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.