Introduction
The human head contains approximately 100,000 hairs, which each grow an estimated half of an inch each month, according to the American Osteopathic College of Dermatology. These hairs do not grow indefinitely. In fact, the average hair only survives for four and a half years, after which a new hair will take its place. The process of hair growth and loss are broken up into three phases.
Anagen Phase
The first phase of hair growth is the active growth phase, also known as the anagen phase. During this phase a group of cells at the bottom of the hair follicle multiply at an accelerated rate. This creates the actual hair that will eventually grow through the follicle to the surface, which is called the hair shaft. To multiply and grow, the hair cells need a constant supply of oxygen and nutrients. These are supplied through papilla. The papilla is a structure located at the root of the follicle that can connect and disconnect with surrounding blood capillaries, which transport the necessary nutrients to the follicle. As long as the hair is in the anagen phase, the cells will continue to grow and form a new hair shaft.
Catagen Phase
The next phase of the hair cycle is the catagen, or transition, phase. In the catagen phase the follicle reduces dramatically in size and becomes cut off from the surrounding capillaries. Without a connection to the nutrients from the capillaries, the hair cells stop growing the hair shaft. The entire process of this phase can take up to two weeks.
Telogen Phase
The final hair growth phase is the telogen phase, or resting phase. During this time the follicle is able to rest and get ready for the new anagen growth cycle to begin to produce a new strand of hair. During this phase, any remaining part of the last hair that has stopped growing will fall out. It is normal for at least 100 of these hairs to fall out each day, according to the American Osteopathic College of Dermatology. If the hair does not fall out during the telogen phase, it will be forced out, as the follicle returns to its anagen phase and a new hair pushes the old hair out of the follicle.
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