Blood is an amazing substance. Inside the body it is a liquid. Outside the body it clots and becomes a solid. The clotting mechanisms that make this happen are very complex.
Clots tend to break into pieces.
If these pieces break away, they may float with the bloodstream until they plug up a blood vessel. A blood clot is called a thrombus. The process of clotting inside a blood vessel is called thrombosis. When clotting occurs in the veins, it is called venous thrombosis or thrombophlebitis. Deep vein thrombosis is a serious condition, sometimes leading to a pulmonary embolism.