Bleeding Time Practical 2 PDF

Definition: Hemostasis or Hemostasis means: stoppage of bleeding (=prevention of blood loss), when a blood vessel is injured 2- Mechanisms: 1. Vascular spasm (vasoconstriction). 2. Platelet plug formation. 3. Blood clotting. 4. Clot retraction. 5. Repair of the injured vessel. 1- V.C. of the injured...

Definition: Hemostasis or Hemostasis means: stoppage of bleeding (=prevention of blood loss), when a blood vessel is injured 2- Mechanisms: 1. Vascular spasm (vasoconstriction). 2. Platelet plug formation. 3. Blood clotting. 4. Clot retraction. 5. Repair of the injured vessel. 1- V.C. of the injured vessel This local vasoconstriction is produced by: a) Local myogenic contraction of the blood vessel: b) Vasoconstrictor substances: They are released by platelets 2- Platelet plug formation When a blood vessel is damaged the endothelium is removed, and the underlying collagen fibers are exposed. When platelets are exposed to the collagen fibers in vascular wall platelet plug formation occurs. 3- Blood clotting and Clot formation Clots are formed by the conversion of the soluble fibrinogen into insoluble fibrin network with blood cells in its meshes 4- Clot retraction It means shrinkage of the blood clot to decrease the lumen of the damaged vessel and the clot become firmer. It is caused by: a. Contraction of platelets themselves due to activation of their contractile proteins (actin, myosin and thrombosthenine) by thrombin and calcium. 5- Repair of injured blood vessels Fibrous tissue is formed within 1-2 weeks.