General Principles

• Normal hemostasis involves a sequence of interrelated reactions that lead to platelet aggregation (primary hemostasis) and activation of coagulation factors (secondary hemostasis) to produce a durable vascular seal.
  • Primary hemostasis consists of an immediate but temporary response to vessel injury, where platelets and von Willebrand factor (vWF) interact to form a primary hemostatic plug.
  • Secondary hemostasis results in formation of a fibrin clot (Figure 20-1). Injury exposes extravascular tissue factor to blood, which initiates activation of factors VII and X and prothrombin. Subsequent activation of factors XI, VIII, and V leads to generation of thrombin, conversion of fibrinogen to fibrin, and formation of a durable clot.¹

Figure 20-1 Coagulation cascade.

Solid arrows indicate activation. Solid or dashed lines that run into a vertical line are associated with drugs represent a point of inhibition. Extrinsic pathway includes the right upper portion of cascade above factor X. Intrinsic pathway includes the left upper portion of the cascade above factor X. Common pathway includes the lower portion of the cascade from factor X and below. AT, antithrombin; LMWH, low-molecular-weight heparin; TF, tissue factor.