Hairs are tiny shafts of a protein called keratin. They are anchored in a group of specialised cells called hair follicles, which supply oxygen and nutrients to the root (or bulb) of the hair, and lubricate the hair with an oily substance called sebum.
The human body is completely covered with hair follicles, except on the palms of the hands, soles of the feet and the lips. Mostly, hair follicles are tiny, and the hairs they produce don't grow long enough to protrude from the pore.
The areas where hairs do protrude (and are visible above the skin) include the armpits, face, around the genitals, the front of the chest, the back, and most profusely, on the scalp of the head. A scalp typically contains about 100,000 hair follicles.
Hair is in a constant cycle of growth, rest, and renewal. Hairs grow and then are shed, but because they grow at different rates they don't all shed at once.
It takes about three years for hair follicles to produce a hair that grows, rests, falls out and then regrows, which means the scalp loses between 50 and 200 hairs a day.
