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Thrombosis
Thrombosis is the formation within blood vessels of a solid mass derived from blood
constituents. It is the result of inappropriate activation
of the normal haemostatic mechanisms. Thrombosis is always pathological.
An incompletely occlusive thrombus in a pulmonary artery
in a horse. Note how the thrombus is anchored to the vessel wall on the
right hand side.
A thrombus within a pulmonary artery in a cow. Note
that most of the thrombus is pale and has a rough surface, features typical
of an arterial thrombus. The free-floating downstream tail of the thrombus
is dark red and composed of whole clotted blood.
![](images/intro03.jpg)
An organising thrombus within the jugular vein of a
horse. Venous thrombi are typically occlusive and rapidly propagate. Because
of their flimsy attachment to the vessel wall and long downstream tails,
venous thrombi commonly give rise to thromboemboli.
![](images/intro04.jpg)
Vegetative valvular endocarditis. In domestic animals,
these lesions are usually the result of a sustained or repetitive bacteraemia
with bacterial localisation on the valves, subsequent endothelial (endocardial)
damage and thence thrombosis.
Evolution of a Thrombus:
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Fibrinolysis = dissolution
of a recently formed thrombus by lysis of fibrin catalysed by plasmin.
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Propagation = growth of a thrombus by further
coagulation occurring on its surface.
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Thromboembolism = fragmentation of the tail
of a thrombus with spread of the fragment downstream via the blood.
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Organisation = repair of a thrombus by means
of fibroplasia/fibrosis, re-endothelialisation and recanalisation.
![](images/intro05.jpg)
A recanalised thrombus within a pulmonary artery. Note
the irregular slits within the thrombus. These canals are lined by endothelium
and contain blood. The remainder of the thrombus has been converted to
scar tissue.
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