaft

Hair grows by forming new cells at the base of the root. These cells multiply to form a rod of tissue in the skin. The rods of cells move upward through the skin as new cells form beneath them. As they move up, they're cut off from their supply of nourishment and start to form a hard protein called keratin. This process is called keratinization (pronounced: ker-uh-tuh-nuh-ZAY-shun). As this happens, the hair cells die. The dead cells and keratin form the shaft of the hair.

Hair grows all over the human body except the palms of the hands, soles of the feet, and lips. Hair grows faster in summer than winter, and slower at night than during the day.

What Do Nails Do?

Nails protect the sensitive tips of fingers and toes. We don't need our nails to survive, but they do support the tips of our fingers and toes, protect them from injury, and help us pick 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 Are the Parts of Nails?

Nails grow out of deep folds in the skin of the fingers and toes. As epidermal cells below the nail root move up to the surface of the skin, they increase in number. Those closest to the nail root get flat and pressed tightly together. Each cell becomes a thin plate; these plates pile into layers to form the nail.

As with hair, nails form by keratinization. When the nail cells accumulate, the nail pushes forward.

The skin below the nail is the matrix. The larger part of the nail, the nail plate, looks pink because of the network of tiny blood vessels in the underlying dermis. The whitish crescent-shaped area at the base of the nail is the lunula (pronounced: LOON-yuh-luh).

Fingernails grow faster than toenails. Like hair, nails grow faster in summer than in winter. A nail that's torn off will regrow if the matrix isn't severely injured.