Hair, Skin, and Nails

Hair, skin, and nails form the outer layer of the body, called the integumentary (in-teh-gyoo-MEN-tah-ree) system.

Hair

What Is Hair and What Does It Do?

Hairs are thin strands of hardened protein packed into layers. Hair has many different jobs.

Hair on the head protects us from the sun and keeps us warm by preserving heat. Hair in the nose, ears, and around the eyes keeps these sensitive areas safe from dust and other small particles. Eyebrows and eyelashes shield the eyes from bright light and small particles that could go into them. The fine hair that covers the body provides warmth and protects the skin.

What Is Hair Made Of?

Hair is made mostly of a hard protein called keratin, which is also in your nails and the outer layer of your skin. Hair doesn’t have nerves or blood, so when you cut it, it doesn’t hurt. It gets its color from a pigment called melanin, which also gives your skin its color.

What Are the Parts of Hair?

Human hair includes the:

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

The shape of the follicle determines whether the hair is curly or straight. At the bottom of the follicle is the papilla (puh-PILL-uh), where the actual hair growth happens. The papilla contains an artery — a kind of blood vessel — that nourishes the root of the hair. As cells multiply and make keratin, they're pushed up the follicle and through the skin's surface as a shaft of hair.

Each hair has three layers:

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

How Does Hair Grow?

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 keratin. This process is called keratinization (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 on the palms of the hands, soles of the feet, and lips. Hair grows faster in summer than winter.

Skin

What Is Skin and What Does It 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 Is Skin Made Of?

Skin is made of proteins and other materials that help protect your body. One important protein is keratin, which makes the outer layer of your skin strong and waterproof. Deeper in the skin, there’s collagen, a stretchy protein that keeps your skin firm and helps it bounce back when you move.

Another protein called elastin works like a rubber band, letting your skin stretch and then return to its shape. Skin also has water, fats, and cells that help it stay soft, heal cuts, and keep germs out.

What Are the Layers of Skin?

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

Epidermis

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 kinds of cells:

Melanocytes (meh-LAH-nuh-sites) make melanin. All people have roughly the same number of melanocytes, but some people make more melanin than others. The more melanin the body produces, the darker the skin. Exposure to sunlight causes the body to make more melanin, which is why people get suntanned or freckled.
Keratinocytes (ker-uh-TIH-no-sites) make keratin. Keratin in the skin's outer layer helps create a protective barrier.
Langerhans (LAHNG-ur-hanz) help protect the body against infection.

The cells in the epidermis are completely replaced about every 28 days. This means minor injuries like cuts and scrapes can heal quickly, since new cells are already on their way to the surface to replace damaged ones.

Dermis

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 (sih-BAY-shiss) glands. These glands make the oil sebum (SEE-bum), which softens the skin and makes it waterproof.

Subcutaneous

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

Nails

What Are Nails and Why Do We Have Them?

Nails protect the sensitive tips of fingers and toes. We don't need our nails to survive, but they do help prevent injuries, and they also help us pick up small objects. Without them, we'd have a hard time scratching an itch or untying a knot.

Nails can be a guage of a person's general health, as different illnesses can affect their growth.

What Are Nails Made Of?

Nails are mostly made of a strong protein called keratin, which helps give them their hard, protective surface. Keratin in nails is packed tightly into layers, making nails tough enough to protect your fingertips.

Nails also contain water and small amounts of calcium and other minerals that help keep them healthy. Underneath the nail, there are living cells that grow and push the nail forward.

What Are the Parts of Nails?

Nails grow out of deep folds in the skin of the fingers and toes. As epidermal cells (cells from the epidermis) 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 gather, 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 (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.