all about hair

What hair really is?

How you come to have the kind of hair you do?

Why some people lose their hair?

Why some people grow their hair long and others cut it off?

Please answer me so that I can do the book report

1 個解答

  • 1 十年前

    Hair is a filamentous outgrowth of dead cells from the skin, found only in mammals. Although many other life forms, especially insects, show filamentous outgrowths, these are not considered "hair" regarding the accepted meaning of the term.

    The primary component of hair fiber is keratin. Keratins are proteins, long chains (polymers) of amino acids. Keratin proteins form the cytoskeleton (miniature skeleton within a cell) of all epidermal cells. Keratin filaments run within a cell from the inside of the outer membrane to weave a "basket" around the nucleus of the cell. Keratins are a principle part of the cells in the epidermis, hair, nails, feathers, hooves, horny tissues and the enamel of teeth.

    In some species, it is absent at certain stages of life. It projects from the epidermis, though it grows from hair follicles deep in the dermis. So-called "hairs" (trichomes) are also found on plants. The projections on arthropods, such as insects and spiders are actually insect bristles. The hair of non-human species is commonly referred to as fur. There are varieties of cats, dogs, and mice bred to have little or no visible hair.

    Head hair

    Traditional 1950s American men's hairstyleThe most noticeable part of human hair is the hair on the head, which can grow longer than on most mammals and is more dense than most hair found elsewhere on the body. The average human head has about 100,000 hair follicles.[2] Its absence is termed alopecia, commonly known as baldness. Anthropologists speculate that the functional significance of long head hair may be adornment, a byproduct of secondary natural selection once other somatic hair had been lost. Another possibility is that long head hair is a result of Fisherian runaway sexual selection, where long lustrous hair is a visible marker for a healthy individual (with good nutrition, waist length hair—approximately 1 meter or 39 inches long—would take around 80 months, or just under 7 years, to grow).

    Each follicle can grow about 20 individual hairs in a person's lifetime.[1] Average hair loss is about 100 strands a day.

    The average human scalp measures approximately 120 sq. inches (770 "Hair density depends upon race and hair color. Caucasians have the highest hair density; the growth rate is average. Blacks have medium density and slowest growing hair. Asians have the lowest density but the fastest growing hair."[2]

    Average number of head hairs (Caucasian)[3]

    color number of hairs diameter

    Blonde 146,000 1/1500th - 1/500th in.

    Black 110,000 1/400th - 1/250th in.

    Brunettes 100,000

    Red 86,000