Pulmonary emboli are blood clots within the arteries of the lungs. These blood clots prevent the flow of blood to the lungs which in turn prevents oxygen from reaching the lung tissue which lowers the oxygen levels in the lungs and increasing the blood pressure in the pulmonary arteries. The longer the clot is blocking the oxygen the more damage can occur in the other organs. An embolism is not the same as a thrombosis. A thrombosis is formed in the vein and stays in the same site until possibly resolved with medication or treatment. If a clot or piece of clot breaks free from the wall of the vein and travels to another site it is an embolus. An emboli moves around through the blood vessels until it reaches a small vessel that it cannot pass through, which causes the blockage.