The hair on our heads isn't just there for looks. It keeps us warm by preserving heat. The hair in the nose, ears, and around the eyes protects these sensitive areas from dust and other small particles that might harm us. Eyebrows and eyelashes protect eyes by decreasing the amount of light and particles that go into them.
The fine hair that covers the body provides warmth and protects the skin. Hair also cushions the body against injury. Human hair consists of the hair shaft, which comes out from the skin's surface, and the root, a soft thickened bulb at the base of the hair embedded in the skin.
The root ends in the hair bulb, which sits in a sac-like pit in the skin called the follicle, from which the hair grows. At the bottom of the follicle is the papilla, where hair growth actually takes place. The papilla contains an artery that nourishes the root of the hair.
As cells multiply and produce keratin to harden the structure, they're pushed up the follicle and through the skin's surface as a shaft of hair. Each hair has three layers: the medulla at the center, which is soft; the cortex, which surrounds the medulla and is the main part of the hair; and the cuticle, the hard outer layer that protects the shaft. Hair grows by forming new cells at the base of the root. These cells multiply to form a rod of tissue in the skin. The rods of cells move upward through the skin as new cells form beneath them. As they move up, they're cut off from their supply of nourishment and start to form a hard protein called keratin in a process called keratinization.
As this process occurs, the hair cells die. The dead cells and keratin form the shaft of the hair. Each hair grows about 1/4 inch (about 6 millimeters) every month and keeps on growing for up to 6 years. The hair then falls out and another grows in its place.
The length of a person's hair depends on the length of the growing phase of the follicle. Follicles are active for 2 to 6 years; they rest for about 3 months after that. A person becomes bald because the scalp follicles become inactive and no longer produce new hair. Thick hair grows out of large follicles; narrow follicles produce thin hair. The color of a person's hair is determined by the amount and distribution of melanin in the cortex of each hair (the same melanin that's found in the epidermis). Hair also contains a yellow-red pigment; people who have blonde or red hair have only a small amount of melanin in their hair.
Hair becomes gray when people age because pigment no longer forms.
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