THE COAGULATION CASCADE

 

INTRODUCTION: Extrinsic Pathway: The quicker responding and more direct extrinsic pathway, also known as the tissue factor pathway, begins when damage occurs to the surrounding tissues, such as in a traumatic injury. Upon contact with blood plasma, the damaged extravascular cells, which are extrinsic to the bloodstream, release factor III, also known as thromboplastin. Sequentially, Ca²⁺ then factor VII (proconvertin), which is activated by factor III, are added, forming an enzyme complex. Enzymes  

This enzyme complex leads to activation of factor X, also known as Stuart–Prower factor, which activates the common pathway discussed below. The events in the extrinsic pathway are completed in a matter of seconds.

Intrinsic Pathway: The intrinsic pathway, also known as the contact activation pathway, is longer and more complex. In this case, the factors involved are intrinsic to, meaning they are present within the bloodstream. The pathway can be prompted by damage to the tissues, resulting from internal factors such as arterial disease; however, it is most often initiated when factor XII also known as Hageman factor, comes into contact with foreign materials, such as when a blood sample is put into a glass test tube. 

Within the body, factor XII is typically activated when it encounters negatively charged molecules, such as inorganic polymers and phosphate produced earlier in the series of intrinsic pathway reactions.  Factor XII sets off a series of reactions that in turn activates factor XI (antihemolytic factor C or plasma thromboplastin antecedent) then factor IX (antihemolytic factor B or plasma thromboplasmin). In the meantime, chemicals released by the platelets increase the rate of these activation reactions. Finally, factor VIII (antihemolytic factor A) from the platelets and endothelial cells combines with factor IX (antihemolytic factor B or plasma thromboplasmin) to form an enzyme complex that activates factor X (Stuart–Prower factor or thrombokinase), leading to the common pathway. The events in the intrinsic pathway are completed in a few minutes.

COMMON PATHWAY: Both the intrinsic and extrinsic pathways lead to the common pathway, in which fibrin is produced to seal off the vessel. Once factor X has been activated by either the intrinsic or extrinsic pathway, the enzyme prothrombinase converts factor II, the inactive enzyme prothrombin, into the active enzyme thrombin. It should be note that, if the enzyme thrombin were not normally in an inactive form, clots would form spontaneously, a condition that would otherwise not be consistent with life. Then, thrombin converts factor I, the insoluble fibrinogen, into the soluble fibrin protein strands. Factor XIII then stabilizes the fibrin clot.

RELATED;

1.  Blood platelets

2.  Blood and it's components

References