The clot travels upwards in the veins, through the heart, and goes into the blood vessels in the lungs. The blood vessels in the lungs get smaller and smaller and the clot finally stops where the blood vessel is so small that the clot can not get through it. It obstructs the flow of blood at that point and causes an embolus. This embolus cuts off the blood supply to a part of the lung. Depending on where the clot stops and how large the clot is, the part of the lung which has lost its blood supply will no longer function.
Many cases occur among people already hospitalized for other reasons.
A person is more likely to have clots after surgery, stroke, long-term bed rest or inactivity, or fractures of the hip or legs. There is a somewhat higher risk among overweight people and during pregnancy. There is also an increased risk among women who take birth control pills and smoke cigarettes. If a clot lodges in the lungs, there may be sudden shortness of breath, chest pain, coughing up blood, and a low-grade fever. The patient sometimes tightens the chest muscles or avoids deep breathing to diminish pain.