The hair cycle
Hair grows in groups of 1 to 4 hairs called hair follicles. We are born with all the hair follicles we will ever have in the scalp(approximately 100,000). Hair follicles replenish themselves by going through three phases, the Anagen phase, Catagen phase and Telogen phase, known as the hair cycle.
This is the reason why hair grows to a particular length and thickness. The duration of the Anagen phase, between 2 to 6 years, determines the length of the hair and the hair bulb determines the hair shaft diameter. The growth rate is approximately 1cm per month.