Hair - definition and function
Hair are the fibers that grow from the hair follicles. Not far above the root of the hair follicle the cells that form the hair fiber are dead, hardened into their shape through compression within the follicle as new cells emerging from the hair’s root push them upward. A hair fiber is five or six cells in thickness and varies in length, depending on its location. Hair on the head can grow to several feet in length, whereas the hair of the eyelashes is generally no longer than about a quarter of an inch long. The hair does not require nourishment from the body, though the secretions of the sebaceous glands help moisturize the hair fibers to keep them supple.
Genetic encoding determines the characteristics of the hair, from how rapidly it grows to whether it is curly or straight. Hair covers all SKIN surfaces except the palms of the hands and the soles of the feet, though is most prominent on the head and, after PUBERTY, in the pubic region, on the legs, and under the arms. Men typically have darker, coarser body hair than women. Specialized hairs line the auditory canals and the inside of the NOSE, functioning to remove debris from these structures.
Like the skin, the hair provides clues to the health of the body. Numerous conditions can change the characteristics of the hair. Such changes reflect circumstances that affect the hair follicles in some way, from physical damage, such as BURNS or scars that can destroy follicles, to immune or disease processes that attack the follicles and disrupt hair growth. Physical stress such as the body experiences with major injury, illness, or surgery can cause various changes in the hair, from altered color and consistency to hair loss.
The hair’s characteristics also change with aging. By midlife the hair typically starts to lose the melanocytes that give it color. Sebum (the natural oil that lubricates the hair follicle) production slows, allowing the hair to become dry. Sun exposure also can alter the hair, lightening its color or extracting moisture to make it brittle. Hair-care products can help restore moisture to the hair on the head as well as to the skin of the scalp.
| Health Conditions that May Involve the Hair | |
|---|---|
| ADVERSE REACTION to a DRUG | ALOPECIA |
| ALOPECIA AREATA | ERYTHEMATOSUS (SLE) |
| DANDRUFF DISCOID LUPUS | ERYTHEMATOSUS (DLE) |
| FILLICULTIS | FURUNCLE |
| HIRSUTISM | HYPOTHYROIDISM |
| INGROWN HAIR | KERATOSIS PILARIS |
| LICHEN PLANUS | MENOPAUSE |
| nutritional deficiencies | PREGNANCY |
| PUBERTY | ringworm |
| stress | SYSTEMIC LUPUS ERYTHEMATOSUS (SLE) |
| toxic exposure | TRICHOTILLOMANIA |
For further discussion of the hair within the context of integumentary structure and function please see the overview section, “The Integumentary System.”
See also AGING, INTEGUMENTARY CHANGES THAT OCCUR WITH; BEZOAR; MELANOCYTE; NAILS; SEBACEOUS GLAND; SWEAT GLANDS.
