How the Lungs Work

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

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

The bronchioles end in tiny air sacs called alveoli. These air sacs 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 that you've just breathed in reaches these air sacs, the oxygen in the air passes through the air sac walls into the blood in the capillaries. The blood then flows into larger veins, which carry it to the heart. Your heart then pumps the oxygen-rich blood to all your body's organs. These organs can't function without an ongoing supply of oxygen.

The animation below shows how the lungs work. Click the "start" button to play the animation. Written and spoken explanations are provided with each frame. Use the buttons in the lower right corner to pause, restart, or replay the animation, or use the scroll bar below the buttons to move through the frames.

The animation shows how the lungs inhale oxygen and transfer it to the blood. It also shows how carbon dioxide (a waste product) is removed from the blood and exhaled.

In idiopathic pulmonary fibrosis, scarring begins in the air sac walls and the spaces around them. It makes the walls of the air sacs thicker. As a result, oxygen can't move from the air sacs into the bloodstream.

For more information about lung function, see the Diseases and Conditions Index How the Lungs Work article.