An aneurism is a bulge in an artery wall. Arterial blood is under pressure - not all that high in the case of the pulmonary artery and its branches into the lungs compared with the aorta to the rest of the body. The wall of arteries is constructed very strongly, with muscle, elastic tissue and a tough fibrous layer on the ouside to resist this pressure. Occasionally the wall has a weak bit (and what's caused that...??) which makes it give way and bulge out. I guess the muscle/elastic layer had gone but the outer fibrous layer is holding. Ther really dangerous one is the aortic aneurism which, if it bursts altogether can be fatal almost instantly. I'd think that a bronchial artery aneurism is less dangerous but still needs seeing to. That's about the limit of my knowledge - it's not really on the A-level Biology syllabus! However, if I can be of any more help.... All the best Steve
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