The number of hairs on an adult is around 100,000, and can say that each hair grows through several cycles of growth. There is a period in which hair grows and another that stops growing and rests, and a final in which hair falls out.
Each hair begins to grow, reaching a stage where the growth is interrupted as the hair lies, after the break, the growth continues, this is repeated until the hair dies and falls, driven by new hair that pushes them to get out the scalp (which happens in a normal, healthy hair). The hair cycle is not synchronized in humans, meaning that while some are falling, others are growing.
The stage of hair growth has an obvious length greater than the resting stage or falling, and this duration is related to the length that will ultimately each hair.
In general the number of hairs during growth is much higher than those who have served their stage, it is normal to lose 50 to 100 hairs a day.
When hair loss, ie the amount of hair loss becomes apparent, the case that a greater amount of hair has entered the period of decline, and may be due to different factors.
