A pulmonary embolus is a bloodclot in one of the bloodvessels in the lung. It forms in the veins of the calves or pelvis and then dislodges and travels in the blood stream to the vessels in the lung. The clinical picture depends on the size of the embolus and on the underlying state of the heart and lungs. Deep venous thrombi and Pulmonary emboli develop in people with prolonged bed rest, surgery, severe trauma, heart failure, women on birth control with a high estrogen content and in cancer patients.