anions in the extracellular fluid and
blood, and by phosphate anions inside
cells (see acid–base balance).
ionizerA device that produces ions
(electrically charged particles). Ionizers
that produce negative ions can be used
to neutralize positive ions in the
atmosphere. Some people believe that
use of an ionizer reduces symptoms,
such as headaches and fatigue, that
may result from a build-up of positive
ions generated by electrical machines.
ipecacuanhaA drug (also called ipecac)
used to induce vomiting in the treatment
of types of poisoning.
ipratropium bromideA bronchodilator
drugused to treat breathing difficulties.
IQThe abbreviation for intelligence
quotient, an age-related measure of
intelligence (see intelligence tests).
iridectomyA procedure performed on
the eye to remove part of the iris. The
most common type of iridectomy, known
as a “peripheral iridectomy”, is usually
performed to treat acute glaucoma. A
small opening is made, surgically or
with a laser, near the outer edge of the
iris to form a channel through which
aqueous humour can drain.
iridocyclitisInflammation of the iris
and ciliary body. Iridocyclitis is more
usually known as “anterior uveitis”. (See
also eye, disorders of.)
iridotomyA surgical procedure per-
formed on the eye, in which an incision
is made in the irisusing a knife or a
laser. (See also iridectomy.)
irisThe coloured part of the eye, made
up of a loose framework of transparent
collagenand muscle fibres, that lies
behind the cornea and in front of themay be needed before a successful
pregnancy is achieved. Modifications of
the technique, such as gamete intrafal-
lopian transfer(GIFT), are simpler and
cheaper than the original method.
in vivoBiological processes occurring
within the body. (See alsoin vitro.)
involuntary movementsUncontrolled
movements of the body. These move-
ments occur spontaneously and may be
slow and writhing (see athetosis); rapid,
jerky, and random (see chorea); or pre-
dictable, stereotyped, and affecting 1
part of the body, usually the face (see
tic). They may be a feature of a disease
(for example, Huntington’s disease) or a
side effect of certain drugs used to treat
psychiatric conditions.
iodineAn element essential for forma-
tion of the thyroid hormones, triiodothy-
ronine (T3) and thyroxine (T4), which
control the rate of metabolism(internal
chemistry) and growth and development.
Dietary shortage may lead to goitre or
hypothyroidism. Deficiency in the new-
born can, if left untreated, lead to
cretinism. Shortages are very rare in deve-
lopedcountries due to bread and table
salt being fortified with iodide or iodate.
Radioactive iodine is sometimes used
to reduce thyroid gland activity in cases
of thyrotoxicosis and in the treatment of
thyroid cancer. Iodine compounds are
used as antiseptics, in radiopaque con-
trast media in some X-ray procedures
(see imaging techniques), and in some
cough remedies.
ionA particle that carries an electrical
charge; positive ions are called cations
and negative ions are called anions.
Many vital body processes, such as the
transmission of nerve impulses, depend
on the movement of ions across cell
membranes. Sodium is the principal
cation in the fluid that bathes all cells
(extracellular fluid). It affects the flow of
water into and out of cells (see osmosis),
thereby influencing the concentration
of body fluids.
The acidity of blood and other body
fluids depends on the level of hydrogen
cations, which are produced by meta-
bolic processes. To prevent the fluids
from becoming too acidic, hydrogen
cations are neutralized by bicarbonateIN VIVO IRISI
Iris PupilIRIS