 Vascular malformations in the lungs may cause a number of problems . The lungs normally " filter out " bacteria and blood clots from the bloodstream ; AVMs bypass the capillary network of the lungs and allow these to migrate to the brain , where bacteria may cause a brain abscess and blood clots may lead to stroke . HHT is the most common cause of lung AVMs : out of all people found to have lung AVMs , 70 – 80 % are due to HHT . Bleeding from lung AVMs is relatively unusual , but may cause hemoptysis ( coughing up blood ) or hemothorax ( blood accumulating in the chest cavity ) . Large vascular malformations in the lung allow oxygen @-@ depleted blood from the right ventricle to bypass the alveoli , meaning that this blood does not have an opportunity to absorb fresh oxygen . This may lead to breathlessness . Large AVMs may lead to <unk> , difficulty in breathing that is more marked when sitting up compared to lying down ; this probably reflects changes in blood flow associated with positioning . Very large AVMs cause a marked inability to absorb oxygen , which may be noted by cyanosis ( bluish discoloration of the lips and skin ) , clubbing of the fingernails ( often encountered in chronically low oxygen levels ) , and a humming noise over the affected part of the lung detectable by stethoscope . 
