Hair is made up of a protein called keratin and it's simple in structure. The hair follicle holds the hair and keeps it connected to the surface of the skin.
The hair bulb forms around the hair follicle. Inside of the hair bulb, the cells divide and build the hair shaft to create the full hair strand. Blood vessels deliver the nutrients that the hair needs in order to grow and structure itself.
Hair growth takes place in three forms: anagen, catagen and telogen. The anagen phase is also known as the growth phase because the hair grows continuously. Each hair on the head will spend multiple years in this phase before moving into the next phase. Catagen is known as the transitional phase and is when the hair growth begins to slow while the hair follicle shrinks. The telogen phase is the final phase and consists of hair growth coming to a complete stop, with the old hair falling out and new hair coming in.
Different people have different rates of hair growth, although the most common growth rate is about one half inch every month. Hair color is determined by the pigment cells that produce melanin. As a person ages, their hair will turn gray or white because the pigment cells will die off.