On the average human scalp, there are approximately 100,000 hair follicles, tiny little pocket-like holes whose sole purpose is to grow hair.

Each tunnel-shaped hair follicle can be found on the epidermis, or outer layer, of the skin. Hair starts growing at the bottom of the hair follicle and the root is composed of protein cells that are fed by the blood flowing through the vessels just below the scalp.

Growing Hair

Fed by the proteins from the blood, the hair follicle slowly grows, pushing up and out past the skin. The average person grows hair at a rate of about 0.3 to 0.4 millimeters per day, or about one-tenth of an inch per week or 6 inches per year.

As the hair grows, tiny glands known as the sebaceous glands near the hair follicle secretes oil, which nourishes both the hair and the scalp.

Hair grows in three cycles. The first is the anagen phase, or growth phase, when the hair starts growing out of the root. This stage typically lasts anywhere from three to seven years.

The second phase is the catagen, or transitional, phase. This is when hair growth slows down and the follicle shrinks a little bit. This phase lasts between two to four months.

Finally, there is the telogen, or resting, phase. This is when old hair falls out and is replaced with new hair growing from the same hair follicle. This phase lasts anywhere from three to four months.

All at Once

Typically, all of your hair follicles are experiencing different phases at the same time. Some of your hair follicles right now are in the growth phase while others are in the resting phase. People constantly are losing and regrowing hair throughout their entire lives.

The average person loses an estimated 100 strands of hair per day, most of which are immediately replaced with new hair.