An outer covering of skin protects the body from injury, germs, water loss, and sunlight. Skin is also an important sensory organ, full of receptor cells sensitive to touch, heat, cold, and pain.
Skin gets its strength from a supple outer layer called the epidermis, made largely of dead cells packed with a hard protein called keratin. The epidermis is continually wearing away and renewing itself. Its bottom layer of living cells keeps dividing, producing new cells that slowly move to the surface. As they travel up, they flatten, harden, and die.
Beneath the epidermis is the dermis, a layer of tissue containing blood vessels, nerves, sensory receptors, sweat glands, and hair roots. Under the dermis is a layer of fat cells that cushions the skin and traps heat deep within the body.
When the body is hot, sweat glands secrete a watery fluid onto the skin. The water draws heat from the skin as it evaporates. At the same time, blood vessels in the skin widen to release excess heat. When the body is cold, the skin’s blood vessels narrow to reduce heat loss, and tiny muscles pull hairs erect, trapping warm air over the skin.
Skin color comes from the pigment melanin, made by cells called melanocytes in the epidermis. Everyone has the same number of melanocytes, but they are much more active in people with dark skin. Melanocytes also become more active after exposure to the sun, producing pigment to protect the skin from damage from the sun.
Nails are made of overlapping plates of dead cells filled with the protein keratin. Keratin is tough and waterproof and is also found in hair and skin. Nails protect the sensitive tips of the fingers and toes.