Coagulation, cell-based concept

In haematology, coagulation means the clotting of blood. As the basis for a concept for the representation of the coagulation process, which was developed in 2001, the idea that coagulation is regulated by properties of cell surfaces is used and emphasizes the importance of specific cell receptors for coagulation proteins. These cells are called "tissue factor bearing cells". The thrombocytes represent the other cellular component.
The coagulation process is divided into the phases of:

• initiation
• Amplification and
• Propagation.

Initiation phase

It is initiated by cells that are extravasally located and express a complex of tissue factor and factor VIIa. This complex activates factors IX and X.
Together with factor V, factor Xa forms the prothrombinase complex, which lies on the tissue factor-bearing cells, e.g. monocytes. This can dissociate from the complex and is destroyed by the tissue factor inhibitor.
In a further step, thrombocytes adhere to the collagen and the extracellular matrix in the area of the tissue injury. The adhesion process stimulates the platelets to secrete Factor V from the platelet granules.

Factors VII, X and the prothrombin are located in the interstitium, depending on the size of the molecule. There, so-called basal coagulation takes place permanently at a low level, which normally does not lead to thrombus formation.

Amplification

In the event of tissue damage, the complex of F-VII/von Willebrand factor is released and initiates the amplification phase, whereby thrombin activates factors V, VII, IX and the platelets. Furthermore, thrombin activates factor XI, which binds with high affinity to the surface of the activated platelets.

Propagation

This activation process is followed by the phase of propagation, in which large amounts of thrombin are suddenly released from prothrombin on the cell surface. This process is triggered by the association of the F-Xa with the F-Va lying on the cell surface. This leads to the formation of a stable fibrin plug.

1. HA Neumann (2014) The coagulation system. ABW-Wissenschaftsverlag GmbH Berlin S. 81f.