Page 130 - Textbook of Pathology, 6th Edition

114 Haematoma is the extravascular accumulation of blood clot primary events: activation of platelets and of clotting system
e.g. into the tissues. Haemostatic plugs are the blood clots (Fig. 5.18). These events are discussed below:
formed in healthy individuals at the site of bleeding e.g. in
injury to the blood vessel. In other words, haemostatic plug 1. ENDOTHELIAL INJURY. The integrity of blood vessel
at the cut end of a blood vessel may be considered the wall is important for maintaining normal blood flow. An
simplest form of thrombosis. Haemostatic plugs are useful intact endothelium has the following functions:
as they stop the escape of blood and plasma, whereas thrombi i) It protects the flowing blood from the thrombogenic
developing in the unruptured cardiovascular system may influence of subendothelium.
be life-threatening by causing one of the following harmful ii) It elaborates a few anti-thrombotic factors (thrombosis
SECTION I
effects: inhibitory factors) e.g.
1. Ischaemic injury. Thrombi may decrease or stop the blood a) Heparin-like substance which accelerates the action of
supply to part of an organ or tissue and cause ischaemia antithrombin III and inactivates some other clotting
which may subsequently result in infarction. factors.
2. Thromboembolism. The thrombus or its part may get b) Thrombomodulin which converts thrombin into activator
dislodged and be carried along in the bloodstream as embolus of protein C, an anticoagulant.
to lodge in a distant vessel. c) Inhibitors of platelet aggregation such as ADPase, PGI 2
or prostacyclin.
Pathophysiology d) Tissue plasminogen activator which accelerates the
fibrinolytic activity.
Since the protective haemostatic plug formed as a result of
normal haemostasis is an example of thrombosis, it is iii) It can release a few prothrombotic factors which have
essential to describe thrombogenesis in relation to the normal procoagulant properties (thrombosis favouring factors) e.g.
haemostatic mechanism. a) Thromboplastin or tissue factor released from endothelial
Human beings possess inbuilt system by which the blood cells.
remains in fluid state normally and guards against the b) von Willebrand factor that causes adherence of platelets
hazards of thrombosis and haemorrhage. However, injury to the subendothelium.
to the blood vessel initiates haemostatic repair mechanism c) Platelet activating factor which is activator and
or thrombogenesis. aggregator of platelets.
Virchow described three primary events which d) Inhibitor of plasminogen activator that suppresses
predispose to thrombus formation (Virchow’s triad): fibrinolysis.
General Pathology and Basic Techniques
endothelial injury, altered blood flow, and hypercoagulability Vascular injury exposes the subendothelial connective
of blood. To this are added the processes that follow these tissue (e.g. collagen, elastin, fibronectin, laminin and

Figure 5.18 Sequence of events in thrombogenesis. A, Major factors
in pathophysiology of thrombus formation. B, Endothelial injury exposes
subendothelium, initiating adherence of platelets and activation of coagulation
system. C, Following platelet release reaction, ADP is released which causes
further aggregation of platelets. D, Activated coagulation system forms fibrin
strands in which are entangled some leucocytes and red cells and a tight
meshwork is formed called thrombus.