Hair is an amazing and complex structure that grows from a small hole in the skin, called a hair follicle (pronounced: puh-FOL-cle). It carries a huge amount of genetic information and can be influenced by lifestyle choices. The color and texture of hair can be an indicator of biological sex, age and ethnic ancestry. It also has social significance, as hairstyle and color can be associated with specific stereotypes for both men and women.
Hair has a wide variety of physical and chemical properties, but its primary function is to serve as insulation against heat loss from the body. It can also trap air to provide a source of warmth in cold climates, and it serves as a mark of dominance or aggression in certain species.
In humans, the hair follicle is surrounded by a special type of skin called the epidermis. At the base of each follicle is an oil gland called the sebaceous gland, which produces a lipid-rich substance known as sebum, which naturally protects and moisturizes hair and skin.
A hair cell begins to develop inside the follicle and, as it grows, the older cells die and are pushed up through the epidermis to the surface of the skin. The resulting structure is filled with a protein called keratin, which gives hair its strength and elasticity. This process is called keratinization (kair-ehtih-ZAY-shen). The follicle is then sealed off by the cuticle, which acts as a protective sheath.
When hair is combed, it pulls through the cuticle and is sheared away from the follicle by a tiny muscle called the arrector pili (ar-REK-tor pye-li). The hair then falls to the floor of the scalp, where it is abrased to remove any excess sebum and the cell structure is shed. This cycle repeats itself over and over again until the follicle enters its resting phase, called telogen (teh-LEH-gen).
Hair is a highly evolved adaptation that has served many functions over time. It is an indicator of sex, age and ethnicity and has social significance in many cultures around the world. In addition, it provides warmth and protection against UV rays. Unlike most mammals, humans have very little body hair, which has likely been an evolutionary adaptation to decrease sweating and help cool the body in hot environments. Hair can even act as a filter, as the fine hairs in the nose and ears help prevent dust and other particles from entering the body through these sensitive areas. Moreover, the hair in the eyes and around the mouth helps to protect these regions from light and dust.