Deep vein thrombosis occurs when a blood clot forms in the deep veins of the leg and obstructs the flow of blood back to the heart. This can lead to swelling of the leg and pain in the calf muscle, although sometimes there are no symptoms. Pulmonary embolism occurs when blood clots leave the veins where they developed, travel through the right side of the heart, and lodge in the small or large branches of the blood vessels going to the lung (pulmonary arteries). This can cause symptoms such as chest pain, difficulty breathing, or coughing up blood. In severe cases, it may result in collapse and sudden death. Long airplane flights or multiple flights in a short period can be associated with deep vein thrombosis and pulmonary embolism. Any situation in which the leg is bent at the knee for prolonged periods without much active motion may lead to a reduction of blood flow and increase the risk of blood clots. Other factors can increase this risk, such as recent surgery, taking oral contraceptives or hormone therapy, pregnancy, cancer, heart problems, and older age. Inherited genetic factors may also play a role.