A blood clot is a clump of blood that forms in a blood vessel, such as a vein or an artery. If a clot gets stuck in a blood vessel, it can cause serious problems like a deep vein thrombosis (DVT) or a pulmonary embolism.

A DVT is a blood clot in certain veins of the legs, pelvis, or arms. It most often occurs in the legs. Blood clots in these veins need to be treated, because they can get bigger, break loose, and travel through the bloodstream to the heart and then to the lungs. This causes a pulmonary embolism.

A pulmonary embolism is a sudden blockage of an artery in the lung. Blood clots in the deep veins of the leg are the most common cause of a pulmonary embolism. In many cases, the clots are small. They may damage the lung. But if the clot is large and stops blood flow to the lung, it can be deadly.