When you breathe in, air enters your body through your nose and mouth. This upper airway warms, humidifies, and filters the air you breathe. The air continues down through the trachea (windpipe), into your bronchi (airways), and into your lungs. The air we breathe on earth contains about 21% oxygen.
At the end of each bronchi is a cluster of alveoli (air sacs). The walls of the alveoli are about as thin as a soap bubble. Oxygen goes through these thin walls and into the blood where it is carried throughout your body. Oxygen helps fuel all the work your body must do to stay alive, move, and get you through your day. A healthy adult lung contains an estimated 300 million alveoli!
After your body uses the oxygen, it becomes carbon dioxide, which travels back to your lungs and is breathed out.
Your lungs have a system, hair-like sweepers called cilia, to clean out the mucus. These cilia move mucus upwards, kind of like a conveyor belt.
When smoke and other pollutants enter the lungs, the cilia are often paralyzed or destroyed. This causes airways to become swollen and narrowed. The lungs should be elastic, able to stretch and then return back to normal size. Over time, lungs with Chronic Obstructive Pulmonary Disease lose their elasticity, get big and floppy, and make it hard for you to get air out.