Here's a reply to a question about the lungs, from my Support Group.
"In Pulmonary Arterial Hypertension (PAH) lung tissue damage occurs inside the capillaries and arteries so it is microscopic. The endothelium, the lining of the arteries, is damaged. There is no test to directly measure this damage so it’s of no use to look at lung tissue and risk the surgery necessary to obtain samples.
What affects the lungs also affects the heart and, in PAH heart damage is secondary but it is the heart damage that kills us. So doctors do tests on the heart instead.
During a heart catherisation, doctors measure the pressure in the pulmonary artery and that gives them a good idea of what is happening to the other arteries in the lung. Doctors only want to do tests that can measure disease progression and be as non-invasive as possible. They could biopsy pieces of lung tissue and that would be of scientific interest, but it would yield no useful information about the progression of your disease, and of how your lungs are reacting to treatment.
