Bma Illustrated Medical Dictionary

injury. First, small blood vessels con-
strict. Second, small blood cellscalled
platelets aggregate and plug the bleed-
ing points. Third, the plasma coagulates,
forming filaments of a substance called
fibrin, which help to seal the damaged
blood vessel (see blood clotting). Defects
in any of these mechanisms can cause
a bleeding disorder.
haemostatic drugsA group of drugs
used to treat bleeding disorders and to
control bleeding. Haemostatic prepara-
tions that help blood clotting are given
to people who have deficiencies of nat-
ural clotting factors. For example, factor
VIIIis used to treat haemophilia. Drugs
that prevent the breakdown of fibrin in
clots, such as tranexamic acid, can also
improve haemostasis.
haemothoraxA collection of blood in
the pleural cavity (see pleura). Haemo-
thorax is most commonly caused by chest
injury, but it may arise spontaneously in
people with defects of blood coagula-
tion or as a result of cancer. Symptoms
include pain in the affected side of the
chest and upper abdomen, and breath-
lessness. If extensive, there may be
partial lung collapse. Blood in the pleu-
ral cavity is withdrawn through a needle.
hairA thread-like structure composed of
dead cells containing keratin, a fibrous
protein. The root of each hair is embed-
ded in a tiny pit in the dermis layer of
the skincalled a hair follicle. Each shaft
of hair consists of a spongy semihollow
core (the medulla), a surrounding layer
of long, thin fibres (the cortex), and, on
the outside, several layers of overlap-
ping cells (the cuticle). While a hair is
growing, the root is enclosed by tissue
called a bulb, which supplies the hair
with keratin. Once the hair has stopped
growing, the bulb retracts from the root
and the hair eventually falls out.
Hair is involved in the regulation of
body temperature (known as thermo-
regulation). If the body is too cold,
arrector pili muscles in the skin con-
tract, pulling the hairs upright to form
goose pimples. Erect hairs trap an insu-
lating layer of air next to the skin.
Brittle hair may be due to excessive
styling, hypothyroidism, or severe vita-
min or mineral deficiency. Very dry hair

can be caused by malnutrition. Ingrown

hairs occur when the free-growing end

of the hair penetrates the skin near the

follicle, which may cause inflammation.

(See also hirsutism; hypertrichosis.)

hairballA ball of hair in the stomach,

found in people who nervously suck or

chew their hair (see bezoar).

hairiness, excessive See hirsutism;

hypertrichosis.

hair removalHair is usually removed

from the body for cosmetic reasons. It

may also be shaved from around an

incision site before surgery. Temporary

methods include shaving, waxing, dep-

ilatorycreams, and waxing; electrolysisis

the only permanent method of removal.

hair transplantA cosmetic operation

in which hairy sections of scalp are

removed and transplanted to hairless

areas to treat alopecia(baldness). There

are several different techniques.

In strip grafting, a strip of skin and hair

is taken from a donor site, usually at the

back of the scalp or behind the ears. The

removed hairs and their follicles are then

inserted into numerous incisions made

in a bald area, known as the recipient site.

The procedure usually takes 60–90 min-

utes. The patient is given a mild sedative

and anaesthetic on the donor and recipi-

ent sites. The donorsite heals in about

HAEMOSTATIC DRUGS HAIR TRANSPLANT

H

Sebaceous

gland

Fat

Sweat gland

Epidermis

Hair shaft

HAIR

Dermis

Arrector pili

muscle

Hair

bulb

Hair

follicle