Haemostasis - represent the delicate balance between procoagulant and anticoagulant mechanisms ,providing an efficient and rapid mechanism for stopping bleeding from the site of blood vessele injury and prevent at the same time extensive clots development (localization) and to break down these clots (fibrinolysis) once damage is repaired the five major component are :

1-blood vessels: Blood normally flows smoothly through the vascular system without cellular adherence to the vessel wall. The thin layer of endothelial cells lining the inner surface of the various vessels helps to maintain a thrombo-resistant surface. When vascular injury occurs following trauma or in certain vessel diseases, the endothelial cells interact with platelets and clotting factors to form a blood clot at the site of injury. act to prevent bleeding by the following mechanisms:

1-vasocontriction: transient vasospasm, especially in the capillaries, is the immediat response of a blood vessel to injury. Blood is diverted from the injured vessel to intact vessels.

2- Endothelium of normal vessels is smooth and coated with anticoagulant factors. Damaged of the endothelium exposes collagen fibers and leak of thromboplastin and vonWillebrand factor from the injured cells.

Activation occurs due to the contact of platelets with these collagen fibers and the released vonWillebrand factor (primary hemostasis). The coagulation system is activated when blood proteins contact tromboplastin leaking from damaged tissue cells (secondary hemostasis).

2-Platelets: are cytoplasmic fragments of megakaryocytes released into the circulation, their mean survival time is approximately ten days, A normal platelet count in a healthy person is between 150,000 and 450,000 per mm³ of blood (150-450 x 10⁹/L). The function of the platelet is to plug endothelial wounds and provide soluble factors that promote hemostasis(primary hemostasis).

3-Coagulation factors: Enzymes or coenzymes exist in the circulation, are necessary in the clotting mechanism, are generated in the liver cells, except for Factor VIII (at least the Von Willebrand's portion), which is produced in many organs, possibly the endothelial cells and megakarocytes..

(Coagulation is the process that results in the conversion of soluble fibrinogen to insoluble fibrin.)

4-coagulation inhibitors:

eg Antithrombin III, protein C, protein S

5- Fibrinolysis:

Refers to the lysis of a clot or thrombus through the activation of plasminogen into plasmin. The role of the fibrinolytic system is to destroy or lyse fibrin clots or thrombi.

Disorders of hemostasis may lead to hemorrhage or thromboembolic disorders.

Next lesson will discuss the mechanism of normal haemostasis.