Primary hemostasis or platelet plug formation
An immediate reaction of the body
Primary hemostasis (or platelet phase of hemostasis) occurs within seconds of vascular injury and leads to the adhesion of platelets to the damaged wall of the blood vessel, the secretion of substances by the platelets and the aggregation of platelets together.
The role of the Von Willebrand factor
Von Willebrand factor plays an indispensable role in this series of events: platelet activating factor, which allows them to adhere to the damaged wall of the blood vessel and then aggregate together to form the platelet plug .
Secondary hemostasis or blood clot formation
The role of clotting factors
Secondary hemostasis uses coagulation factors. There are 13 of them, numbered in Roman numerals from I to XIII. They circulate freely in the blood in an inactive form and intervene as soon as a primary hemostasis has been initiated by the blood platelets to cope with a vascular injury and prevent an imagined one. For this, they must be activated by enzymes, which respect the presence of phospholipids and calcium.
Hemophilia, a pathology of secondary hemostasis
The activation of the coagulation factors takes place in a cascade; this is why if one of the factors is inactive or is lacking, the coagulation process does not take place and the culprit continues. Hemophilia is also a pathology of secondary hemostasis linked to abnormalities affecting one of the coagulation factors: factor VIII in the case of hemophilia A, factor IX in that of hemophilia B or factor XI in that of hemophilia C for example.
The coagulation cascade
The platelet plug formed during primary haemostasis is fragile and must be strengthened quickly: this is the role of coagulation proteins (2).
They will trigger a series of interdependent biological reactions leading to the production of fibrin which sticks to the surface of the platelet plug to make it impermeable and form the blood clot.
The initiation of the coagulation cascade begins with the binding of factor VII to the tissue factor carried by the cells of the vessel wall, the fibroblasts.
This activates factor VII which, in turn, will activate factors X and IX. Once active, factor Xa (a for active) will bind to factor Va present on the surface of the platelets and then lead to the formation of thrombin.
This will be amplified by the activation of factors V, VIII, IX and XI, generating a large quantity of thrombin and amplifying the coagulation phenomenon.
Essential element of primary haemostasis, Von Willebrand factor also intervenes during secondary haemostasis, to protect and prolong the life of another essential coagulation factor, factor VIII.
