hair is also an important biomaterial primarily composed of protein, notably keratin.
Hair can grow on most areas of the body, except palms of the hands and soles of the feet.
Hair is defined as a protein filament that grows through the epidermis from follicles deep within the dermis. Hair can grow on most areas of the human body, except on the palms of the hands and the soles of the feet (among other areas), but hair is most noticeable in most people in a small number of areas, which are also the ones that are most commonly trimmed, plucked, or shaved. These include the face, nose, ears, head, eyebrows, eyelashes, legs and armpits, as well as the pubic region.
Most common interest in hair is focused on hair growth, hair types and hair care, but hair is also an important biomaterial primarily composed of protein, notably keratin.
Hair can be divided into three parts length-wise:
In cross-section, there are also three parts:
Human scalp hair normally grows at a rate of 0.4 mm /day (incidentally human scalp hair grows at a rate four times that of human nails. Human nails grow at a rate of 0.1 mm/day). It is commonly accepted that emotional distress, especially that caused by verbal abuse, etc. often is a contributing factor to a slower hair growth rate.
Dandruff is due to excessive shedding of dead skin cells from the scalp, and can also be caused by frequent exposure to extreme heat and cold. As it is normal for skin cells to die and flake off, a small amount of flaking is normal and in fact quite common. Some people, however, either chronically or as a result of certain triggers, experience an unusually large amount of flaking, which can also be accompanied by redness and irritation.
Usually defined as fine, medium, coarse, wiry or frizzy hair depending on the hair diameter.
Within these four texture ranges hair can also be thin, medium or thick density and it can be straight, curly, 'kinky' (tightly coiled), or wavy.
Hair conditioner also affects hair texture. Hair can be healthy, normal, oily, dry, damaged or a combination. Hair texture can also be affected by hair styling equipment such as straighteners, crimper's, or curlers.
Curly and/or 'kinky' hair has a different biological structure from straight hair. It tends to be much drier than straight hair because the oils secreted into the hair shaft by the sebaceous glands can more easily travel down the shaft of straight hair. People with very curly hair may find that this hair type can be dry and often frizzy.
Gray hair is considered to be a characteristic of normal aging.
The tendency of older people to develop gray hair is due to a build up of hydrogen peroxide due to wear and tear of the hair follicles. The age at which this occurs varies from person to person, but in general nearly everyone 75 years or older has gray hair, and in general men tend to become gray at younger ages than women.
Red hair usually doesn't turn gray with age; rather it becomes a sandy color and afterward turns white.
Often drugs used in cancer chemotherapy will frequently cause a temporary loss of hair, most noticeable on the head and eyebrows, because they kill all rapidly dividing cells, not just the cancerous ones.
Other diseases and traumas can cause temporary or permanent loss of hair, either generally or in patches. Patients with Hyperthyroidism or Hypothyroidism can experience hair loss until their hormone levels are regulated.
Involves the state of lacking hair where it often grows, especially on the head. The most common form of baldness is a progressive hair thinning condition called androgenic alopecia. The amount and patterns of baldness can vary greatly; it ranges from male and female pattern alopecia (androgenic alopecia, also called androgenetic alopecia or alopecia androgenetica), alopecia areata, which involves the loss of some of the hair from the head, and alopecia totalis, which involves the loss of all head hair, to the most extreme form, alopecia universalis, which involves the loss of all hair from the head and the body.
Treatments for various forms of alopecia have limited success. Some hair loss sufferers make use of clinically proven treatments such as finasteride and topically applied minoxidil in an attempt to prevent further loss and regrow hair. As a general rule, it is easier to maintain remaining hair than it is to regrow; however, the treatments mentioned may prevent hair loss from Androgenetic alopecia, and there are new technologies in cosmetic transplant surgery and hair replacement systems that can be completely undetectable.
In the USA, there are only two drug-based treatments that have been approved by the U.S. Food and Drug Administration (FDA) and one product that has been cleared by the FDA for the treatment of androgenetic alopecia, they are finasteride (marketed for hair loss as Propecia) and minoxidil.
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