Integumentary system: skin

Your skin is your body's largest organ. In an adult it covers about 2 square meters and weighs several kilograms. It is the first thing the world touches, and it does far more than hold you together. The skin, along with hair and nails, makes up the integumentary system.

Skin has two main layers, with a third layer of tissue below. The epidermis is the thin outer layer. Its top cells are dead, flat, and filled with keratin, a tough protein that makes skin waterproof and hard to damage. Deep in the epidermis, new cells are constantly made and pushed upward, so you replace your outer skin about every month. Cells called melanocytes make melanin, the pigment that gives skin its color and absorbs some harmful ultraviolet (UV) light. Below it lies the dermis. It is thicker and holds blood vessels, nerve endings, sweat glands, oil glands, and hair follicles. Beneath the skin, the hypodermis is a layer of fat and connective tissue that cushions you and holds in heat.

Skin has four main jobs. It protects: it blocks germs, keeps water in, and shields against UV light. It helps regulate temperature: sweat cools you as it evaporates, and blood vessels in the dermis widen to release heat or narrow to hold it in. It senses: nerve endings detect touch, pressure, pain, heat, and cold. Fingertips have many more of these endings than your back, so they are far more sensitive. And skin makes vitamin D when sunlight strikes it, a vitamin your body needs to absorb calcium for strong bones.

The ridges on your fingertips form before birth, and no two people have the same fingerprints, not even identical twins. When skin is cut, blood forms a clot, and new epidermal cells grow in to close the wound. Deep burns that destroy the dermis can damage nerve endings and glands, which is why they heal slowly and may leave scars. Sunscreen, protective clothing, and cleaning cuts all help your skin do its protective job.

A common mistake is to think skin is only a covering. It is a living organ with several tissues and many jobs. Another is to think the dead cells on the surface are useless; they form the tough, waterproof barrier that protects the living cells underneath. Next lesson: the skeleton.