Hair is a keratinized structure that grows out of the skin. It is the defining feature of many mammals, though it is much more sparse on humans than other animals such as rabbits who have a thick coat of fur or whales and hippopotamuses who have very long hairs covering their bodies. Hair serves a number of purposes including protection, communication, thermoregulation and sensory input. Hair on the head provides heat insulation and protects the skull from sunlight and sunburn. Hair around the eyes and nose (eyelashes) protects the cornea from dust particles, and hair on the arms prevents sweat from dripping onto the face.
Hair consists of dead cells that have been filled with and surrounded by keratin, a protein. The keratin is compacted and fused together, creating a tough and durable structure that resists the wear and tear of life and is resistant to chemical damage. It is the same material that is used for the nails, hooves and feathers of birds.
Your hair starts in an area of your skin called a hair follicle, a small cystic hole in the surface of the skin. Each hair follicle has a sebaceous gland that produces a lipid-rich substance, called oil, that naturally protects and moisturizes the hair. At the base of the follicle is a bulb that produces new hair cells. As these new cells grow, they push the older ones out of the follicle, resulting in the strand-like appearance of your hair. The strands of hair are then covered with a tough, scale-like layer of the skin called the cuticle.
Within the hair follicle, stem cells continue to divide rapidly and eventually become long enough to be visible above the surface of the skin. As your hair grows, it is constantly nourished by blood vessels in the follicle at the bottom of the hair follicle called the papilla. The pigment cells of the hair bulb produce the colour of your hair and determine if it is straight, wavy or curly.
When your hair reaches a length that is above the top of the hair follicle, it enters the active growth phase of the hair cycle, called anagen. During the anagen phase, the hair cell is fed by the blood vessel in the follicle and it continues to grow until it reaches a point where it is ready to be shed, entering the catagen phase of the hair cycle.
The anagen phase lasts for years on the scalp and for less time on other parts of the body. After the anagen phase ends, the hair enters a resting period of the hair cycle, which is called catagen. During this phase, the hair stops growing, the cell division slows down and blood flow to the follicle is cut off.
The hair follicles then produce a club hair, which is a short club-shaped piece of hair that can be seen in the hairline. This is the hair that you can pull back into a ponytail or comb over to disguise a bald spot.