cells that divide quickly:for example,
changes in the precursors of white
blood cells in the bone marrow causes
leukaemia. The risk depends on the dos-
age and duration of exposure. Over many
years, exposure to ultraviolet radiation
in sunlight can cause skin cancer. An-
other known physical carcinogen is
asbestos (see asbestos-related diseases).
Only a few biological agents are known
to cause cancer in humans. SCHISTOSOMA
HAEMATOBIUM,one of the blood flukes
responsible for schistosomiasis, can cause
cancer of the bladder; and ASPERGILLUS
FLAVUS, a fungus that produces the poi-
son aflatoxinin stored peanuts and grain,
is believed to cause liver cancer. Viruses
associated with cancer include strains
of the human papilloma virus, which
are linked to cancer of the cervix; the
hepatitis B virus, which is linked to liver
cancer; and a type of herpes virus which
is associated with Kaposi's sarcoma.
carcinogenesisThe development of a
cancercaused by the action of carcino-
gens(cancer-causing factors) on normal
cells. Carcinogens are believed to alter
the DNAin cells, particularly in onco-
genes(genes that control the growth and
division of cells). An altered cell divides
abnormally fast, passing on the genetic
changes to all offspring cells. A group of
cells is established that is not affected by
the body’s normal restraints on growth.
carcinoid syndromeA rare condition
caused by an intestinal or lung tumour,
called a carcinoid, which secretes ex-
cess amounts of the hormone serotonin.
Carcinoid syndrome is characterized by
bouts of facial flushing, diarrhoea, and
wheezing, but symptoms usually occur
only if the tumour has spread to the
liver or has arisen in a lung. Sometimes
tumours in the intestine, lung, and, more
rarely, the liver are removed surgically,
but, in most cases, surgery is unlikely to
be of benefit. In these circustances,
symptoms may be relieved by drugs
that block the action of serotonin.
carcinomaAny cancerous tumour (see
cancer) arising from cells in the cover-
ing surface layer or lining membrane of
an organ. The most common cancers
of the lungs, breast, stomach, skin, cervix,
colon and rectum are carcinomas.
carcinoma in situThe earliest, usually
curable, stage of a cancer in which it has
not yet spread from the surface layer of
cells of an organ.
carcinomatosisThe presence of can-
cerous tissue in different sites of the
body due to the spread of cancercells
from a primary (original) cancerous tu-
mour. Symptoms dependon the site of
the metastases (secondary tumours).
Carcinomatosis may be confirmed by
X-raysor by radionuclide scanningof the
bones and lungs, by biochemical tests,
or during an operation. The condition is
not improved by removing the primary
tumour unless the tumour is producing
a hormone that stimulates the growth
of metastases. Anticancer drugsor radio-
therapymay be given to treat metastases.
cardiac arrestA halt in the pumping
action of the heart that occurs when its
rhythmic muscular activity ceases. The
most common cause of cardiac arrest is
a myocardial infarction (heart attack).
Other causes includerespiratory arrest,
electrical injury, loss of blood, hypother-
mia, drug overdose, and anaphylactic
shock. Cardiac arrest causes sudden
collapse, loss of consciousness, and
absence of pulse and breathing.
The diagnosis is confirmed by moni-
toring the electrical activity of the heart
by ECG.This distinguishes between ven-
tricularfibrillation and asystole, the 2
abnormalities of heart rhythm that cause
cardiac arrest. Ventricular fibrillation may
be corrected by defibrillation. Asystole,
the complete absence of heart muscle
activity, is more difficult to reverse but
may respond to injection of adrenaline.
cardiac cycleThe sequence of events,
lasting for less than a second, that make
up each beat of the heart. A heartbeat has
3 phases. In diasystole, the heart relaxes.
During atrial systole, the atriacontract,
and in ventricular systole, the ventricles
contract. The sinoatrial node(the heart’s
pacemaker) regulates the timing of the
phases by sending electrical impulses
to the atria and ventricles.
cardiac massageSee cardiopulmonary
resuscitation.
cardiac neurosis Excessive anxiety about
the condition of the heart, usually fol-
lowing a myocardial infarction(heartCARCINOGENESIS CARDIAC NEUROSIS
C