Each single hair consists of a shaft with a bulbous root. The root produces the shaft, which is mostly made up of a protein called keratin. The shaft itself has a tough outside layer called the cuticle, made up of overlapping scaly cells. When lotions and treatments are applied externally, they simply "glue down" these cells, making the hair appear healthier and shinier.

The hair's middle layer is called the cortex, and it contains the pigment that gives hair its color. A central core called the medulla may not be present in very fine hair.

The root is encased in a follicle, a tube under the skin. Inside the base of the follicle is the papilla, a cone-shaped elevation that fits into the root. The papilla, and its central matrix, is the hair's lifeline. It contains the network of nerves and blood sources which induce the hair to grow and provide nourishment to it. A healthy papilla will continue to produce new hair, which is why supplements that provide specific nourishment for growing hair can be so helpful.

Hair doesn't grow constantly. Hair follicles are the only organs in the body that engage in cyclic growth, including a period when no hair grows at all. Awareness of this cycle is helpful in understanding hair loss and its treatments.

The first stage of the cycle is called the anagen phase. This is when the hair is actively growing. Normally, around 85% of all scalp follicles are in this phase.

The length of the anagen phase varies -- from as little as two years to as many as six. This explains why some people are able to grow hair to their waists, while other people can leave their hair uncut and still never have it go much more than to their shoulders. People who can grow very long hair simply have a longer anagen phase.

When the anagen phase is over, the follicle begins its catagen phase. The hair stops growing and the follicle begins to "shut down." The papilla shrinks and the matrix cells disappear. A fine, short "club hair" forms inside the follicle shaft. Pigment formation stops. The catagen phase lasts anywhere from a few days to one or two weeks. Normally, about 2% of all head hair is in the catagen phase.

The final level is the telogen phase, or "resting period". In normal hair growth, this period lasts about six weeks, with about 13% of all scalp follicles in the phase. During the telogen phase, the hair is shed. These hairs fall out at the rate of about 100 per day. Since a full head of hair contains between 90,000 and 120,000 hairs (mostly depending on color, with red at the low end and blonde the highest), the loss isn't noticeable. Once the telogen phase is complete, the follicle becomes active again. It re-enters the anagen stage, and that particular hair begins to grow.