Have you ever wondered why the hair on your head can grow long while your eyebrows remain fairly uniform in length?
Each hair on your body grows from its own individual hair follicle. A follicle will produce new cells for a certain period of time depending on where it is located on your body.
All the hair on our bodies grow in three phases.
During the first phase, called the Anagen Phase, the hair is actively growing and contains a number of microstructures not present during any other time. Two of these structures, the bulge and the papilla, are believed to control hair growth. It is during this stage that the hair follicle is directly connected to the blood vessel. It is important to note that only during this stage that we can permanently disable hair growth. The life of the hair is in the blood! If we curtail the blood supply, we can prevent re-growth.
During the second phase, called the Catagen Phase, the hair has stopped growing and is beginning to pull away from the papilla. The bulge is reabsorbed into the body at this time as well. While we still see the hair, it is no longer connected to the blood supply. Thus, while the hair can be destroyed during this phase, it cannot be permanently disabled. The hair will grow back in time.
During the third phase, the Telogen Phase, the hair has become completely dormant. It is no longer connected to anything substantial and it is preparing to fall out. This hair will shed once a new Anagen hair begins to form below.
Not all body hair is alike. For example, at any given time, 85% of the hair on your scalp is in the Anagen Phase and the Anagen Phase on the scalp lasts between two to six years. So, most of the hair on your head is actively growing at any given time, and this also explains why the hair on your head can grow so long. On the other hand, with your eyebrows, only 10% of the hair is in the Anagen Phase at any one time and the phase lasts only between three to nine weeks.
It is interesting that animals that shed have hair follicles that synchronize their rest phase. This means that all of the follicles enter the rest phase at the same time. Not surprisingly, all of the hair falls out at one time. For instance, a dog that sheds will lose its hair in large clumps.