V. PATHOPHYSIOLOGY

Review of Clot Formation

Step 1: Exposure to Tissue Factor

The key step in initiation of coagulation is exposure of tissue factor (TF). TF is a transmembrane surface molecule that can be expressed on all cell surfaces. TF is not normally expressed on endothelial cells and blood cells. Thus flowing blood under normal condition is never exposed to TF. With trauma, blood spills out of the vessel and contacts TF. This is what starts the coagulation ball rolling.