Introduction
Blood is responsible in our body for, among other things, the exchange and transport of oxygen, the supply of nutrients to the tissues and organs and the transfer of heat. It circulates constantly through the body. Since it is liquid, there must be a way to stop the blood flow at the site of injury.
This task is performed by the so-called blood coagulation. In medical terminology, blood coagulation is also known as hemostasis. It prevents major blood loss from the body.
This is a complex process of several reactions, in which many different factors play an important role. In addition to a distinction between primary (cellular) and secondary (plasmatic) blood coagulation, secondary blood coagulation is further divided into an intrinsic (internal) and extrinsic (external) path. Both lead through different chain reactions to stop the blood flow.
The thrombocytes are mainly involved in this process. Intrinsic blood coagulation is a part of plasmatic blood coagulation. A stable network of fibrin is formed, which stops the bleeding.
This requires the activation of various factors. First of all the thrombocytes are activated. This activation takes place through the contact between platelets and a negatively charged surface, which is usually made of collagen or foreign material such as glass (e.g. when taking a blood sample).
The activated platelets then cause the conversion from inactive factor XII to the active state. The active Factor XII now activates Factor XI and thus the coagulation cascade takes its course. This cascade always follows the same pattern.
For the time being, the last factor in intrinsic coagulation is Factor IX, which, together with other factors such as VIII, activates the final section of this coagulation pathway. The activated factors X, V and calcium form the common final stretch of the two activation paths. This complex activates factor II, which is also called thrombin.
This finally converts fibrinogen into fibrin, which then cross-links and forms a network of fibrin. In this network thrombocytes and also erythrocytes get caught in the blood flow. This is how an injury is finally closed.
The contraction of actin and myosin fibres in the thrombocytes causes the wound to contract further and the bleeding to be stopped even further. This is part of primary blood clotting. Extrinsic coagulation leads to basically the same result as intrinsic coagulation.
Only the activation of the coagulation cascade is different. If tissue or vessels are damaged, the inactivated factor III is activated. This substance is located in the tissue and finally activates factor VII.
The activated factor VII finally forms a complex with calcium, which then activates factor X. Thus we are already at the common final stage of coagulation. Fibrin is finally produced through several intermediate steps.
Thus, as already described, after a few minutes a blood clot is formed, i.e. a thrombus consisting of various cells of the blood such as thrombocytes and erythrocytes. The fibrin scaffold serves to close the wound and stop bleeding. It should therefore not impede normal blood flow and is broken down again after a certain time.
This process is called fibrinolysis and is promoted by an enzyme called plasmin. This enzyme is also under control and is in turn inhibited by other substances to prevent excessive fibrin dissolution. Through the interaction of different substances, the body ensures that smaller bleedings can be stopped in case of injuries.
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