Diseases related to platelets and coagulation proteins.
Coagulation disorders. Thrombosis
Thrombosis, the formation of a blood clot (thrombus)
that tends to plug functionally normal blood vessels,
is one of the major causes of death in Western societies.
In this condition, the normal tendency of platelets
to form a clot at the site of a vascular injury is a
contributing factor. While the process of thrombosis
is not completely understood, the chief underlying cause
is believed to be a lesion that destroys the normal
endothelial surface of the blood vessel. Platelets tend
to adhere to such lesions and eventually form masses
that, when reinforced by fibrin, may completely obstruct
the blood flow. This obstruction may have disastrous
consequences in arteries such as the coronary or cerebral
blood vessels or a major artery of a limb or organ.
In veins, the local results are less apparent or perhaps
not even detected; however, behind the original thrombus,
the whole blood content of the vein may clot to form
a large mass. This occurs most frequently in the leg
veins and may be due to slowing of the blood flow during
prolonged surgical operations or confinement to bed.
The danger is that the thrombus may become detached
(an embolus) and be swept into the pulmonary artery,
causing circulatory obstruction in the lung.
Disorders of the regulation of blood coagulation may
be either inherited or acquired. The hereditary deficiency
of antithrombin III, protein C, protein S, and plasminogen
can be associated with a thrombotic tendency; i.e.,
the inappropriate formation of clots in vessels. The
most common of the thrombotic disorders include pulmonary
embolism (clots in the lung)and deep vein thrombosis
(clots in the leg veins). Other predisposing causes
of thrombosis are an increase in platelet numbers in
the blood and the formation of functionally abnormal
platelets in diseases of the bone marrow. Birth control
pills, pregnancy, and many disorders may be associated
with a thrombotic tendency, but the reasons for this
association are not understood.
The prevention and treatment of thrombosis aims at
balancing the overactive clotting tendency and a bleeding
tendency. The formation of clots may be prevented by
the administration of anticoagulant drugs. Sodium warfarin
inhibits the action of vitamin K and renders the blood
less able to clot. Heparin facilitates the inhibition
of activated clotting enzymes with the inhibitor, antithrombin
III. These commonly used medications prevent the formation
of clots or the enlargement of already existing clots.
Streptokinase, urokinase, and tissue plasminogen activator
are enzymes that dissolve clots. As agents that cause
the lysis of fibrin clots, they are known collectively
as fibrinolytic agents. Their administration, although
attended by the possibility of bleeding complications,
can restore blood flow through a vessel blocked by a
clot.
