The hair fiber is an extremely stable structure that can resist breakdown thousands of years after a person’s death. About 50–100 microns in diameter, it consists of a 3 main layers - cuticle, cortex and medulla. The cortex provides the majority of hair fiber mass and is responsible for its strength. It is made of elongated cells, tightly packed together. These cells are composed of long filaments, or microfibrils, which contain rods of keratin embedded in an amorphous matrix. These proteins are remarkable for a proportionately high level of the intermolecular cross-linked amino acid, cystine, the disulfide bonds of which give hair its great tensile strength. Other protein intermolecular bonds, such as salt bonds and hydrogen bonds also exist in keratins. The hair cortex is covered by an external cuticle, which accounts for 10% of the hair fiber’s weight. The cuticle consists of overlapping layers of scales, each about 0.5 micron thick. In a newly formed human hair, up to 10 scales can overlap in one cross-section. The cuticular scales protect the underlying cortex and act as a barrier. When the scales are intact, their smooth surface reflects light, so the hair looks shiny and healthy. Finally, the medulla consists of specialized cells that contain air spaces. Only thick terminal hairs have a medulla, which may be discontinuous.

Biochemical structure of the hair structure: 1 - hair cuticula (scales); 2 - hair cortex; 3 - hair cortex fiber (fibril); 4 - hair cortex fiber's mikrofiber (microfibril) bundle; 5 - hair cortex fibers starting mikrofiber (microfibril); 6 - hair cortex mikrofiber initial fiber (protofibril) 7 - the original protein spiral fibril; 8 - amino acids, 9 - atoms.

Only 3 percent of the hair consists of water, the remaining 97 percent is a proteinous material called keratin, containing  of iron, copper, zinc, chromium, permanganate, sulfur and other trace elements and also vitamins A, B, C, T and P. Therefore healthy hair require to get enough protein with food. Keratin is formed of resistant to wear long compressed protein molecules with a regular structure which is sufficiently strong and flexible. These large protein molecules are made of significantly smaller highly concentrated xhained specific amino acid molecules. Hair keratin consists of nineteen amino acids. When amino acids are bonded in a healthy hair state, they form a polypeptide chain. One of the amino acids - Cysteine - is cross-linked with disulfide. These disulfide links provide keratin with a space for the structure (shape) and are responsible for hair elasticity. Disulphide link disruptions (chemical hair alterations - perming, straightening, coloring, etc.) reduce hair elasticity and allow hair to change their shape.

Chemical composition of the hair structure formula shows as C-O-N-H-S. The names of these chemical elements and their average percentages are: C - Carbon - 49.6%, O - Oxygen - 23.2%, N - Nitrogen - 16.8%, H - Hydrogen - 6.4%, S - Sulfur - 4.0%. Besides hair has some magnesia, arsenic, iron, phosphorus, chromium and other metals and minerals.

 

Hair keratin amino acids names and their percentages: Cysteine - 17.5%, Serine - 11.7%, Glutamic acid - 11.1%, Threonine - 6.9%, Glycine - 6.5%, Leucine - 6.1 %, Valine - 5.9%, Arginine - 5.6%, Aspartic acid - 5.0%, Alamine - 4.8%, Proline - 3.6%, Isoleucine - 2.7% Tyrosine - 1.9 % Phenylanine - 1.4%, Histidine - 0.8%, Methionine - 0.5%.

Hair natural color depends on their chemical composition. Darker hair contain more carbohydrates and less oxygen, blonder hair have more oxygen than carbohydrates. For example medium-dark hair have up to 50.65 percent of carbohydrates, 6.36 percent of hydrogen, 17.14 percent of nitrogen, 5.0 percent of sulfur and 20.85 percent of oxygen. Hair molecules are bounded up to the zigzag chains of molecules. These molecular chains are very close each other’s and form tiny fibers called miceles, that create fibrous elongated form cells. Molecular chains inter-connect with micels due to the sulfur bridges. These chemical elements create "keratin" and "protein". Besides hair contain a certain amount of fatty substances, cholesterol, mineral compounds and arsenic. Keratin is a highly viscous and elastic protein that stimulates the hair cell hardening. The pigment contained in hair is called melanin. It is a protein complex combinations (triptofan, pirol, etc.). This pigment protein contains nitrogen, sulfur, oxygen and a small amount of iron and arsenic. Melanin component concentration ratio determines each unique human hair color.