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Hair

Human Hair

Hair is a protein filament that grows from follicles found in the dermis, or skin. Hair is one of the defining characteristics of mammals. The human body, apart from areas of glabrous skin, is covered in follicles which produce thick terminal and fine vellus hair. 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

Attitudes towards different hair, such as hairstyles and hair removal, vary widely across different cultures and historical periods, but it is often used to indicate a person’s personal beliefs or social position, such as their age, sex life, or religion

Function

Many mammals have fur and other hairs that serve different functions. Hair provides thermal regulation and camouflage for many animals; for others it provides signals to other animals such as warnings, mating, or other communicative displays; and for some animals hair provides defensive functions and, rarely, even offensive protection.

Hair also has a sensory function, extending the sense of touch beyond the surface of the skin. Guard hairs give warnings that may trigger a recoiling reaction.

Warmth

Polar bears use their fur for warmth and while their skin is black, their transparent fur appears white and provides camouflage while hunting and serves as protection by hiding cubs in the snow.

Humans have developed clothing and other means of keeping warm, 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 function of hair in other parts of the body has been questioned. Hats and coats are still required while doing outdoor activities in cold weather to prevent frostbite and hypothermia, but the hair on the human body does help to keep the internal temperature regulated.

When the body is too cold, the arrector pili muscles found attached to hair follicles stand up, causing the hair in these follicles to do the same. These hairs then form a heat-trapping layer above the epidermis.

This process is formally called piloerection, derived from the Latin words ‘pilus’ (‘hair’) and ‘erectio’ (‘rising up’), but is more commonly known as ‘having goose bumps‘ in English.[12] This is more effective in other mammals whose fur fluffs up to create air pockets between hairs that insulate the body from the cold.

The opposite actions occur when the body is too warm; the arrector muscles make the hair lie flat on the skin which allows heat to leave.

Protection

In some mammals, such as hedgehogs and porcupines, the hairs have been modified into hard spines or quills.

These are covered with thick plates of keratin and serve as protection against predators.

Thick hair such as that of the lion’s mane and grizzly bear’s fur do offer some protection from physical damages such as bites and scratches.

Touch sense

Displacement and vibration of hair shafts are detected by hair follicle nerve receptors and nerve receptors within the skin.

Hairs can sense movements of air as well as touch by physical objects and they provide sensory awareness of the presence of ectoparasites.[13] Some hairs, such as eyelashes, are especially sensitive to the presence of potentially harmful matter.

Eyebrows and eyelashes

Eyelashes and eyebrows help to protect the eyes from dust, dirt, and sweat.

The eyebrows provide moderate protection to the eyes from dirt, sweat and rain. They also play a key role in non-verbal communication by displaying emotions such as sadness, anger, surprise and excitement. In many other mammals, they contain much longer, whisker-like hairs that act as tactile sensors.

 

 

The eyelash grows at the edges of the eyelid and protects the eye from dirt. The eyelash is to humans, camels, horses, ostriches etc., what whiskers are to cats; they are used to sense when dirt, dust, or any other potentially harmful object is too close to the eye.[18] The eye reflexively closes as a result of this sensation.

Structure

The word “hair” usually refers to two distinct structures:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

  1. the part beneath the skin, called the hair follicle, or, when pulled from the skin, the bulb. This organ is located in the dermis and maintains stem cells, which not only re-grow the hair after it falls out, but also are recruited to regrow skin after a wound.[2]
  2. the shaft, which is the hard filamentous part that extends above the skin surface.

A cross section of the hair shaft may be divided roughly into three zones.

Hair fibers have a structure consisting of several layers, starting from the outside: Each strand of hair is made up of the medulla, cortex, and cuticle.

 

 

 

 

 

 

 

 

 

 

Medulla

the medulla, a disorganized and open area at the fiber’s center

The innermost region, the medulla, is not always present and is an open, unstructured region.[5]

Cortex

the cortex, which contains the keratin bundles in cell structures that remain roughly rod-like.

The highly structural and organized cortex, or middle layer of the hair, is the primary source of mechanical strength and water uptake.

The cortex contains melanin, which colors the fiber based on the number, distribution and types 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.

People with straight hair have round hair fibers. Oval and other shaped fibers are generally more wavy or curly.

Cuticle

the cuticle, which consists of several layers of flat, thin cells laid out overlapping one another as roof shingles,

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.[4]

The diameter of human hair varies from 0.017 to 0.18 millimeters (0.00067 to 0.00709 in).[6] There are two million small, tubular glands and sweat glands that produce watery fluids that cool the body by evaporation.

Hair growth begins inside the hair follicle. The only “living” portion of the hair is found in the follicle. The hair that is visible is the hair shaft, which exhibits no biochemical activity and is considered “dead”. The base of a hair’s root (the “bulb”) contains the cells that produce the hair shaft.[8]

Other structures of the hair follicle include the oil producing sebaceous gland which lubricates the hair and the arrector pili muscles, which are responsible for causing hairs to stand up. In humans with little body hair, the effect results in goose bumps.

Hair Growth

Hair is made of a tough protein called keratin. A hair follicle anchors each hair into the skin. The hair bulb forms the base of the hair follicle.

Living cells in the hair bulb divide and grow to build the hair shaft. Blood vessels nourish the cells in the hair bulb, and deliver hormones that modify hair growth and structure at different times of life.

Hair growth occurs in cycles consisting of three phases:

 

 

 

 

 

 

  • Anagen (growth phase): Most hair is growing at any given time. Each hair spends several years in this phase.
  • Catagen (transitional phase): Over a few weeks, hair growth slows and the hair follicle shrinks.
  • Telogen (resting phase): Over months, hair growth stops and the old hair detaches from the hair follicle. A new hair begins the growth phase, pushing the old hair out.

Hair grows at different rates in different people; the average rate is around one-half inch per month.

Hair color

Hair color is created by pigment cells producing melanin in the hair follicle. With aging, pigment cells die, and hair turns gray.

The pigmentation of hair follicles due to two types of melanin: eumelanin and pheomelanin.

 

Eumelanin, which has two subtypes of black or brown, determines the darkness of the hair color.

A low concentration of brown eumelanin results in blond hair, whereas a higher concentration of brown eumelanin results in brown hair. High amounts of black eumelanin result in black hair, while low concentrations result in gray hair.

Generally, if more eumelanin is present, the color of the hair is darker; if less eumelanin is present, the hair is lighter. The darker a person’s natural hair color is, the more individual hair follicles they have on their scalp.

Pheomelanin colors hair orange and red. All humans have some pheomelanin in their hair.

Pheomelanin is more chemically stable than black eumelanin, but less chemically stable than brown eumelanin, so it breaks down more slowly when oxidized. This is why bleach gives darker hair a reddish tinge during the artificial coloring process. As the pheomelanin continues to break down, the hair will gradually become red, then orange, then yellow, and finally white.

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 on the same person.

Particular hair colors are associated with ethnic groups. Gray or white hair is associated with age.

The genetics of hair colors are not yet firmly established. According to one theory, at least two gene pairs control human hair color.

The Fischer–Saller scale, named after Eugen Fischer and Karl Saller (de), is used in physical anthropology and medicine to determine the shades of hair color. The scale uses the following designations: A (very light blond), B to E (light blond), F to L (blond), M to O (dark blond), P to T (light brown to brown), U to Y (dark brown/black) and Roman numerals I to IV (red) and V to VI (red blond).[1]

 

 

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