How the Lungs Work

The air that you breathe in through your nose or mouth travels down through your windpipe (trachea) into tubes in your lungs called bronchial tubes, or airways.

The airways are shaped like an upside-down tree with many branches. The trachea is the trunk. It splits into tubes, called bronchi. Thinner tubes, called bronchioles, branch out of the bronchi.

The bronchioles end in tiny air sacs called alveoli. The alveoli have very thin walls, and small blood vessels called capillaries run through them. There are about 300 million alveoli in a normal lung.

When the air reaches the alveoli, the oxygen in the air passes through the thin walls of the alveoli into the blood in the capillaries. From there, it flows into larger veins and arteries, which carry it to your heart. The heart then pumps the oxygen-rich blood to all of your body's organs. Your heart and other organs can't do their jobs without an ongoing supply of oxygen.

In babies who develop bronchopulmonary dysplasia, the airways aren't yet fully developed. The alveoli are larger than normal, and there are fewer of them. The capillaries also may not be fully developed. As a result, the lungs can't move enough oxygen into the bloodstream to support the heart and other body organs.