Human hair grows everywhere on the body except for the soles of the feet, the lips, the palms of the hands, and the eyelids, apart from eyelashes. Like skin, hair is a stratified squamous, keratinized epithelium made of multi-layered, flat cells with overlying keratin (a protein), whose rope-like filaments provide structure and strength to the hair shaft.

Hair follows a specific growth cycle with three distinct and concurrent phases: anagen, catagen, and telogen phases. Each phase has specific characteristics that determine the length of the hair. All three phases occur simultaneously; one strand of hair may be in the anagen phase, while another is in the telogen phase.


The body has different types of hair, including vellus hair and androgenic hair, each with its own type of cellular construction. The different construction gives the hair unique characteristics, serving specific purposes, mainly warmth and protection

Normal growth

There is no one regimen a person can follow that ensures a healthy head of hair. One's lifestyle, diet and hair type should all be considered when attempting to adopt a routine to develop and maintain healthy hair.
[edit] Exercise

Exercise stimulates the blood to flow. This nutrient-rich blood nourishes each hair follicle, promoting healthy hair growth.[1] Lack of this sufficient blood flow would yield the growth of duller, more brittle hair.[2] Apart from supplying the follicle with nutrient-rich blood, exercise also helps to alleviate stress, which is known to cause hair loss.[3]
[edit] Growth cycle

The three stages of hair growth are the anagen, catagen, and telogen phases. Each strand of hair on the human body is at its own stage of development. Once the cycle is complete, it restarts and a new strand of hair begins to form. The rate or speed of hair growth is about 1.25 centimeters or 0.5 inches per month, or about 15 centimeters or 6 inches per year.[4]
[edit] Anagen phase

The anagen phase is known as the growth phase.[5] It begins in the papilla and can last up to eight years.[6] The span at which the hair remains in this stage of growth is determined by genetics.[7] The longer the hair stays in the anagen phase, the faster and longer it will grow. During this phase, the cells in the papilla divide to produce new hair fibers, and the follicle buries itself into the dermal layer of the skin to nourish the strand.[6] About 85% of the hairs on one's head are in the anagen phase at a given time.[5]
[edit] Catagen phase

Signals sent out by the body determine when the anagen phase ends and the catagen phase begins. The catagen phase, also known as the transitional phase, allows the follicle to, in a sense, renew itself. During this time, which lasts about two weeks, the hair follicle shrinks due to disintegration and the papilla detaches and "rests", cutting the hair strand off from its nourishing blood supply. Ultimately, the follicle is 1/6 its original length, causing the hair shaft to be pushed upward. While hair is not growing during this phase, the length of the terminal fibers increase when the follicle pushes them upward.[6]
[edit] Telogen phase

During the telogen, or resting, phase the hair and follicle remain dormant anywhere from 1–4 months.[6] Ten to fifteen percent of the hairs on one's head are in this phase of growth in any given time.[5] The anagen phase begins again once the telogen phase is complete. The preceding hair strand is pushed up and out by the new, growing strand. The process causes the normal hair loss known as shedding.
[edit] Growth inhibitors and disorders
[edit] Chemotherapy

Most chemotherapy drugs work by attacking fast-replicating cells. Rapid cell replication is one of the hallmarks of cancers, however hair follicle cells also grow and divide quickly; consequently, the chemotherapy drugs usually inhibit hair growth.[8] The dose and type of medicine determine how severe a person experiences hair loss, but once the chemotherapy has ended, one can begin to experience new hair growth after three to 10 months.[8]
[edit] Alopecia-related syndromes

Alopecia is a hair loss disease that can occur in anyone at any stage of life. Specifically Alopecia areata is an autoimmune disease that causes hair to spontaneously fall out. It is mainly characterized by bald patches on the scalp or other parts of the body, and can ultimately cause baldness across the entire body.[9] This disease interferes with the hair growth cycle by causing a follicle to prematurely leave the anagen, or active growth, phase and enter the resting, or telogen phase. The hair growth in the affected follicles is lessened or stopped completely.

Traction alopecia is caused by adding too much strain on the hair on one's head. Tight ponytails and other styles that require added tension to the hair are often what cause this disease. It can also occur on the face in areas where the hair is often styled.[10] Plucking or waxing one's eyebrows frequently, for example, can yield suppressed hair growth in the area.

On the scalp, the hair is usually known to be lost around the hair line, leaving the densest amount of hair at the crown. Small vellus hair will often replace the hair that is lost. In most people, scalp hair growth will halt due to follicle devitalization after reaching a length of generally two or three feet. Exceptions to this rule can be observed in individuals with hair development abnormalities, which may cause an unusual length of hair growth. [10]

References

    ^ "Hair Growth Guidelines to Promote Hair Growth". Stoppinghairlossnow.com. Retrieved 2010-04-21.
    ^ "The Correlation Of Exercise & Healthy Hair - Help you body and hair become more healthy". Healthyhairplus.com. Retrieved 2010-04-21.
    ^ [1]
    ^ How Fast Does Hair Grow?, hairfinder.com, accessed 5 January 2011
    ^ a b c (Hair Structure and Hair Life Cycle)
    ^ a b c d "How Does Hair Grow?" Web. 9 February 2010.
    ^ [2]
    ^ a b "Chemotherapy and hair loss: What to expect during treatment". MayoClinic.com. Retrieved 2010-04-21.
    ^ (Alopecia Areata - Alopecia Areata Overview)
    ^ a b (Traction Alopecia)

http://en.wikipedia.org/wiki/Human_hair_growth