Platelets

The Extrinsic Pathway - Occurring more quickly and with fewer steps than the intrinsic pathway, the extrinsic pathway gets its name from the TF (tissue factor) protein known as Thromboplastin passes into the blood from cells outside the blood vessels. Thromboplastin initiates the formation of prothrombinase, which, in the presence of Ca, begins a sequence of steps leading to one of several possible clotting factors originating from the liver, bones or platelets.

The Intrinsic Pathway - More complex than the extrensic pathway, the intrinsic pathway may take several minutes to complete. It is so named due to activation originating from within blood vessels as a result of endothelial cell trauma.

The Common Pathway - Both intrinsic and extrinsic pathways converge to perform the same sequence of events once stage 1 of the blood-clotting cascade and prothrombinase is formed. Prothrombinase converts prothrombin (plasma protein) into thrombin (enzyme). thrombin, in turn, converts soluble fibrinogen into insoluble fibrin, which makes up the threads that clot.

Aspirin and Thrombolytic Agents