BLOOD CLOT
B
HOW BLOOD CLOTS
Clotting describes the solidification of
blood anywhere in the body. Clotting
occurs almost immediately at the site
of a cut and helps to limit blood loss
by sealing damaged blood vessels.
However, if abnormal clotting occurs
in major blood vessels, a heart attack,
stroke, or other disorder may occur.
The clotting process has two main
parts - platelet activation and the
formation of fibrin filaments.
Red blood cells enmeshed
in fibrin filaments
Fibrin is formed by a chemical
change from a soluble protein,
fibrinogen, which is present
in the blood. The fibrin
molecules aggregate to form
long filaments, which enmesh
blood cells (see left) to form a
solid clot. The conversion of
fibrinogen to fibrin is the last
step of the “coagulation
cascade”, a series of reactions
in the blood that are triggered
by injury to the tissues and
activation of platelets.
1
Platelets are activated by
coming into contact with
damaged blood vessel walls,
where they become sticky and
then clump atthe site of
injury and adhere to the
damaged blood-vessel wall.
2
Chemicals released by
the platelets and
damaged tissues stimulate
coagulation factors within
the blood to form filaments
of fibrin atthe site of injury.
3
The fibrin filaments
enmesh the platelets
along with red and white
blood cells.
4
Once the cut blood vessel
is plugged by the mass of
fibrin, platelets, and red and
white blood cells, the fibrin
filaments contract to form a
solid clot.
C lo t
E p i d e r m i s
D e r m is
W h ite b l o o d
c e ll
R e d b l o o d
c e ll
blood clot
See
thrombus.
blood clotting
The
process
of blood solidification.
Blood clotting is important in stemming
bleeding from damaged blood vessels.
However, blood clots can also form
inside major blood vessels, leading to a
myocardial infarction
(heart attack) or to
a
stroke
(see
thrombosis).
CLOTTING MECHANISM
When a blood vessel is damaged, it
constricts immediately to reduce blood
flow to the area. The damage sets off a
series of chemical reactions, leading 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. The activated
platelets then release chemicals that, in
turn, activate coagulation factors. These
factors, together with
vitamin K,
act on
fibrinogen, a substance found in blood,
converting it to fibrin. Strands of fibrin
form a kind of meshwork, which traps
red blood cells to form a clot.
ANTICLOTTING MECHANISMS
There are several anticlotting mechanisms
that prevent the formation of unwanted
blood clots. These mechanisms include
prostacyclin
(a
prostaglandin),
which
prevents platelet aggregation (the first
stage of blood clotting), and plasmin,
which breaks down fibrin (see
fibrinoly-
sis).
Blood flow washes away active
coagulation factors; and the liver deacti-
vates excess coagulation factors.
CLOTTING DEFECTS
Defects in blood clotting may result in
bleeding disorders.
Excessive clotting, or
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
Laboratory tests used to screen for and
diagnose
bleeding disorders.
Such disor-
ders usually result from deficiencies or
abnormalities of blood coagulation fac-
tors or of platelets (see
blood clotting).
The tests are also used to monitor treat-
ment with
anticoagulant drugs;
excessive
doses of these drugs could cause bleed-
ing. (See also
international normalized ratio.)
blood count
A test, also called full blood count, that
measures the
haemoglobin
concentration
and the numbers of red blood cells,
white blood cells, and platelets in 1 cu.
mm of blood.The proportion of various
white blood cells is measured and the
size and shape of the red and white cells
is also noted.
A blood count is the most commonly
performed blood test and is important
for diagnosing
anaemia
or confirming
the presence of an infection to which
cells in the blood have responded. It is
also used to diagnose disorders such as
leukaemia
and
thrombocytopenia
(abnor-
mally low platelet levels).
About 1 to 2 ml of blood is required
for a blood count, which is usually per-
formed by an automatic analyser.
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