If a blood clot becomes lodged in the lungs because it broke off of a larger clot somewhere else in the body it can stop the blood flow to that part of the lung tissue. This can cause the tissue not recieving blood to infarct or die. It can also cause a shunt or a change in where the blood goes in the lung which can cause a "high blood pressure" in the lung and can cause strain on the heart.
Bllod clots in the lungs vary in severity depending on size and location. Some are small and not as severe and can be easily treated while others cause sudden death.
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I wanted to add to my answer. Usually blood clots are not diagnosed well by XRAY. Better by CT scan or VQ scan. On Xray sometimes (rarely) you can see an area of infarction that is wedge shaped. This does not look like a nodule.
If you have a pulmonary nodule (or something that you are not sure about on chest XRAY) you should get a follow up CT scan of your chest.
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