Hair

Hair
Hair, is a characteristic of mammalian class of living beings.Hair follicle in the skin is the place where hair grows.
The active living portion of the hair is the follicle. The hair that is visible is the hair shaft. It has no biochemical activity and is non active.  The active portion of the hair is found in the follicle.The base or the bulb contains the cells that produce the hair shaft.  Other structures of the hair follicle include the oil producing sebaceous gland which lubricates the hair and the erector pili muscles, which are responsible for causing hairs to stand-up.  
  The strand of hair is made of the medulla, cortex and the cuticle. The cortex, or middle layer of the hair, is the primary source of mechanical strength. The cortex contains melanin which gives colour to the hair. The colour depends on the number, distribution and the type of melanin granules.
The shape of the follicle determines the shape of the cortex, and the shape of the fiber is related to how straight or curly the hair is. Asian hair typically has a round fiber and is quite straight. Oval and irregularly-shaped fibers are generally more wavy or even curly. The cuticle is the outer covering. Its complex structure slides as the hair swells and is covered with a single molecular layer of lipid that makes the hair repel water.The diameter of human hair varies from 17 to 180 µm (0.00067 to 0.0071 in).
All natural hair colors are the result of two types of hair pigment eumelanin and pheomelanin. Both of these pigments are melanin types, produced inside the hair follicle. Generally, if more melanin is present, the color of the hair is darker; if less melanin is present, the hair is lighter. Levels of melanin can vary over time causing a person’s hair color to change, and it is possible to have hair follicles of more than one color.Eumelanin is the dominant pigment in dark-blond, brown, and black hair, and pheomelanin is dominant in red hair.Blond hair is the result of having little pigmentation in the hair strand.

Gray hair occurs when melanin decreases or disappears.

The hair found on the head serves as primary sources of heat insulation and cooling when sweat evaporates from soaked hair, as well as protection from ultra-violet radiation exposure.The hair on the human body does help to keep the internal temperature regulated. When the body is too cold, the arrector pili muscles attached to hair follicles cause the hair to stand up. These hairs then form a heat-trapping layer above the epidermis. This process is called piloerection. The opposite actions occur when the body is too warm. The arrector muscles make the hair lay flat on the skin which allows heat to leave.
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