The human body contains approximately five million hair follicles while the scalp (prior to any kind of hair loss) contains 100,000-150,000 hair follicles. The normal growth rate of scalp hair is one-fourth to one-half inch per month. Understanding hair growth is important in understanding hair loss.
THE NORMAL HAIR GROWTH CYCLE
A FOLLICULAR UNIT IS COMPRISED OF THE HAIR SHAFT AND THE 
FOLLICLE OR ROOT. The youthful hair follicle has all the requirements--hormone receptors, diverse cell structure and genetic code-- to produce the hair on your head. Each hair consists of a series of interconnected proteins known as keratins.
Periods of growth (anagen) last between two and eight years and are followed by a brief period, two to four weeks, in which the hair (not the follicle) begins to separate from the follicle (catagen). Metabolic activity ceases and the resting phase (telogen) begins lasting two to four months. Shedding of the hair occurs only after the next growth cycle (anagen) begins and a new hair shaft begins to emerge. On average 100 telogen hairs are shed every day. This is normal hair loss and accounts for the hair loss seen every day in the shower and with hair combing. In a healthy scalp, these hairs will regrow.
The three phases of the cycle are:
- Anagen= growth phase
- Catagen= degradation phase
- Telogen= resting phase.
Human hair is produced in a continuous cyclical pattern of growth and rest known as the "hair growth cycle."
Androgens (testosterone,dihydrotestosterone) are normal hormones present 
in both men and women. They are the most important regulators of human hair growth. Androgens must be present for the growth of beard, axillary (underarm), and pubic hair, however they DO NOT cause growth of scalp hair. Dihydrotestosterone is the more potent of the two. In fact, they have the opposite effect on susceptible hairs. A susceptible hair is a hair with a follicle that is genetically coded to die when exposed to an androgen. Most people have a combination of susceptible and non-susceptible hairs. The distribution and ratio of these hairs determines the visible pattern of hair loss. In men, these susceptible follicles are typically present on the top of the scalp, whereas in women they are interspersed throughout the scalp.