In the United States, venous thromboembolism is associated with high mortality and morbidity affecting as many as 900 000 people (1-2 per 1000) each year. Estimates suggest that 60 000 to 100 000 Americans die of deep venous thrombosis/pulmonary embolism and 10% to 30% die within 1 month of diagnosis, with sudden death being the first symptom in approximately 25% of population with pulmonary embolism. One-half of the patients with deep venous thrombosis will have postthrombotic syndrome, which includes swelling, pain, discoloration, and scaling in the affected limb. Approximately 33% of patients will have a recurrence within 10 years. It is important to understand the anatomy of the pulmonary circulation and the pathophysiology of the disease as it leads to understanding of the potential devastating clinical consequences.