Pulmonary embolism (PE) is a condition in which a blood clot, usually originating from the deep veins of the lower limbs or pelvis, travels through the venous circulation and lodges in a pulmonary artery or one of its branches. The resulting obstruction produces important pathophysiological changes in pulmonary circulation, gas exchange, cardiac function and tissue oxygenation. The severity depends on the size and location of the embolus, the extent of vascular obstruction and the patient’s underlying cardiopulmonary condition. The primary pathophysiological change is sudden obstruction of pulmonary blood flow. When an embolus blocks a pulmonary artery, ventilation may continue in the affected region, but perfusion is reduced or completely absent. This produces a ventilation-perfusion (V/Q) mismatch because air reaches alveoli that are no longer adequately supplied with blood. These areas become physiologically similar to dead space. As a result, effective gas exchange decreases and arterial oxygen levels may fall. The embolus also causes pulmonary vasoconstriction. Substances such as thromboxane and serotonin released from platelets and injured vascular tissue produce constriction of pulmonary vessels.