Human hair is a complex biological material. These strands play roles in thermoregulation, sensory perception, and protection from environmental factors like UV radiation. Understanding its fundamental composition reveals how chemical components are organized into distinct physical layers. This structure allows hair to exhibit a range of physical properties, influencing its appearance and resilience.
The Chemical Makeup of Hair
The primary constituent of human hair is protein, predominantly a fibrous structural protein called keratin. Keratin makes up up to 95% of the hair fiber by weight, providing its strength and resilience. This protein’s properties arise from its high content of the amino acid cysteine, which forms strong disulfide bonds between protein chains. These cross-links are responsible for hair’s rigidity and ability to maintain its shape, influencing characteristics like curl and straightness.
Beyond its protein backbone, hair also contains a significant amount of water, ranging from 12% to 15% of its total composition. This hydration functions as a natural plasticizer, maintaining hair’s flexibility and softness. Adequate moisture helps prevent the hair from becoming brittle and contributes to its healthy appearance.
Lipids are present both within and on the surface of the hair. These include triglycerides, waxes, phospholipids, cholesterol, squalene, and free fatty acids. These lipids contribute to hair’s natural lubrication, serving as a protective barrier that helps seal the hair’s outer layer and reduce moisture loss. This layer provides a smooth, shiny surface and helps protect the hair from environmental damage.
Hair color is determined by melanin pigments, which make up about 1-3% of human hair by weight. Two main types are: eumelanin, which produces black and brown shades, and pheomelanin, responsible for red and yellow tones. Their ratio and concentration determine the wide spectrum of natural hair colors. Melanin granules are integrated into the hair shaft during its growth.
Hair also contains trace amounts of various elements, reflecting an individual’s internal environment. These include carbon, oxygen, nitrogen, hydrogen, and sulfur. Minerals like calcium, magnesium, zinc, copper, iron, and lead are also found, absorbed from the body.
Hair’s Layered Structure
The chemical components of hair are organized into three distinct physical layers within the hair shaft, each contributing to its integrity and function. The outermost layer is the cuticle, composed of several overlapping, flattened cells that resemble scales. These translucent cells lie flat against the hair shaft, pointing towards the tip, and their primary function is to protect the inner layers from damage and regulate moisture exchange.
The condition of the cuticle influences hair’s appearance and feel. When cuticle scales lie smoothly, light reflects evenly, giving hair a shiny, healthy look. Conversely, damage to the cuticle, from factors like chemical treatments or excessive heat, can cause the scales to lift, resulting in a rough texture, increased tangling, and greater susceptibility to moisture loss. This condition often makes the hair appear dull.
Beneath the cuticle lies the cortex, which forms the thickest part of the hair shaft, accounting for 75-90% of its total diameter and weight. The cortex is made of organized bundles of keratin fibers aligned lengthwise. These keratin bundles provide hair with its tensile strength and elasticity, allowing it to stretch and return to its original shape without breaking.
The cortex also houses the melanin granules that determine hair color. The size, number, and distribution of these pigment granules within the cortical cells influence the depth and hue of the hair. The arrangement of keratin fibers within the cortex also plays a role in determining the hair’s natural curl pattern, with more asymmetrical distributions leading to curlier hair.
The innermost layer of the hair shaft is the medulla, though its presence varies among hair types and even along the length of a single hair. When present, the medulla can appear as a continuous, interrupted, or fragmented core. Its structure varies, sometimes being a hollow canal or filled with loosely packed cells. Its function in human hair is less understood, but it may contribute to the hair’s strength and insulation properties.
How Composition Influences Hair Characteristics
The interplay between hair’s chemical makeup and its layered structure dictates its observable characteristics. Hair’s strength and elasticity are due to the keratin proteins within the cortex. The numerous disulfide bonds linking keratin chains create a resilient internal network, allowing hair to withstand pulling forces and stretching before breaking.
Hair’s porosity, its ability to absorb and retain moisture, is governed by the condition of its outermost cuticle layer. When the cuticle scales lie flat and are tightly sealed, hair is less porous and resists water absorption, retaining its moisture. Conversely, a damaged or lifted cuticle increases porosity, allowing water to enter and exit more easily, which can lead to dryness or frizz.
The natural color of hair is a result of the type and amount of melanin produced and integrated into the cortical layer. Eumelanin concentration dictates the range from black to brown, while pheomelanin contributes to red and yellow tones. The absence or significant reduction of melanin production leads to white or gray hair.
The texture of hair—whether it is straight, wavy, or curly—is influenced by the shape of the hair follicle and the distribution of keratin within the cortex. A round follicle produces straight hair with an even keratin distribution. In contrast, an oval or flattened follicle, coupled with uneven keratin placement and the formation of more disulfide bonds, results in wavy or curly strands.