The lungs are part of the respiratory system, which allows us to breathe. They bring oxygen into our bodies when we inhale and remove carbon dioxide when we exhale. The diaphragm is the main muscle for breathing, and it expands and contracts the chest to draw air in and out of the lungs. The lungs contain airways that carry air in and out, blood vessels surrounding the lungs, and the muscles that help us breathe. Oxygen from the air we breathe moves to our blood, and carbon dioxide, a waste gas, is removed from our blood. The respiratory system prevents harmful substances from entering our lungs. The lungs and respiratory system automatically perform the vital process of gas exchange.