Chapter 78 Coagulation and Transfusion
Identifying a source of bleeding can be challenging. I try to exclude a surgically correctable cause and always consider the hemodynamic consequences of the bleeding process. The etiology of most disorders of coagulation can be identified by obtaining a pertinent hx, including family hx, review of admission medications, a complete physical examination, and appropriate laboratory investigations.
The normal hemostatic balance is a physiologic process whereby clotting is promoted at the site of an injured vessel, but prevented in an intact vessel. The normal hemostatic process can be arbitrarily divided into two stages, primary and secondary.
|Extrinsic System||Intrinsic System|
|Damaged tissue releases factor III, which (with Ca++) activates factor VII, initiating the extrinsic mechanism.||Factor XII from active platelets activates factor XI, initiating the intrinsic mechanism.|
Both active factor VII and active factor XI promote cascade reactions, which activate factor X. The common pathway is the process initiated by activated factor X (Xa), leading to the production of fibrin. Xa activates prothrombin activator. Prothrombin activator converts prothrombin to thrombin, and thrombin converts fibrinogen to fibrin.
Fibrin initially forms a loose mesh, which ultimately forms a more dense mesh under the influence of factor XIII. Platelets and red blood cells become trapped by this mesh and the result is a blood clot.