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Hair

From Academic Kids

Long braided hair
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Long braided hair

Hair is the filamentous outgrowth of the epidermis found in mammals. It is a characteristic of all mammals, though in some species hair is absent at certain stages of life. "Hairs" are also found on plants, the technical term for which is trichomes (see for further discussion of plant hairs).

Hair serves a number of different functions. It provides insulation from cold weather and, in some species, from particularly hot weather. Because hair is often pigmented, it provides coloration. This might serve to camouflage an individual; in some mammals, the pigmentation changes with the seasons, becoming white during the snowy winter, for example.

In modern Western societies, it is considered manly for men to have hair on their faces, arms, chests and legs, but the hair growing from the top of the head is generally kept short, relatively speaking; equally, it is considered feminine for women to have no hair on their bodies, with the exception of pubic hair, but to have a lot of it on the tops of their heads. This is a fairly recent development. Before the First World War men generally had long hair and beards. The trench warfare between 1914 and 1918 exposed men to lice and flea infestation which caused the order to be given for the routine cutting of hair to a severely short length. The shorter style became the new normality and has never entirely gone away since.

The hair of non-human animal species is commonly referred to as fur.

Contents

Human hair

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Typically, humans have more hair on the top of the head, and also hair where extremities meet the torso (axillary (arm-pit) hair, and pubic hair), on the eyelids and above them (eyebrows). In most societies people shave, style or adorn their hair for aesthetic reasons.

Sometimes, the term body hair is used, to distinguish hair on the body from hair on the head. All hairs alternate regular periods of growth and dormancy. During the growth portion of the cycle, hair follicles are long and bulbous, and the hair advances outward at about a third of a millimeter per day. After three to six months, body hair growth stops (the pubic and axillary areas having the longest growth period). The follicle shrinks and the root of the hair rigidifies. Following a period of dormancy, another growth cycle starts, and eventually a new hair pushes the old one out of the follicle from beneath. Head hair, by comparison, grows for a long duration and to a great length before being shed. Anthropologists speculate that the functional significance of long head hair may be adornment, a by-product of secondary natural selection once other somatic hair had been lost.

Unlike other animals, human beings often have their hair cut or remove it by shaving or other means.

It is important to note that hair grows across all areas of the skin on humans regardless of sex or race except in the following locations: the lips, the nipples, the palms of hands, the soles of feet, certain external genital areas, and the navel. Some people may seem to have less body hair than others. In fact the difference is that some people have shorter and thinner body hair than others. Overall coverage in terms of number of folicles is relatively constant.

Several theories have been advanced to explain the unique features of human hair. One theory suggests that nature selected humans for shorter and thinner body hair as part of a set of adaptations including bipedal locomotion and an upright posture. There are several problems with this theory, not least of which is that cursorial hunting is used by (other) animals that do not show any thinning of hair, and that hair similar to Chimpanzees and Gorillas also shades the skin from radiant heat and protects it from hot winds, and thus another mechanism for heat loss is not required. Another problem is that bipedal locomotion possibly predates hominids moving from a forest environment to a savanna environment. A more recent theory for human hair loss has to do with a possible period of bipedal wading in a salt marsh in the Danakil region of Ethiopia, possibly occurring in the hominid lineage between 5 and 7 million years ago. As a wading animal, it was more efficient to develop short body hair and a layer of subcutaneous fat for streamlining and insulation in the aquatic environment; the eccrine sweat glands developed later after the hominids left the water; see Aquatic ape hypothesis. One problem with this theory is that both Chimpanzees and Gorillas have the same density and distribution of the eccrine glands, but that they have not been developed for sweat production. A third theory proposes that Sexual Selection played a role, possibly in conjunction with Neoteny, with the more juvenile appearing females being selected by males as more desirable; see #Types_of_hair and Vellus hair. This would also explain the sexual dimorphism in human body hair. At this point the evidence is inconclusive as to the cause of the unique features of human hair.

Structure

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Hair is a biological polymer; over 90% of its dry weight is made up of proteins called keratins. Under normal conditions, human hair contains around 10% water, which modifies its mechanical properties considerably. Hair proteins are held together by disulfide bonds, from the amino acid cysteine. These links are very robust; for example, virtually intact hair has been recovered from ancient Egyptian tombs. Different parts of the hair have different cysteine levels, leading to harder or softer material.

Structurally, hair consists of an inner cortex, comprising spindle-shaped cells, and an outer sheath, called the cuticle. Within each cortical cell are many fibrils, running parallel to the fibre axis, and between the fibrils is a softer material called the matrix. It grows from a hair follicle.

The cuticle is responsible for much of the mechanical strength of the hair fibre. It consists of scale-shaped layers. Human hair typically has 6-8 layers of cuticle. Wool has only one, and other animal hair may have many more layers. Hair responds to its environment, and to its mechanical and chemical history. For example, hair which is wetted, styled and then dried, acquires a temporary 'set', which can hold it in style. This style is lost when the hair gets wet again. For more permanent styling, chemical treatments (perms) break and re-form the disulphide links within the hair structure.

The diameter of a human hair ranges from about 18 ?m to 180 ?m. In people of European descent, blond hair and black hair are at the thinner end of the scale, while red hair is the thickest. The hair of people of Asian descent is typically thicker in diameter than the hair of other groups.

Cross-section shape of human hair is typically round in people of Asian descent, round to oval in European descent, and nearly flat in African peoples; it is that flatness which allows African hair to attain its frizzly form. In contrast, hair that has a round cross-section will be straight. A strand of straight round cross-section hair that has been flattened, for example, with an edge of a coin, will curl up into a micro-afro.

Some persons of African descent have hair that if left alone will grow in tight curls and develop into a unique hairstyle known as the afro; since the development of the concept of Black Power, there has been a debate as to whether such persons should use products that straighten their hair.

The speed of growth is roughly 11 cm/yr = 0.3 mm/day = 3 nm/s. The cells at the base of the hair follicle divide and grow extremely rapidly. This is why people under chemotherapy sometimes lose their hair; the treatment targets any rapidly-dividing cell, not just the cancerous ones.

Hair is strong. A single strand can hold 100g (3.5oz) of weight. A head of hair could support 12 tonnes. It is equivalent in strength to aluminium or Kevlar. Wet hair however is very fragile.

Types of hair

On most adult humans there are two main types of hair: terminal hair, and vellus hair. A third type, lanugo hair, is present in the fetus, and some newborn babies. It can also be seen on the bodies of those who are extremely emaciated.

Terminal hair grows thick and long, and is what grows on the head, armpits and pubic area, as well as on the face, chest, arms and legs (better evident in men).

Vellus hair is a very soft and short hair that grows most places in the body in both sexes. In Caucasians it is often colourless, or blonde. It is best seen in women and children, as they have less terminal hair to obscure it.

Hair change with aging

Older people tend to develop gray hair (actually colorless) because the pigmentation in the hair gets lost and the hair becomes colorless. The age on which this occurs varies from person to person, but in general nearly everyone from 75 years or older, has gray hair, and in general men tend to become gray at younger ages than women.

The older a person is, the more the chance is that he or she has gray hair, and above 85 almost nobody has the original hair color. Gray hair is considered to be a characteristic of normal aging.

People starting out with very pale blond hair usually develop white hair instead of grey hair when aging.

Androgenic hair

The hair follicles on much of the body respond to androgens (primarily testosterone and its derivatives). The rate of hair growth increases and the heaviness of the hairs increases. However, different areas respond with different sensitivities. As testosterone level increases (normally at puberty), the sequence of appearance of sexual (androgenic) hair reflects the gradations of androgen sensitivity. The pubic area is most sensitive, and heavier hair usually grows there first in response to androgens. The following regions also respond to androgens, in order of decreasing sensitivity: axillary and perianal areas, sideburns, above the upper lip, periareolar areas, chin and beard areas, center of chest, arms and legs, across the chest, shoulders, buttocks, back, and abdomen.

It is the hair in these areas that appears earlier or grows to excess in disorders of excess androgen (e.g., precocious puberty, late-onset congenital adrenal hyperplasia, and polycystic ovary syndrome).

Other information

Notable variations in physical appearance of the top and back of the head are:

Hair spray, gel, etc. may be used for fixation of the arrangement and may also make it shiny.

It is commonly claimed that hair and nails will continue growing for several days after death. This is a myth; the appearance of growth is actually caused by the retraction of skin as the surrounding tissue dehydrates, making nails and hair more prominent.


Integumentary system
Skin - Sweat glands - Hair - Nails
Skin
Epidermis (Stratum corneum, Stratum lucidum, Stratum granulosum, Stratum spinosum, Stratum germinativum/basale)
Dermis - Subcutis
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