It’s everywhere—on your head, covering various parts of your body, and perhaps even clogging your shower drain. Hair is such a ubiquitous part of human anatomy that we rarely stop to question its presence. But really, what is hair? What is its function and how does it grow? For answers to these burning questions and more, look no further.
According to MedlinePlus, an affiliation of the US National Library of Medicine, the average human body is covered with five million hairs that grow everywhere except on the lips, palms, and soles of the feet. These hairs are comprised of two parts: the hair follicle in the skin and the shaft visible to the human eye. Hair follicles, likened to tunnels in the skin, contain the growing hair shafts and the blood vessels that provide nutrients. The hair shaft itself stems from a region called the bulb, located at the base of the follicle, which undergoes cell division every one to three days.
Hair shafts consist of dead skin cells and are separated into three layers—the inner medulla, the middle cortex, and the outer cuticle. The cortex makes up most of the individual hairs on the body and, along with the medulla, contributes to hair color.
Image Source: Dr. Don Fawcett
Hair serves multiple purposes, but its main functions involve protection and warmth. It keeps foreign particles in the air from gaining access to the body through the ears, eyes, and nose among other areas. The American Academy of Dermatology goes one step further and identifies the roles of specific regions of hair, such as eyelashes, to keep dust from entering the eyes, and eyebrows to prevent eye irritation from sweat.
Human hair growth is random, not cyclical, and depends on location. Hair on the head is estimated to lengthen about half an inch per month, or six inches per year. Facial, arm, and leg hair, however, remain much shorter due to a limited active growth phase called anagen. For scalp hair, the average anagen occurs anywhere from two to six years while other regions experience active growth for a mere 30 to 45 days. If you experience trouble growing your scalp hair to a certain length, your anagen is often shorter. On the other hand, the longer your hair is, the more likely you are to have a more extensive anagen. The other two stages of hair growth, catagen and telogen, serve as respective transitional and resting phases. Hair growth stops during catagen, and hair shedding typically occurs in the midst of telogen.
Overall, hair plays an important role in everyday wellness; it just does so at a microscopic level. It may seem like something of a nuisance, especially when you have to shave your face or your legs, but hair also does a world of good in protecting what lies beneath it.
