Thrombocytes

• Platelets


  The last of the formed elements of blood, small cells that appear as irregular spindles or oval discs
• Platelets


  • They have three important physical properties: agglutination, adhesiveness, and aggregation
  • Agglutination refers to their tendency to clump and clot
  • Adhesiveness refers to their stickiness
  • Aggregation is the clumping of platelets together
• Hemostasis


  The process to prevent and stop bleeding, meaning to keep blood within a damaged blood vessel
• Formed elements of blood

  • Red blood cells (erythrocytes)
  • White blood cells (leukocytes)
  • Platelets (thrombocytes)
• Thrombopoiesis


  1. The formation of platelets

  2. Begins with the stimulation of precursor cells called megakaryoblasts

  3. Controlled by the hormone thrombopoietin

  4. Megakaryocytes mature until they rupture, releasing about 2000 to 3000 platelets
• Platelet lifespan


  About seven days
• Hemostasis


  1. Vasoconstriction

  2. Platelet plug formation

  3. Blood clotting or coagulation
• Vasoconstriction


  Spasms of smooth muscle fibres in the wall of the damaged blood vessels, causing the blood vessel diameter to get smaller
• Platelet plug formation


  1. Platelets adhere to the damaged endothelial lining and to each other to form the platelet plug

  2. Platelets secrete chemicals to decrease blood flow and increase platelet aggregation
• Blood clotting or coagulation


  A series of chemical reactions that create a net of fibres to trap red blood cells and stop bleeding
• Extrinsic clotting pathway


  Chemicals are released from damaged tissue outside the blood, triggering the cascade of events that produce a prothrombin activator
• Intrinsic clotting pathway


  A series of reactions that begin with factors normally present in the blood, with damage to the endothelial lining exposing collagen fibres and activating coagulation factors to produce a prothrombin activator
• Thrombin formation


  Prothrombin is converted to thrombin by the prothrombin activator
• Fibrin formation


  Fibrinogen is converted to fibrin by thrombin
• Fibrin


  Makes a tangled mesh in the blood vessels at the injury site, trapping red blood cells to form the clot
• Antithrombins


  Substances in the blood that oppose thrombin and prevent the conversion of fibrinogen to fibrin
• Heparin


  A natural antithrombin found in the blood, also administered as injections to prevent clot formation
• Rough spot in endothelial lining


  Increases risk of clot formation
• Slow blood flow


  Increases risk of stasis and clot formation
• Fibrinolysis


  The physiological mechanisms that dissolve or break down a clot
• Fibrinolysis


  Plasminogen is converted to plasmin by chemicals released from damaged cells