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What Is Skin?

Skin is the body's largest organ. Every inch of a person’s skin is made of millions of cells. Inside the skin are glands, nerve endings, and blood vessels.

What Does Skin Do?

Skin does many important jobs. It:

  • protects the organs inside our bodies
  • forms a barrier that prevents harmful substances and germs from entering the body
  • cushions body tissues against injury
  • helps control body temperature (through sweating when we’re hot and by keeping heat inside the body when we’re cold)
  • prevents water that's inside our bodies from evaporating and keeps extra water out (so we don’t absorb water like a sponge when we bathe or swim)

The nerve endings in the skin let us feel warmth, cold, pain, and other sensations.

What Are the Parts of Skin?

Skin is made of three layers: the epidermis (ep-ih-DUR-mis), dermis (DUR-mis), and the subcutaneous (sub-kyoo-TAY-nee-us) tissue. The layers work together to allow the skin to do its jobs.

What Happens in the Epidermis?

The epidermis is the tough, protective outer layer of skin that we can see. It is thin in some areas, like the eyelids, and thicker in areas that need more protection, like the soles of the feet.

The epidermis constantly makes new cells. Cells form at the bottom and work their way up to the surface of the skin, where they flake off. This process takes about 28 days.

The epidermis has three main types of cells:

  • Melanocytes (meh-LAN-eh-sites) make a pigment called melanin (MEL-eh-nen) that gives skin its color. People with darker skin have more melanin and those with lighter skin have less melanin. Spending time in the sunlight increases the production of melanin. That’s why people can get darker or freckled after spending time in the sun.
  • Keratinocytes (ker-AT-eh-neh-sites) make keratin, a type of protein that's in hair, skin, and nails. Keratin helps create a protective barrier against injury and germs.
  • Langerhans (LAHNG-ur-hanz) cells help protect the body against infection.

What Happens in the Dermis?

The middle layer of skin is the dermis. It’s made of two types of fibers — collagen and elastin — that help skin stretch and stay firm. The dermis contains blood vessels, nerve endings, sweat glands, sebaceous glands, and hair follicles. Each of these have different jobs:

  • Blood vessels in the dermis provide nutrients to the bottom of the epidermis so that new cells can be made there.
  • Nerve endings let us sense pain, temperature, and pressure when our skin touches things.
  • Sweat glands play an important role in controlling our body’s temperature.
  • Hair follicles are where strands of hair grow from.
  • Sebaceous (sih-BAY-shiss) glands make the oil sebum (SEE-bum), which softens the skin and makes it waterproof.

What Happens in the Subcutaneous Tissue?

The deepest layer of skin is the subcutaneous tissue. This layer stores fat. It helps cushion the body from injuries and keeps the body warm.