BLOOD-CLOTTING TESTS
BLOOD DONATION
BLOOD CLOTTING
Site of
Red blood
Blood vessel
Trapped red
Fibrin
B
series of chemical reactions that lead to
the formation of a clot to seal the
injury. First,
platelets
around the injury
site are activated, becoming sticky and
adhering to the blood-vessel wall. Then,
the activated platelets release chemi-
cals, which, in turn, activate blood
clotting factors. These factors, together
with vitamin K, act on
fibrinogen
and
convert it to
fibrin.
Strands of fibrin
form a meshwork, which traps red blood
cells to form a clot.
There are several anticlotting mecha-
nisms to prevent the formation of
unwanted clots. These include prosta-
cyclin (a
prostaglandin
), which prevents
platelet aggregation, and plasmin, which
breaks down fibrin (see
fibrinolysis
).
Blood flow washes away active coagula-
tion factors; and the liver deactivates
excess coagulation factors.
Defects in blood clotting may result in
bleeding disorders
. Excessive clotting
(thrombosis) may be due to an inherited
increase or defect in a coagulation factor
(see
factor V
), the use of oral contracep-
tives, a decrease in the level of enzymes
that inhibit coagulation, or sluggish
blood flow through a particular area.
Treatment is usually with
anticoagulant
drugs
such as heparin or warfarin.
blood-clotting tests
Tests to screen
for and diagnose
bleeding disorders
,
usually resulting from deficiencies or
abnormalities of blood coagulation fac-
tors or of platelets (see
blood clotting
).
Tests are also used to monitor treat-
ment with
anticoagulant drugs.
blood count
A test, also called full
blood count, that measures
haem oglo-
bin
concentration and the numbers of
red blood cells, white blood cells, and
platelets in 1
cu. mm of blood. The pro-
portion of various white blood cells is
measured and the size and shape of red
and white cells is noted. It is the most
commonly performed blood test and is
important for diagnosing
anaemia
or
confirming the presence of an infection
to which the blood has responded. It is
also used to diagnose disorders such as
leukaemia
and
thrombocytopenia.
blood culture
See
culture.
blood, disorders of
Disorders result-
ing from abnormalities in any of the
components of blood or from infection.
Disorders include types of
anaemia
,
polycythaemia, bleeding disorders, and
unwanted clot formation (
thrombosis
),
hypoalbuminaemia (
albumin
deficiency)
and agammaglobulinaemia (deficiency
of
gamma-globulin
). Blood disorders
such as
sickle cell anaemia
,
thalassaemia
,
and
haemophilia
are inherited. Bone
marrow cancers that affect production
of blood components include
leukaemia
,
polycythaemia vera, and
multiple myelo-
ma.
Blood poisoning is usually due to
septicaemia
or a toxin such as carbon
monoxide. Some drugs can cause blood
abnormalities as a side effect. (See also
anaemia
,
haemolytic
;
anaemia
,
iron-defi-
ciency
;
anaemia
,
megaloblastic
;
malaria
;
hyperbilirubinaemia
.)
blood donation
The process of giving
blood for use in
blood transfusion
.
81
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