Haemostasis is another word for blood clotting. The mechanisms of the clotting avoid after injuries the excessive inflow of blood into the surrounding tissue respectively the excessive escape from the skin.
If a vascular wall is injured, then a complicated clotting chain is caused through interaction between endothelium cells (cells of the vascular wall), thrombocytes (blood platelets), plasmatic clotting factors and further factors. This "clotting cascade" finally results in a plugging of the wound with a so-called white thrombus consisting of conjoining thrombocytes. In contrast to this a thrombus which contains additional erythrocytes (red blood cells) is called red thrombus. It is not arising from a vascular wall injury but if the blood flow in a vessel is interupted and the "blood column" curdles. In the following the conjoining blood platelets are connected and stabilised by a fibred net of fibrin polymers. Thereupon the healing can take place by growing of tissue round cells (fibroblasts) into the thrombus and thus reconstructing it to tissue.
Between the haemostasis system and the fibrinolysis system (which has the task to resolve the red thrombus in the vessels) adjusted balances are existing. Minor disorders of these balances can lead to haemorrhages or to a thrombus resp. thrombosis in other than the injured regions.
In healthy persons the heamostasis has to fulful following conditions: it has to be activated in the case of an injury quick enough, i.e. within a few minutes, and lead to a bleeding stop and it must be restricted to the injured area.
The fulfilment of these requirements is assured and regulated by using different activating systems, by positive and negative feedback and by disabling mechanisms.
In the following the complex mechanisms of haemostasis are discribed from the medical and biomedical point of view. The haemostasis reactions can be devided into different stages. One can differentiate between cellular (primary haemostasis) and plasmatic clotting (secondary haemostasis). There are tight functional and temporal interactions between these systems.