General pathology · Hemodynamic disorders
Pulmonary thromboembolism: where it comes from and what it does !
- Mechanism
A clot from the leg veins travels through the right heart to the pulmonary artery. By its size it is silent, causes an infarct, or kills.
Updated 2026-11-01
What it is
Pulmonary thromboembolism (PE) happens when a thrombus, almost always from the proximal deep veins of the legs, breaks off and lodges in the pulmonary arteries. Its effects depend on the size of the embolus and on the state of the heart and lungs: most are small and silent; a large embolus can cause acute right ventricular failure and sudden death. The lung seldom infarcts because it has a dual blood supply (pulmonary and bronchial).
Step by step
- A thrombus forms in a deep vein of the leg or pelvis (Virchow’s triad).
- A fragment breaks off and rises through the inferior vena cava.
- It crosses the right atrium and ventricle and enters the pulmonary artery.
- It lodges according to its size: main trunk, lobar branches or small arteries.
- The lung ventilates areas that get no blood (dead space) and hypoxemia appears.
- If the block is large, the right ventricle dilates and fails: obstructive shock.
- If small arteries are blocked and the bronchial circulation is not enough, a hemorrhagic infarct follows.
Clinical notes
- Sudden breathlessness, fast heart rate and pleuritic chest pain are commonest; syncope or shock suggest a massive embolism.
- Risk factors: surgery (especially orthopaedic), immobilisation, cancer, pregnancy and the postpartum period, estrogens, thrombophilias.
- Many emboli happen during a hospital stay or soon after surgery; that is why thrombosis risk is assessed on admission.
In the lab
- D-dimer: very sensitive, not specific; a normal value helps rule it out in low-probability patients.
- CT pulmonary angiography shows the filling defect; arterial blood gases with hypoxemia and respiratory alkalosis.
Test yourself
Think of the answer, then tap the question to see it.
Where do most pulmonary emboli come from?
From thrombi in the proximal deep veins of the legs (popliteal, femoral and iliac).
Why do most pulmonary emboli not cause an infarct?
Because the lung has a dual supply: the bronchial arteries keep the tissue alive if cardiac function is good.
What does a pulmonary infarct look like grossly?
Hemorrhagic and wedge-shaped, with its base toward the pleura, which often shows fibrinous pleuritis.
What is a paradoxical embolism?
A venous embolus that crosses to the arterial side through a defect such as a patent foramen ovale and causes, for example, a stroke.
Related
- Thrombosis and Virchow's triad
- What happens to a thrombus: propagation, embolism, dissolution, organization
- Infarction: red vs white, and what decides the damage
- Shock: types, stages and the organs it damages
- Fat, air and amniotic fluid embolism
Related conditions
Elsewhere in DurgiPedia
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Pulmonary embolus: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2025); About Venous Thromboembolism (Blood Clots) (Centers for Disease Control and Prevention (CDC), 2025); Venous Thromboembolism - What Is Venous Thromboembolism? | NHLBI, NIH (National Heart, Lung, and Blood Institute (NHLBI), 2022).
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Representative model, for educational purposes only.