Human hair is a complex biological structure that provides thermal insulation and protection for the scalp. It also serves as a sensitive extension of the touch sense, detecting subtle air movements or contact. A strand of hair is composed of a non-living protein filament that emerges from a living structure embedded in the skin. Understanding its composition and architecture explains its unique strength, texture, and color.
Chemical Composition
Hair is primarily made of protein, specifically alpha-keratin, which constitutes approximately 80 to 95 percent of its total weight. Keratin is a long chain of amino acids. The elemental composition is dominated by carbon, oxygen, nitrogen, hydrogen, and sulfur, which form the keratin structure.
The high sulfur content comes from the amino acid cysteine. These cysteine molecules link together to form strong chemical bridges known as disulfide bonds, which are responsible for the hair fiber’s strength, durability, and resistance.
Beyond protein, hair contains small percentages of water and lipids (fats) such as free fatty acids and cholesterol. These lipids contribute to the hair’s flexibility and ability to retain moisture. Trace elements and minerals, including zinc, iron, copper, and magnesium, are also incorporated, often reflecting a person’s nutritional status.
Physical Structure
Each strand of hair originates from a hair follicle embedded in the dermis layer of the skin. The living part, the root or bulb, is located at the base of the follicle where cells rapidly divide. As these cells move upward, they undergo keratinization, filling with keratin and losing their nucleus to form the dead fiber that emerges from the scalp. The visible hair shaft is composed of three concentric layers.
The Cuticle
The outermost layer is the cuticle, consisting of several layers of flat, overlapping cells. Its main function is to protect the inner structure and regulate the hair’s water content.
The Cortex
Beneath the cuticle lies the cortex, which makes up about 90 percent of the hair’s total weight. It is the main source of the hair’s strength and elasticity, containing tightly packed bundles of keratin filaments. The cortex also houses the pigment granules, called melanin, which determine the hair’s natural color.
There are two primary types of melanin: eumelanin (brown and black shades) and pheomelanin (yellow, blonde, and red hues).
The Medulla and Texture
The innermost layer is the medulla, a central core of loosely arranged cells often absent in fine hair. Hair texture is determined by the shape of the follicle; round follicles produce straight hair, while oval or flattened follicles produce wavy or curly hair.
The Hair Life Cycle
Hair growth is a cyclical process that occurs independently in each follicle, ensuring continuous renewal. The cycle involves four distinct phases: anagen, catagen, telogen, and exogen.
- The anagen phase is the active growth period and the longest stage, typically lasting two to seven years. Cells in the hair bulb divide rapidly, and the hair shaft actively grows. Approximately 85 to 90 percent of scalp hair is in this phase.
- The catagen phase is a short transitional stage, lasting one to three weeks. It marks the end of active growth as the hair follicle shrinks and detaches from the blood supply.
- The telogen phase is a resting period for the follicle, lasting around three to four months. The hair remains anchored in the follicle but does not grow.
- The cycle concludes with the exogen phase, where the old hair is released and shed, allowing the follicle to re-enter the anagen phase and begin growing a new hair.