in which the atria beat regularly and
very rapidly. Symptoms and treatment
are the same as for atrial fibrillation.
atrial natriuretic peptideA substance
produced in special cells in the muscu-
lar wall of the atria (see atrium) of the
heart. Atrial natriuretic peptide is re-
leased into the bloodstream in response
to swelling of the atrial muscle due, for
example, to heart failureor hypertension.
It lowers blood pressure by increasing
the amount of sodium excreted in the
urine, which reduces blood volume.
atrial septal defect (ASD)A congeni-
tal heart abnormality (see heart disease,
congenital) in which there is a hole in
the dividing wall (see septal defect)
between the heart’s 2 upper chambers,
or atria (see atrium).
atrioventricular nodeA small knot of
specialized muscle cells in the right
atrium of the heart. Electrical impulses
from the sinoatrial nodepass through
the atrioventricular node and along
conducting fibres to the ventricles,
causing them to contract.
atriumEither of the 2 (right and left)
upper chambers of the heartthat collect
blood from the body and lungs res-
pectively. The atria
open directly into
the ventricles.
atrophyWasting
away or shrinkage
of a normally de-
veloped tissue or
organ due to a re-
duction in the size
or number of its
cells. Atrophy is
commonly caused
by disuse or inade-
quate cell nutrition
due to poor blood
circulation. It may
also occur during
prolonged illness,
when the body needs to use up the pro-
tein reserves in muscles. In some
circumstances, atrophy is a normal pro-
cess, as in ovarian atrophy in women
who have passed the menopause.
atropineAn anticholinergic drugder-
ived from belladonna. Atropine is used
to dilate the pupil in eye conditionssuch as iritis(inflammation of the iris)
and corneal ulcer. It is also used in chil-
dren before eye examination. Atropine
may be included in a premedication
before general anaesthesia to reduce
respiratory secretions and is also used
as an emergency treatment for brady-
cardia(abnormally slow heartbeat). It is
sometimes combined with an anti-
diarrhoeal drugto relieve abdominal
cramps accompanying diarrhoea.
Adverse effects include dry mouth,
blurred vision, retention of urine, and,
in the elderly, confusion. Atropine eye-
drops are rarely given to adults because
they cause disturbance of vision lasting
2–3 weeks and may precipitate acute
glaucomain susceptible people.
attachmentAn affectionate bond be-
tween individuals, especially between a
parent and child (see bonding), or a per-
son and an object, as in a young child
and a security blanket. The term is also
used to refer to the site at which a mus-
cle or tendon is attached to a bone.
attention deficit hyperactivity dis-
order (ADHD)A behavioural disorder
in which a child has a consistently high
level of activity and/or difficulty in
attending to tasks. Attention deficit
hyperactivity, or hyperkinetic, disorder
affects up to 1 in 20 children in the UK.
The disorder, which is more common in
boys, should not be confused with the
normal boisterous conduct of a healthy
child. Children with ADHD consistently
show abnormal patterns of behaviour
over a period of time. An affected child
is likely to be restless, unable to sit still
for more than a few moments, inatten-
tive, and impulsive.
The causes of ADHD are not fully
understood, but the disorder often runs
in families, which suggests that genetic
factors may be involved. ADHD is not,
as popularly believed, a result of poor
parenting or abuse.
Symptoms develop in early childhood,
usually between the ages of 3 and 7,
and may include inability to finish
tasks; short attention span; inability to
concentrate in class; difficulty in follow-
ing instructions; a tendency to talk
excessively, frequently interrupting other
people; difficulty in waiting or takingATRIAL NATRIURETIC PEPTIDE ATTENTION DEFICITA
Left atrium
of heartATRIUMRight
atrium of
heart
STRUCTURE OF HEART