Hair is a cylindrical filament that comprises a function part of the integumentary system of mammals. Loved primarily for its cosmetic value, hair comes in all colors, lengths, textures, and types. The average person has approximately 1.4 million hairs on their entire body,  and hair can be found on every part of the body except the palms of the hands, the soles of the feet, and the lips. Hair is made of former cells filled with protein that originate from follicles deep within the dermis, which grows up through the epidermis. It acts as insulators, sensors, and protectors of the skin. Thus, these seemingly uncomplicated and common pieces of the body do much more than what meets the eye.
Human hair is unlike the fur possessed by animals including other primates and researchers have started asking why. Fur stops growing at a certain length, but hair keeps on growing. Even on the human body leg hairs grow for five or six months and then fall out. Hair on the head keeps growing for two to six years. If a hair follicle from the head is transplanted to the leg, it keeps growing for a longer time though not as long as if it were on the head. A leg hair transplanted to the head does not grow long, so hair transplants always move hair from one part of the head to another.
The keratin protein in human hair is different from the equivalent protein in chimpanzees and gorillas, and chimpanzees never need hair cuts. Evolutionists previously wondered how to explain why humans do not have body fur, and now they will need to explain how human hair developed.
Anatomically, a hair can be split into two general regions: the root and the shaft. The root consists of the portion of hair beneath the surface of the skin, while the shaft is the part above the skin. The root contains the hair follicle, the pocket of epithelial tissue out of which a hair grows. The hair follicle is composed of two layers, the internal and external epithelial root sheaths, both of which are wrapped in the dermal root sheath. The base of the root, known as the hair bulb, houses the matrix, a mass of unspecialized or "generic" cells; upon differentiation, these cells are keratinized (filled with protein) and become part of one of the hair layers. Melanocytes, also in the hair bulb, produce the pigment melanin. Using phagocytosis, the matrix cells incorporate the melanin, giving hair its color. The darker the color, the more melanin in the matrix cells. Red hair is slightly different in that it is a mixture of melanin and red pigment, with the melanin deepening the red hue from the pigment.
A hair has three concentric layers composed primarily of a protein called keratin: the medulla, the cortex, and the cuticle. The medulla, the innermost layer and central core, may or may not be found in a human hair. This space can contain nothing but air, but often it is formless and vague in appearance, unlike in animal hairs where it is usually distinct. The medulla structure, when located in human hair, is classified as continuous, broken/discontinuous, or fragmentary/trace. The next layer, the cortex, is made up of spindle-shaped elongated cells and composes most of a hair's mass. Pigments, cortical fusi, and/or ovoid bodies ("large oval-to-round-shaped structures") may be found in the cortex. Finally, the cuticle, the outermost layer of the hair, covers the hair shaft with translucent scales; these scales always point toward the end of the hair away from the hair's root. Flattened or imbricate scales, which are narrow and overlapped, are the most common type found in human hairs.  As the cells in the three regions begin to keratinize and harden, they are integrated into the dead (keratinized) portion of the hair fiber; new hair cells in the hair bulb region push the dead shaft upwards towards the skin's surface. 
As previously mentioned, hair is chiefly composed of keratin proteins (88%). At the microscopic level, keratin is made up of a polypeptide chain called the "alpha helix." Three alpha helices coiled together form a protofibril; a bundle of nine protofibrils in a circle produce a cable known as a microfibril; an irregular grouping of hundreds of microfibrils in a sulfurous matrix of proteins composes a macrofibril; and these macrofibrils together comprise the main body (cortex) of the hair layers (from the alpha helix in a keratin protein to protofibrils, microfibrils, macrofibrils, and cortex layers.) The following molecular bonds hold all these hair layers together:
- Hydrogen bond--this bond gives hair 35% of its strength and 50% or more of its elasticity, or its ability to stretch and still revert back to its original state. Electrolytically controlled, hydrogen bonds are a weak intermolecular attachment that is the easiest to be formed and the easiest to be broken down. This kind of bond also enables water to temporarily change the shape of the hair. 
- Ionic bond-- Salts are held together by what is called an ionic bond (electrolytic). This bond is responsible for approximately 35% of hair's strength as well as the remaining percentage of its elasticity, and is positioned parallel to the alpha helix axis line.
- Cystine bond--because this bond is perpendicular to the hair's axis, it contributes to hair's resistance to abrasion and its overall toughness as it holds hair fibers together. Called by other names including the disulfide bond, sulfur or S bond, the cystine bond has cross-links between amino acids in the hair fiber that allow for hair perms.
- Sugar bond--the sugar bond is responsible for only a little amount of hair's strength (5%), but its position perpendicular to the axis of the hair (like the cystine bond) enables it to make hair tougher. This bond also gives hair some degree of moisture. 
The hair follicle, out of which hair grows, contains numerous vessels and glands that are essential to hair growth. The lymph and blood vessels supply necessary nutrients to the follicle and the budding hair in the anagen (growing) phase. The sebaceous glands secrete sebum, an oil-based substance that prevents microorganisms from living on the surface of the skin and protects hair from becoming brittle and dry. Attached to the hair follicle, the Arrector pilli muscle controls the hair's position, making it stand on end to trap heat (an action visible in the form of goosebumps). At the junction of the follicle and this muscle are stem cells that contribute to the production of hair in the anagen phase of hair growth. 
Hair on the scalp is not only a cosmetic characteristic of the body, but also a shield to shade the head from the sun. In regards to the beauty aspect of hair, it usually aids in the differentiation of men and women. One of the chief functions of hair is insulation, where it acts as a physical barrier between the bare skin and the external air.  In addition, hair keeps the body warm through the formation of goose bumps, which results in the erection of hair follicles that act as a form of insulator near the skin. Hair protects the skin from external factors like dirt and dust, as well as chapped skin resulting from wind damage. During physical activity, hair absorbs and redirects sweat so that it evaporates more easily, cools the body, and doesn't irritate the skin or other parts of the body.  Hair also serves as a sensory structure that detects movement. The follicles are wrapped in sensory receptors that sense changes in the position of the hair. When hairs moves, it stimulates the receptor that then triggers a nerve impulse (action potential) to the central nervous system. Specific regions of hair also have specialized purposes.
Nasal and Ear hair
Commonly deemed a nuisance, nose hair is vital to the body's defense. During inspiration through the nose, nasal hair helps to trap airborne pathogens (spores, germs, fungus, etc) and other solid particles in thick mucus; this filtration keeps unwanted substances out of the lungs and body. Also important to the respiratory system, nasal hairs in conjunction with the mucus humidify the air, giving it moisture and heat as it enters the body.  In the ear, hair and wax filter out foreign particles, including little bugs, from reaching the inner components of the ear; thus, ear hair acts as a defense against infections in the ear. 
Eyelashes and Eyebrows
Because eyelashes are sensitive to tactile sensations, they protect the eye by initiating signals that close the eyelid reflexively when an object is too close to the eye (causing the eye to blink). Eyelashes form a barrier to prevent debris, sweat, and dirt and dust particles from entering the eye, in addition to shading the eyes from too much sunlight. Cosmetically, lush and long eyelashes are valued as especially desirable.  Due to their arched shape, eyebrows divert water and sweat droplets from entering the eyes; instead, the liquid runs along the sides of the face. This function is important because water blurs vision and the salts in sweat irritate the eye. As a facial feature, eyebrows give expression, often conveying emotions. Eyebrows are also increasing in aesthetic significance, where thin and sculpted ones are considered more attractive than bushy and unkempt eyebrows. 
Although often unpleasant or embarrassing, underarm hair does appear to have some function. One popular theory is that this hair catches and collects pheromones (chemicals secreted by the body in connection with sexual arousal), producing an odor similar to sweat. Another explanation is that underarm hair helps to reduce the friction in the armpits during movement. As there is no discomfort resulting from friction experienced by people with shaved armpits, this explanation is merely a theory. 
Pubic hair is a sign of puberty in both men and women. Functionally, it protects the skin from irritation or redness resulting from friction, particularly during intercourse. Like underarm hair, pubic hair is also thought to retain pheromones. Many people hold that pubic hair protects against infection, although this claim is not scientifically supported. What is known however is that pubic hair prevents dirt or dust from entering the vagina (women) or from irritating the penis (men); these foreign particles are then rinsed out from the hair during bathing. 
The hair growth cycle consists of three stages: anagen (growing phase), catagen (transitional phase), and telogen (resting phase). On average, a person will undergo this cycle approximately twenty times, beginning at birth and continuing without ceasing until death.  In a year, the scalp hairs grow about six inches, or 0.3-0.4 mm a day. The number of hairs undergoing a specific phase is random, and not based on a particular pattern of hairs or the season.
- Anagen phase--This initial stage is subdivided into three segments: proanagen, mesanagen and metanagen. DNA and RNA synthesis in the hair follicle lead to rapid growth as the follicle reaches its maximum girth and length. At this point, eight concentric layers of multiple cell varieties compose the hair follicle.  This new hair pushes the old club hair up and out of the hair follicle. The scalp hair stays in this stage from anywhere between two to six years. Hair on other parts of the body, including eyebrows, eyelashes, arms, and legs, has a relatively short active growth stage of 30-45 days, thus explaining its shortness compared to hair found on the head. People with longer hair typically have a longer anagen phase than those with naturally shorter hair.  In general, hair grows more rapidly in the winter months than in the summer months. Throughout the anagen phase, the hair bulb produces melanin, or pigment, and hairs only begin to appear white as people age and melanin production gradually decreases.  Minerals and nutrients are supplied to the budding hairs by the circulatory system, giving the new hairs a nourished and thick appearance. Roughly 80-90% of the scalp hair follicles are in this stage at any given moment.
- Catagen phase--A brief intermediate period, this stage lasts between two to four weeks. During this time, the hair follicle detaches itself from its blood supply, steadily shrinking to one-sixth of its original size. The hair bulb gets prodded towards the skin's surface by the formation of new hair, and the bulb ceases pigment production.  As the outer root sheath diminishes and attaches to the hair root, a club hair is formed. Approximately 3% of all hairs are in the catagen phase at one time.
- Telogen phase--This final stage is called the resting phase because the club hair is formed in its entirety and the hair follicle is at rest. For hairs on the scalp, this phase lasts for approximately 100 days; on other areas of the body (eyelashes, eyebrows, legs, arms, etc.) the telogen phase is even longer. During this time, the hair follicles slowly weaken to the point where they are pushed out by new hair shafts. Typically, 25-100 hairs in this stage are shed every day, although fewer are shed in the spring and summer than in the winter or fall. This shedding occurs painlessly and naturally, often during brushing or washing of the hair. Of the hairs on the head, around 10% are in this phase. The emergence of new hairs resets the cycle back to the anagen phase.
Various factors contribute to the overall growth cycle of each individual hair. Nutrition and bodily health are widely accepted as essential for healthy follicle growth, as starvation, serious anemia, and malnutrition result in poor hair growth. Hormone levels in the bloodstream also affect the hair cycle. For example, thyroid hormone stimulates faster growth in resting hair follicles. Vitamin B, or panthenol, improves the hair shaft's gloss, strength, and elasticity. Zinc also appears significant in healthy hair growth as a lack of this mineral causes hair loss or the formation of thinner, finer hair.
Perhaps more than any other factor however, androgens (male hormones found in both sexes in differing ratios) affect the hair of the scalp. Androgens regulate the thickness of the hair shaft in addition to the controlling much of the hair cycle in general. Oestrogens (female hormones also present in both genders) extend the anagen phase, but slow down the rate of growth within this stage. These hormones explain why adolescents experience sudden increases in hair growth as their hormone levels rise. During puberty, underarm and pubic arm begin to grow. For men, chest hair growth peaks later in life, and facial hair does not grow strongly until men are in their thirties. Ear and nose hair also experience the most growth in the mid- or later years. Women observe changes in hair growth during pregnancy as oestrogen and other hormone levels increase. The growth of hair then, is chiefly controlled by a balance of androgens and oestrogens in the body.
Hair growth begins in the womb with the formation of trichocysts three months after conception. As the baby grows, the trichocysts develop into hair follicles which produce short downy hairs after the child's birth. Because the number of follicles is determined prior to birth, the number of hairs never increases after birth; rather, they decrease over time. The number of hairs is unique to each person. On the scalp, a person can have anywhere from 60,000 to 150,000 hairs. 
Diseases and Disorders
Alopecia, or abnormal hair loss, typically occurs in one of three forms:
- Androgenic Alopecia: The most common instance of alopecia, androgenic alopecia is a condition in which the terminal hair diminishes into vellus hair. In women, this results in thinning of the hair on the top regions and crown of the head; approximately 20 million American women suffer from this condition. As for men, they also experience this general thinning, but it often progresses until only a fringe of hair remains around the sides of the head. Affecting an estimated 40 million American men, this male form of androgenic alopecia is often called 'male-pattern baldness.' Androgenic alopecia in both genders results from a variety of factors, including hormone changes, the aging process, and genetics. Although people can experience this disorder in their teenage years, it is more commonly seen around the age of forty.
- Alopecia Areata: Technically, alopecia areata is a relatively common skin condition, but it's a disorder in which the hair follicles are attacked by the body's autoimmune system. This disorder usually appears in childhood, although it can affect anyone of any age, gender, or race. The hair loss resulting from alopecia areata is rather sporadic and unpredictable in size or duration. Lost in patches, sometimes the hair will completely regrow in a bald region or the region may spread and/or "progress" over time. There are varying levels of severity in this disorder, including complete baldness on the head (alopecia totalis), or the loss of hair anywhere on the body (alopecia universalis). Among men, 10% experience alopecia barbae, and are unable to grow facial hair. The unpredictable nature of this disorder can significantly impact a person's ability to function normally at work, in school, and in society. In the U.S., more than 4.7 million people have some form of alopecia areata.
- Postpartum Alopecia (or Telogen Effluvium): A newborn mother may experience this kind of hair loss as a result of the fluctuation of hormones during pregnancy. The hormones affect the regular growth cycle of the hair follicles, which can lead to sudden and significant hair loss for three to nine months following the pregnancy. Fortunately, despite the trauma incurred by this hair loss, the growth pattern usually resumes its normal cycle within a year.
Trichorrhexis Nodosa (Trichonodosis)
When studied under a microscope, this focal defect in the hair fiber reveals many swollen and frayed nodes along the hair fiber. Of all hair shaft defects, trichorrhexis nodosa is one of the most common. The focal defects result from exposure of the cortex layer of the hair fiber in the absence of the protective cuticle layer. Lacking structural support and protection from chemicals and environmental factors, the cortex layer is left exposed and damaged. Chemicals reduce the fiber's strength and the hair becomes weak and flexible, often fraying or splitting.
Trichorrhexis nodosa comes in two forms: proximal and distal. Proximal trichorrhexis nodosa typically affects Afro-Caribbean people due to the great amount of stress on their tight, curly hair. As the most severe type of this disorder, it increases the fragility of hair. Also frequently found in black people, a hair problem called acne keloidalis nuchae leads to the formation of severe pustules on the posterior of the scalp or on the neck. Distal trichorrhexis nodosa is mild in comparison, causing only a few whitish nodes on the hair shaft and overall brittleness, dullness, or dryness of the hair's surface. Asian and Caucasians experience this form more often because it affects straight hair.
Trichorrhexis nodosa can either be acquired or congenital. The acquired type stems from over-processing or "excessive hair manipulation" from perming, styling, dying, brushing, etc. that places too much stress on the hair fibers. Far less common than the acquired form, congenital trichorrhexis is typically hereditary, developing early in life and present from birth.
Loose Anagen Syndrome
In loose anagen syndrome, the hair fiber is not properly anchored in the follicle and the root sheaths of the hair shaft don't develop normally. Thus, the hair falls out of the follicle easily. This condition is found primarily in young (2-5 year old) blond-haired children, particularly girls. The disorder leads to thin head hair, especially on the back of the scalp where it encounters more friction (repeated contact with a pillow at night.) People with loose anagen syndrome have short hair that can be unmanageable, and rarely do they require a haircut. Although most common in children, adults can develop this condition, and those that do typically experience more persistent hair loss. With age, the disorder gradually improves. It is thought that there may be a genetic link to loose anagen syndrome, but as of yet there is no scientific evidence to support this theory.
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