particles). Aggregationis the 1st stage
of blood clotting and helpsto plug
injured vessels. Inappropriate aggre-
gation can have adverse effects; for
example, if aggregation occurs in an
artery, it may result in a thrombosis.
aggressionA general term for a wide
variety of acts of hostility. A number of
factors, including human evolutionary
survival strategies, are thought to be
involved in aggression. Androgen hor-
mones, the male sexhormones, seem to
promote aggression, whereas oestrogen
hormones,the femalesex hormones,
actively suppress it. Age is another
factor; aggression is more common
among teenagers and young adults.
Sometimes, a brain tumour or head
injury leads to aggressive behaviour.
Psychiatric conditions associated with
aggressive outbursts are schizophrenia,
antisocial personality disorder, mania,
and abuse of amfetamines or alcohol.
Temporal lobe epilepsy, hypoglycaemia,
and confusiondue to physical illnesses
are other, less common, medical causes.
agingThe physical and mental changes
that occur with the passing of time.
Aging is associated with degenerative
changes in various organs and tissues,
such as loss of elasticity in the skin and
a progressive decline in organ function.
Mechanical wear and tear causes cumu-
lative damage to the joints, and the
muscles lose bulk and strength. Wound
healing and resistance to infection also
decline. Gradual loss of nerve cells can
lead to reduced sensory acuity and dif-
ficulties with learning and memory.
However, dementiaoccurs in only a
minority of elderly people.
Heredity is an important determinant
of life expectancy, but physical degener-
ation may be accelerated by factors
such as smoking, excessive alcohol in-
take, poor diet, and insufficient exercise.
agitationRestless inability to keep still,
usually as a result of anxiety or tension.
Agitated people engage in aimless,
repetitive behaviour, such as pacing up
and down or wringing their hands, and
they often start tasks without complet-
ing them. Persistent agitation is seen
in anxiety disorders, especially if there is
an underlying physical cause such asalcohol withdrawal. Depressionmay be
accompanied by agitation.
agnosiaAn inability to recognize objects
despite adequate sensory information
about them reaching the brain via the
eyes or ears or through touch. Agnosia
is caused by damage to areas of the
brain that are involved in interpretative
and recall functions. The most common
causes of this kind of damage are stroke
or head injury.
Agnosia is usually associated with just
one of the senses of vision, hearing, or
touch and is described as visual, audit-
ory, or tactile respectively. Some people,
after a stroke that damages the right
cerebral hemisphere, seem unaware of
any disability in their affected left limbs.
This is called anosognosia or sensory
inattention. There is no specific treat-
ment for agnosia, but some interpretative
ability may return eventually.
agonistHaving a stimulating effect. An
agonist drug, sometimes known as an
activator, is one that binds to a sensory
nerve cell (receptor)and triggers or in-
creases a particular activity in that cell.
agoraphobiaFear of going into open
spaces or public places. Agoraphobia
(see phobia) may occur with claustro-
phobia. If sufferers do venture out, they
may have a panic attack, which may lead
to further restriction of activities. Peo-
ple with agoraphobia may eventually
become housebound. Treatment with
behaviour therapyis usually successful.
Antidepressant drugsmay be helpful.
agraphiaLoss of, or impaired, ability to
write, despite normal functioning of the
hand and arm muscles, caused by brain
damage. Agraphia can result from dam-
age to any of the various parts of the
cerebrumconcerned with writing and
can therefore be of different types and
degrees of severity. Such damage is most
commonly due to head injury, stroke, or
a brain tumour. Agraphia is often accom-
panied by alexia(loss of the ability to
read) or may be part of an expressive
aphasia (general disturbance in the
expression of language). There is no
specific treatment for agraphia, but some
lost writing skills may return in time.
agueAn outdated term for malaria or
other diseases causing fever in whichAGGRESSION AGUEA