What Is Hair, Really?

The adult human body averages five million hairs, of which 100,000 to 150,000 are on the scalp. Hair is composed of keratin, the same protein that makes up nails and the outer layer of our skin. The part seen rising out of the skin is called hair shaft or strand. Each strand consist of three layers. The outermost protective layer (cuticle) is thin and colorless. The middle layer, or cortex, is the thicknest. It provides strength, determines your hair color and whether your hair is straight or curly.

Hair color is determined by melanin from your pigment cells. The more pigment granules there are, and the more tightly packed, the darker the hair. Two kinds of melanin contribute to hair color. Eumelanin colors hair brown to black, and an iron-rich pigment, pheomelanin colors it yellow-blonde to red. Whether hair is mousy, brown, brunette or black depends on the type and amount of melanin and how densely it's distributed within the hair. For example, deep-black African hair contains closely packed melanin in the cortex, a few in the cuticle. Very dark European hair, quite apart from having more melanin granules than lighter or blonde hair, has more melanin per granule. When pigment-producing cells cease to function, the result is the uncolored white or gray hair.

In Caucasians, true blonds typically have more hair (about 140,000 hair) than brunette (about 105,000) or redhead (about 90,000).

Below your skin is the hair root which is enclosed by a sack-like structure called the hair follicle. Tiny blood vessels at the base of the follicle provide nourishment. A nearby gland secretes a mixture of fats (called sebum) which keep the hair shiny and waterproof to some extent. At the base of the follicle is the papilla, which is the "hair manufacturing plant." The papilla is fed by the blood-stream which carries .

Male hormones or androgens regulate hair growth. Pubic and axillary (armpit) hair are particularly androgen-sensitive and grow at lower androgen levels than hair on the chest or legs. In boys, most pubic hair is grown by age 15, followed by the development of armpit hair two to three years later.

In girls, too, an increase in androgens at puberty triggers growth of pubic and armpit hair. Scalp hair, not directly androgen-responsive, is influenced by local amounts of a testosterone derivative, dihydrotestosterone.

What is the normal cycle of hair growth and loss?
The normal cycle of hair growth lasts for 2 to 6 years. Each hair grows approximately 1 centimeter (less than half an inch) per month during this phase. About 90 percent of the hair on your scalp is growing at any one time. About 10 percent of the hair on your scalp, at any one time, is in a resting phase. After 2 to 3 months, the resting hair falls out and new hair starts to grow in its place.

It is normal to shed some hair each day as part of this cycle. However, some people may experience excessive (more than normal) hair loss. Hair loss of this type can affect men, women and children.

What causes excessive hair loss?
A number of things can cause excessive hair loss. For example, about 3 or 4 months after an illness or a major surgery, you may suddenly lose a large amount of hair. This hair loss is related to the stress of the illness and is temporary.

Hormonal problems may cause hair loss. If your thyroid gland is overactive or underactive, your hair may fall out. This hair loss usually can be helped by treatment of the thyroid. Hair loss may occur if male or female hormones, known as androgens and estrogens, are out of balance. Correcting the hormone imbalance may stop your hair loss.

Many women notice hair loss about 3 months after they have had a baby. This loss is also related to hormones. During pregnancy, high levels of certain hormones cause the body to keep hair that would normally fall out. When the hormones return to pre-pregnancy levels, that hair falls out and the normal cycle of growth and loss starts again.
Some medicines can cause hair loss. This type of hair loss improves when you stop taking the medicine. Medicines that can cause hair loss include blood thinners (also called anticoagulants), medicines used for gout, medicines used in chemotherapy to treat cancer, vitamin A (if too much is taken), birth control pills and antidepressants.

Certain infections can cause hair loss. Fungal infections of the scalp can cause hair loss in children. The infection is easily treated with antifungal medicines or herbal formula.
Finally, hair loss may occur as part of an underlying disease, such as lupus or diabetes. Since hair loss may be an early sign of a disease, it is important to find the cause so that it can be treated.

Can improper care of my hair cause hair loss?
Yes. If you wear pigtails or cornrows or use tight hair rollers, the pull on your hair can cause a type of hair loss called traction alopecia (say: al-oh-pee-sha). If the pulling is stopped before scarring of the scalp develops, your hair will grow back normally. However, scarring can cause permanent hair loss. Hot oil hair treatments or chemicals used in permanents (also called "perms") may cause inflammation (swelling) of the hair follicle, which can result in scarring and hair loss.

What is common baldness?
The term "common baldness" usually means male-pattern baldness, or permanent-pattern baldness. Male-pattern baldness is the most common cause of hair loss in men. Men who have this type of hair loss usually have inherited the trait. Men who start losing their hair at an early age tend to develop more extensive baldness. In male-pattern baldness, hair loss typically results in a receding hair line and baldness on the top of the head.
Women may develop female-pattern baldness. In this form of hair loss, the hair can become thin over the entire scalp.

Can my doctor do something to stop hair loss?
Perhaps. Your doctor will probably ask you some questions about your diet, any medicines you're taking, whether you've had a recent illness and how you take care of your hair. If you are a woman, your doctor may ask questions about your menstrual cycle, pregnancies and menopause. Your doctor may want to do a physical exam to look for other causes of hair loss. Finally, blood tests or a biopsy (taking a small sample of cells to examine under a microscope) of your scalp may be needed.Sources
Partly written by familydoctor.org editorial staff
American Academy of Family Physicians