Encyclopedia of Health and Medicine

H
hair 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 AREATA
DANDRUFF
FILLICULTIS
FURUNCLE
HYPOTHYROIDISM
KERATOSIS PILARIS
MENOPAUSE
PREGNANCY
ringworm
SYSTEMIC LUPUS ERYTHEMATOSUS
(SLE)
ALOPECIA
ERYTHEMATOSUS (SLE)
DISCOID LUPUS ERYTHEMATOSUS
(DLE)
HIRSUTISM
INGROWN HAIR
LICHEN PLANUS
nutritional deficiencies
PUBERTY
stress
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.
hair replacement A surgical procedure, also
called hair transplantation, to relocate viable HAIR
follicles from sites on the scalp where they are
abundant to sites where there has been permanent
hair loss. The most common reason for hair
replacement is androgenic ALOPECIA (male pattern
hair loss). Hair replacement is nearly always a cos-
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