“Blood has a seemingly impossible job: It must flow continuously and smoothly throughout your body for an entire lifetime, but quickly shut off to prevent spills when you get a cut or injury.  Blood clots are healthy and lifesaving when they stop bleeding.  Platelets get “turned on” by triggers released when a blood vessel is damaged. They stick to the walls in the area and each other, changing shape to form a plug that fills in the broken part to stop blood from leaking out. Blood clotting, or coagulation, is an important process that prevents excessive bleeding when a blood vessel is injured.”

WebM.D.