In our discussion of how the heart works, we said that blood is pumped from the right side of your heart to your lungs. Just like the air you inhale moves through passageways, the blood that flows through your lungs moves through a set of branching pipes called pulmonary vessels. The biggest pipes are called pulmonary arteries; as the vessels move deeper into the lungs, they divide into smaller and smaller branches, eventually becoming a large network of very thin-walled vessels calledcapillaries. These vessels are important because they are the site of oxygen transfer from the alveoli, across the alveolar and capillary walls, into the blood. If there is trouble with the pulmonary vessels, you may have difficulty getting oxygen into the blood.