The definition, epidemiology, pathogenesis, and pathophysiology of PE are discussed in detail in this topic. A broad overview of the clinical presentation, evaluation, and diagnosis as well as treatment, prognosis, and follow-up of PE is also provided in this topic; however, a detailed discussion of these issues is provided separately. (See "Clinical presentation, evaluation, and diagnosis of the nonpregnant adult with suspected acute pulmonary embolism" and "Treatment, prognosis, and follow-up of acute pulmonary embolism in adults" .)
To understand why these clots end up in the lungs it helps to talk a little about the anatomy. The veins in the legs come together and blood returns to the right side of the heart through a big vein called the inferior vena cava. From the right heart, the blood then travels to the lungs via the pulmonary arteries to renew its supply of oxygen. Going upstream from the veins in the legs, all of the blood vessels (including the heart) are larger. When the blood enters the lungs, however, the vessels become progressively smaller, and this is where the clots then become "trapped" in one of the pulmonary arteries leading to pulmonary infarction (death) of part of a lung.