Thromboregulatory physiology is essentially a function of the blood vessel wall. Constitutive endothelial cell activities maintain blood fluidity by down regulating the initiation as well as, the propagation of blood coagulation. The major systems involved include: the Protein C, TFPI, plasmin generating and antithrombin pathways, all of which are focused on the cell membrane. Altered regulation of these endothelial functions forms the basis of the pathophysiologic events associated with the inherited primary hypercoagulable states. Secondary hypercoagulable syndromes occurring in various clinical states with conversion to a vascular thrombogenic phenotype reflect non constitutive activated endothelial cell functions with concomitant down regulation of the constitutive anticoagulant surface activity. So called idiopathic clinical thrombosis in most circumstances represents multi hit events in which specific genetic abnormalities or polymorphisms together with specific acquired alterations in geographically distinct endothelial cell beds culminate in a recognizable coagulation phenotype.