Hereditary telangiectasia causes a connection between arteries and veins where there should not be connections. This can happen in the skin, mucus membranes, and other organs in addition to the lungs. A CT scan of the chest is often the first test that identifies this in the lungs.
They can vary greatly in number and size. The effect this has on you will depend upon the number and their size. When they are larger, you are more likely to have blood in your sputum.