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Hair is composed of Keratin, a special protein that also produces our fingernails and toenails the nails and forms the protective outer layer of our skin. Each strand of hair consists of three concentric layers, the cuticle, the cortex and the medulla.
The outer layer is called the cuticle and is thin and colorless, its job is to protect the thicker cortex which contains the melanin. Melanin is responsible for the color of your hair and the actual color depends on what kind of melanin you have . There are two kinds, eumelanin which creates brown or black hair, and pheomelanin which makes hair appear red. Blonde hair is a result of very low amounts of melanin, the shade of the blonde again depends upon which type of melanin you have. Gray hair is a result of a lack of melanin which is often caused by age but can also be caused by stress and illness.
The innermost layer of hair is called the medulla and reflects light giving hair the various color tones it has. That's why hair color looks a lot different in sunlight than it does in the shade.
Whether you have straight or curly hair depends on the shape of the cortex. If the cross section of the cortex is round you will have straight hair, if the cortex is oval shaped you will have curly or wavy hair.
The physical thickness and length of hair depends on what type of hair it is. Vellus hair is the fine fuzz type of hair that's often called peach fuzz. Its very fine and colorless and often almost invisible to the naked eye.
Hair that is dark and very visible is known as Terminal hair. Terminal hair is the hair that we refer to when talk about hair. Whether a hair is a fine vellus hair or a thick dark hair depends entirely on the follicle that is producing the hair. In balding men thick terminal hair is often replaced by fine vellus hair. This is a result of the hair producing equipment, the follicle, suffering physical damage and being unable to produce terminal hair.
The average Caucasian person has 5 million hairs of which 100,000 - 150,000 are on the head. Blondes not only have more fun, they also have more hair, about 140,000 more than average, Brunettes have slightly higher than average hair about 105,000 hairs, and redheads have a little less than average about 90,000 hairs.
Hair grows in a cycle that consists of three phases: Anagen, Catagen and Telogen
Anagen - This is the growth phase that lasts between two and eight years. During the anagen phase the growth cells in the papilla rapidly divide and produce the hair shaft which becomes keratinized as it pushes up and out of the follicle into the pore. At the same time, the follicle grows down into the deeper levels of the dermis (skin) to get nourishment. People who have long anagen growth rates are able to grow very long hair and others have short growth phases and cannot grow very long hair. Hair grows at a rate of about a ½ inch per month, so a hair left uncut will grow to a length of between 12 inches and 48 inches.
Catagen - The Anagen phase is followed by a brief two to four week Catagen phase or transitional phase. This is part of a renewal process where the follicle is literally degraded and the hair stops growing but does not fall out. During the Catagen phase the hair follicle shrinks to about 1/6 of the normal length. The lower part is destroyed, the dermal papilla breaks away, the bulb detaches from the blood supply and the hair shaft eventually is pushed up as the follicle disintegrates.
Telogen - The follicle then goes into the Telogen or resting phase for two to four months, during this time the hair still does not grow but remains attached to the follicle while the dermal papilla is in a resting phase below. Approximately 10-15 percent of all hairs are in this phase at any one time.
After the Telogen phase the cycle is complete and the hair goes back into the Anagen phase. It is at this time when the new hair shaft is forming that the old hair is pushed out and lost.
On average 50-100 hairs are lost due to this natural growth process every day. This is normal hair loss and accounts for the hair loss seen every day in the shower and with hair combing. In healthy follicles these hairs will soon be replaced by new hair.
A variety of factors can alter the normal hair growth cycle and cause temporary or permanent hair loss including medication, radiation, chemotherapy, exposure to chemicals, hormonal and nutritional factors, thyroid disease, generalized or local skin disease, and stress.