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Three factors are thought to contribute to the formation of blood clots (Virchow's Triad): 
-blood vessel damage 
-interrupted blood flow or turbulence 
-congenital or medical problems that cause someone to be predisposed to clots 

Some of the well-known risk factors that health care workers focus on are: sitting down or laying still for extended periods of time, blood clotting disorders, heart arrhythmias, and cancer.
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There are platelets present in the blood. these platelets block or seal the path of flowing blood where ever there is an injury.
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