Pulmonary Embolism
A venous clot, usually from the deep leg veins, lodges in the pulmonary arterial tree and abruptly creates ventilated-but-unperfused lung and a sudden rise in right ventricular afterload, so the clinical picture is driven by gas-exchange mismatch and acute right heart strain rather than by the lung itself being diseased.
First principles
The lung tissue is normal; the problem is a blocked pipe
Pulmonary embolism begins with venous thrombosis, typically in the deep veins of the leg or pelvis, favoured by Virchow's triad: venous stasis (immobility, surgery), endothelial injury, and hypercoagulability (malignancy, pregnancy, oestrogen, inherited thrombophilia). A fragment breaks off and travels through the right heart to lodge in the pulmonary arteries. Unlike pneumonia or asthma, the alveoli themselves are structurally untouched at the moment of embolism: the defect is purely vascular, which is why the chest X-ray is often normal and why the diagnosis rests on imaging the pulmonary vasculature rather than the lung parenchyma.
Educational content pending clinical review. Not medical advice.