What does blood clots on the lungs mean?

Pulmonary embolism: When doctors say blood clots in the lungs, they are usually talking about pulmonary embolism, which is blockages in one or more arteries in the lungs, caused by blood clots from another part of your body, most commonly, your legs. Pulmonary embolism is a potentially life-threatening complication of DVT (deep vein thrombosis), which is clotting in the deep veins.

Can be deadly: The size of the clot dictates how dangerous it is. Very very tiny emboli may not have much clinical consequence, unless it happens frequently. A large embolus that blocks large amounts of flow to the lung can cause symptoms including shortness of breath and even cardiac arrest. Unexpected shortness of breath should always be considered an emergency and evaluated by a doctor right away!