there are many factors from of them.....
1- normal blood viscosity: they are related in the plasma protein content .
2- normal healthy endothelial cells of the blood vessels: they are related in the lipids level in the blood and in some factors such as smoking.
3- anticoagulation factors as heparin preduced from basophils.
4- removal of blood clotting by the mechanisim of lysis.
5- present of prostacyclin which prevent aggregation of blood platelets.

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warning ourself from the future is an important factor for success]