Hair

Threads of keratin growing from follicles in the skin — present on nearly all of the body except the palms, soles, and lips.

Structure

• Shaft — the visible part above the skin (dead, keratinized).
• Root and follicle — the living part below, where the hair is made.
• Associated structures — a tiny arrector pili muscle that raises the hair (“goosebumps”), and a sebaceous gland. Together, the hair, its sebaceous gland, and the arrector muscle form the pilosebaceous unit.

What it does

• Protection — scalp hair shields from UV and impact; nostril and ear hairs filter; eyelashes and eyebrows shield the eyes.
• Sensation — movement of a hair is detectable.
• Heat conservation — limited in humans.

Key idea

Hair grows in cycles of growth, rest, and shedding, and each follicle runs its own cycle — so shedding is continuous rather than seasonal.