A venous thrombosis is a blood clot that forms within a vein.
Thrombosis is a specific medical term for a blood clot that remains in the place where it formed. Superficial venous thromboses can cause discomfort but generally do not cause serious consequences, unlike the deep venous thromboses (DVTs) that form in the deep veins of the legs or in the pelvic veins.
Since the veins return blood to the heart, if a piece of a blood clot formed in a vein breaks off it can be transported to the right side of the heart, and from there into the lungs. A piece of thrombus that is transported in this way is an embolism: the process of forming a thrombus that becomes embolic is called a thromboembolism. An embolism that lodges in the lungs is a pulmonary embolism (PE).
Systemic embolisms of venous origin can occur in patients with an atrial or ventricular septal defect, through which an embolus may pass into the arterial system. This is termed a paradoxical emboli.
A pulmonary embolus is a very serious condition that can be fatal if not recognized and treated promptly.
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