PLATELETS
Platelets are fragments of larger bone marrow cells called megakaryocytes.  As these large cells mature, their cytoplasm breaks off into small fragments, which are the platelets.  Lifespan of platelets in the circulation is only 5 days  They play an important role in the prevention of bleeding..

A normal platelet has a diameter of one to two microns and often has irregular borders.  As the platelet travels through the circulation, it is attracted to breaks or injuries in the vessel walls.  Since it has a sticky surface, the platelet readily adheres to other platelets, piling up quickly to form a "platelet plug."  This plug can initiate the coagulation process.
 

Click on each blood type to learn more:
Red Cells
White Cells
Platelets
Plasma