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Deep vein thrombosis (DVT) occurs when a blood clot forms in a vein, usually in the leg. Since blood in veins is returning to the lungs for oxygenation, the clot can be pushed into the lungs and cause a pulmonary embolism, which can result in partial or full restriction of blood supply to the lungs. Depending on the severity of the embolism, either stress on the heart and lungs or fatality can result.

Deep veins are attached to muscles, leading to the inferior and superior vena cavae. These two central veins represent the main collection source for all blood returning to the lungs. Since the vena cavae are rather large, it is easy for a clot from a smaller vein to quickly pass through them to create a pulmonary embolism. As such, deep vein thrombosis is a medically urgent condition.
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