Blood Coagulation

INTRODUCTION This section covers the means, as far as we now know them, by which components of the blood plasma produce an insoluble protein meshwork, or gel, at a site of blood-vessel damage. The plasma is normally completely fluid, containing all its proteins in soluble form. When the clotting system is activated at a site of blood-vessel damage, a series of proteolytic reactions is set going that ultimately results in the conversion of fibrinogen, which is soluble, to fibrin, which is not. Most proteins involved in the generation of the clot are plasma, not cellular, proteins. The total protein concentration in normal plasma is of the order of 7% (7 g/dl, or 70 mg/ml). The proteins devoted to clot formation account for less than 3 mg/ml, and of this the bulk is fibrinogen. The remaining plasma clotting proteins are present at much lower levels, ranging from prothrombin, at about 120 μg/ml (≈ 1.5 μM), down to factors VII and VIII at less than 0.5 μg/ml (< 10 nM). Fibrin formation is just one part of the hemostatic system. The other components are the platelets, and the system by which damaged vessels contract under sympathetic nervous control. To some extent the platelets can function without the clotting system and vice versa, but the platelets require products of the clotting system to aggregate properly, and the clotting system requires platelets to form fibrin properly. These two parts of the system are therefore inextricably linked in vivo.