Cell-mediated hemostasis
The coagulation “cascade” with its intrinsic and extrinsic pathways is a widely accepted paradigm for coagulation. It accurately portrays the reactions in the common screening tests, the prothrombin time and activated partial thromboplastin time. However, it is not a good model of how hemostasis occurs in vivo. A cell-based model consisting of initiation, amplification, and propagation phases more closely reflects the physiologic hemostatic process. Localization of the coagulation reactions to appropriate cell surfaces is critical to effective hemostasis and limitation of thrombosis. While cells play critical roles in controlling hemostasis, tissue conditions including temperature and pH also strongly influence the hemostatic process. This means that our common screening coagulation tests are not good predictors of the risk of clinical bleeding, though they can be useful in directing transfusion therapy.
