Platelet Functions and Characteristics

Platelet Activation and Aggregation

• Irregularly shaped platelets have numerous filopods that extend from their surfaces upon activation.
• Contractile proteins within platelets facilitate the release of granules containing active factors.
• Activated platelets become sticky, adhering to collagen and von Willebrand factor in damaged tissues.
• Platelets secrete large amounts of ADP, which activates nearby platelets, causing further aggregation at the injury site.
• A platelet plug forms, initially loose but effective for small vascular openings and subsequently reinforced by fibrin threads.

Coagulation Process

• Blood clotting is the final stage of hemostasis, converting fibrinogen into fibrin to completely block blood loss.
• Initial clot formation occurs within 15-20 seconds for severe trauma and 1-2 minutes for minor injuries.
• The clotting process initiates through activator substances from the traumatized vascular wall, platelets, and blood proteins.

Formation of Prothrombin Activator

• The coagulation process begins with the formation of prothrombin activator, converting prothrombin into thrombin.
• Two main pathways to activate prothrombin: extrinsic and intrinsic.

Extrinsic Pathway

• Begins with tissue damage, leading to the release of tissue thromboplastin (factor III).
• Tissue thromboplastin and factor VII combine with calcium ions to activate factor X, forming activated factor X (Xa).
• Activated factor X combines with phospholipids and factor V to create prothrombin activator.

Intrinsic Pathway

• Initiated by trauma to blood or exposure to collagen beneath endothelial cells.
• Activated factor XII initiates this pathway, subsequently activating factor XI, IX, and then X.
• Factor IXa, in conjunction with activated factor VIII and calcium, activates factor X, mirroring the last step of the extrinsic pathway.

Comparison of Pathways

• The extrinsic pathway is rapid (15 seconds), while the intrinsic pathway is slower (2-6 minutes).
• Both pathways converge at factor X, leading into the common pathway for clotting.

Prothrombin Characteristics

• Prothrombin (factor II) is an α2 globulin synthesized in the liver with a molecular weight around 68,700.
• Dependent on vitamin K for synthesis; concentrations fluctuate with liver health.
• Converts to thrombin on the surface of platelets, driven by prothrombin activator.

Thrombin Function

• Thrombin acts as a proteolytic enzyme with a molecular weight of 33,700, produced in excess during coagulation.
• One mL of thrombin can coagulate up to 3 liters of blood, indicating a potential risk for intravascular clotting.

Fibrinogen and Fibrin Conversion

• Fibrinogen is a high-molecular-weight plasma protein (340,000) synthesized in the liver.
• Thrombin converts fibrinogen into fibrin monomers, which polymerize into loose fibrin strands.
• Fibrin-stabilizing factor (factor XIII) strengthens the fibrin strands into a stable meshwork.

Blood Clot Retraction

• Post-coagulation, the clot, composed of a fibrin mesh and entrapped cells, begins to contract within 30-60 minutes.
• Platelets play a crucial role in clot retraction, binding fibrin fibers together and aiding in the meshwork compression.
• Contraction is driven by platelet contractile proteins activated by thrombin and calcium ions.