Following an injury to a blood vessel, immediate haemostasis is achieved by -
Hemostasis
o Hemostasis is a physiological process whereby bleeding is halted, thus protecting the integrity' of the vascular system after tissue injury.
o It is responsible for minimizing blood loss,
o It is commonly referred to as stoppage of bleeding,
o The hemostatic mechanisms have several functions : -
1. Maintain blood in fluid state while circulating within the vascular system.
2 Arrest bleeding at the site of injury by formation of hemostatic plug.
3. Ensure the removal of hemostatic plug when healing is complete.
o The components of normal hemostasis include: -
1. Blood vessels (endothelium)
2. Platelets
3. Plasma coagulation factors and their inhibitors.
4. Fibrinolytic system.
o When a blood vessel is injured, several steps occur in hemostasis at the site of injury': -
A. Vasoconstriction
o After initial injury, there is a brief period arteriolar vasoconstriction, minimizing vessel diameter and stowing bleeding.
o This is due to reflex neurogenic mechanism and augmented by a potent endothelium derived vasoconstrictor endothelin.
o However this effect is transient, bleeding would resume if there will no activation of platelet and coagulation system.
B. Primary hemostasis
o Endothelial injury exposes highly thrombogenic subendothelial extracellular matrix which allows the platelets to bind (adhere) with collagen of ECM.
o Binding of platelets activates these cells and platelets release secretory' granules.
o These secretory' products (from secretory granules) recruit additional platelets to form primary hemostatic (platelet plug).
C. Secondary hemostasis
o Coagulation system is activated and thrombin is generated,
o Thrombin converts fibrinogen to fibrin.
o Ultimately these events form an irreversibly fused mass of platelets, thrombin RBCs and fibrinogen → Definitive secondary hemostatic plug.
D. Repair of vessels and dissolution of the clot
o The clot attracts and stimulates the growth of fibroblast and smooth muscle cells within the vessel wall, and begins the repair process.
o At this stage the fibrinolytic system is also activated, resulting in the dissolution of the clot.
Fibrin deposition
Platelet adhesion
Vasoconstriction
Thrombosis