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The lungs’ job is to get air into and out of the body. Inside the lungs, air travels through a network of branching airways (tubes) made of stretchy tissue. Each airway is wrapped with bands of muscle that help keep it open. The airways branch out and get smaller as they go deeper into the lungs. The smallest airways end in clusters of tiny balloon-like air sacs (alveoli). These clusters are surrounded by blood vessels.
When you inhale (breathe in), air enters the lungs. It travels down through the airways until it reaches the air sacs.
When you exhale (breathe out), air travels up through the airways and out of the lungs.
The air you inhale contains oxygen, a gas your body needs. When this air reaches the air sacs, oxygen passes into the blood vessels. Oxygen-rich blood then leaves the lungs and travels to all parts of the body. As the body uses oxygen, carbon dioxide (a waste gas) is produced. The blood carries this back to the lungs. Carbon dioxide leaves the body with the air you exhale.
The cells in the lining of the airways produce a sticky secretion called mucus. The mucus traps dust, smoke, and other particles in the air you breathe. The cells have tiny hairs called cilia. They sweep mucus up the airways to the throat, where it’s coughed out or exhaled.