Skin, Hair, and Nails

About Skin, Hair and Nails

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. Nails protect the sensitive tips of fingers and toes. Human nails aren't 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.

Skin Basics

Skin is essential in many ways. It forms a barrier that prevents harmful substances and microorganisms from entering the body. It protects body tissues against injury. It also controls the loss of life-sustaining fluids like blood and water, helps regulate body temperature through perspiration, and protects from the sun's damaging ultraviolet rays.

Without the nerve cells in skin, people couldn't feel warmth, cold, or other sensations. For instance, goosebumps form when the erector pili muscles contract to make hairs on the skin stand up straight when someone is cold or frightened — the blood vessels keep the body from losing heat by narrowing as much as possible and keeping the warm blood away from the skin's surface, offering insulation and protection.

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, dermis, and the subcutaneous tissue.

The upper layer of our skin, the epidermis, is the tough, protective outer layer. It's 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 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 produce keratin, a type of protein that is a basic component of hair, skin, nails, and helps create an intact barrier.
• Langerhans 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 next layer of our skin, the dermis, which is made up of blood vessels, nerve endings, and connective tissue. The dermis nourishes the epidermis. Two types of fibers in the dermis — collagen and elastin — help the skin stretch when we bend and reposition itself when we straighten up. Collagen is strong and hard to stretch, and elastin, as its name suggests, is elastic. In older people, some of the elastin-containing fibers degenerate, which is one reason why the skin looks wrinkled.