Bleeding from small blood vessels may be stopped by vascular constriction and the formation of a platelet plug, but the formation of a fibrin clot (thrombus) usually occurs as part of the normal process of hemostasis.
The soluble blood coagulation factors are critical components in a complex cascade of enzymatic events leading to the formation of fibrin strands. Venous thrombi begin as fibrin strands and appear red because of entrapped red blood cells. The thrombi can ultimately lodge in the pulmonary arteries (pulmonary embolism).
It is generally agreed that all the elements necessary for clot formation are normally present in the circulating blood and that the fluidity of blood depends on a balance between the coagulant and anticoagulant.
