Everyone has hair. Hair seems like a simple part of our body, but nothing about the human anatomy is simple. When you look at a hair strand, you are looking at the end product of a complex scientific process.
Dissecting a Strand of Hair
There is much more to hair than what you see. Each hair on your body begins with a multi-layered follicle embedded within your skin. The layer at the base of the follicle is called the papilla. The papilla connects to capillaries, which perform the important function of supplying blood to cells surrounding the bulb. The bulb is the bottom part of the hair strand.
The follicle is surrounded by an inner and outer sheath, both of which provide protection to the hair shaft and help ensure the hair grows in the right direction. The inner sheath stops at the sebaceous glad, which is sometimes referred to as the oil gland. The outer sheath begins at the sebaceous gland and runs to the erector pili muscle, which is the muscle responsible for that "standing on end" feeling when the muscle contracts.
The hair shaft itself consists of keratin, which is made up of three layers of protein cells. The outermost layer of the hair shaft is referred to as the cuticle. This layer is responsible for the sheen and luster aspect of hair appearance. The cortex is the middle layer, and it makes up the bulk of the shaft. The Innermost layer is called the medulla.
The hair on your head grows at a rate of about 4/10 of a millimeter every day. In the course of a year, the average growth for scalp hair is six inches. There are three different phases of hair growth. They are: the catagen phase, the telogen phase and the anagen phase.
The Catagen Phase
The catagen phase of hair growth affects about 3% of hair at a time. It is a transitional period in which there is no hair growth. While in the catagen phase, the outer portion of the sheath at the root of the hair shrinks and adheres to the root itself. Specific hairs stay in this phase between two and three weeks at a time.
The Telogen Phase
The telogen phase affects about 15% of hair at a time. Referred to as the resting phase, the telogen phase lasts for approximately 100 days for scalp hair. The telogen phase lasts longer for other body hair.
The Anagen Phase
The anagen phase typically lasts between two and six years. This is an active phase that involves the division of hair cells and growth of new hair. People who are not able to grow their hair long generally have shorter anagen phases.
Disorders Related to Hair Growth
Hursutism and Hypertrichosis are hair disorders that often affect humans. Hursutism affects women, and causes them to have unusual hair growth in body areas where typically only men have visible hair. It involves excessive growth of thick, dark facial hair, chest hair, areola hair.
Hypertrichosis is also characterized by excessive hair growth, but with this disorder, hair tends to grow on areas of the body where visible hair does not ordinarily grow. There are a number of medical treatments for these hair disorders.