How the Lungs Work

Breathing supplies your cells with oxygen and removes the carbon dioxide they produce as waste. The nasal cavity adds warmth and moisture to air from your nose. Some dust and germs get caught in mucus or tiny hairs. Your mouth provides another entrance. Air passes through the pharynx, or throat, then along the firm windpipe, called the trachea. Two bronchi lead from it, with one entering each lung. These tubes divide repeatedly into narrower bronchioles, forming a branching network like an upside-down tree. Millions of small sacs called alveoli sit at the ends. Tiny blood vessels surround each sac. Oxygen crosses the thin walls into your blood as carbon dioxide moves into the sac. Breathing out removes that carbon dioxide. This exchange continues with each breath.

Here are the key parts of your breathing system, in the order air visits them. The pharynx is your throat, a tube that connects your nose and mouth to the rest of the airway. The larynx is your voice box, a short passage between your throat and your windpipe. The trachea is your windpipe, the tube that carries air down toward your lungs. At the bottom, the trachea splits into two bronchi, which are branches of tissue that each lead into one lung. Inside the lungs, each bronchus splits into smaller airways called bronchioles. At the ends of the bronchioles are the alveoli, tiny air sacs where gas exchange happens. Gas exchange means oxygen from the air you breathed in moves into your blood, and carbon dioxide, the waste gas, moves out of your blood so you can exhale it. The whole team of parts working together is called the respiratory system, the body system that swaps gases between your body and the outside air.

Two lungs sit in your chest, on opposite sides of your heart. They exchange gases between blood and air. Airways in the conducting zone carry air without exchanging gases. These include the windpipe, bronchi and bronchioles up to the terminal bronchioles. The respiratory zone includes respiratory bronchioles and alveoli, where gas exchange occurs. In the alveoli, blood receives oxygen and releases carbon dioxide. Your heart sends oxygen-poor blood to the lungs. After exchanging gases, that blood returns to your heart, ready to be pumped around your body. Protective layers called pleurae surround each lung and help it move smoothly during the thousands of breaths you take daily.

The breathing system has upper and lower parts. In the grouping used here, the upper part includes the mouth, nose, nasal cavity, throat and voice box. It receives air, which gains warmth before travelling further. The lower part begins at the trachea, or windpipe. Firm cartilage rings stop this tube from closing during breathing. It forks into two bronchi, with a branch serving each lung. Further divisions form thinner bronchioles spread throughout the lungs like tree twigs. The smallest branches lead to clusters of alveoli. These thin-walled sacs sit beside tiny vessels called capillaries. Oxygen crosses the sac and capillary walls into blood. Carbon dioxide crosses in the opposite direction and leaves when you breathe out.

Oxygen crossing from an alveolus into blood mostly attaches to haemoglobin, a protein held in red blood cells. Haemoglobin collects oxygen in the lungs and carries it around your body. At tissues needing oxygen, it releases its cargo. The oxygen enters cells, including working leg muscles and your brain. Cells need it to release energy from food. They also produce carbon dioxide as waste. This gas passes into blood for the return trip to your lungs. There, it enters the alveoli and leaves your body when you breathe out. Blood keeps making these journeys through the day and night, bringing oxygen to cells and taking carbon dioxide away.