How do the lungs work? When you breathe in, your diaphragm tightens and moves down, and air flows through your nose or mouth, down your windpipe and through branching tubes into tiny air sacs called alveoli. There, oxygen passes into your blood and carbon dioxide passes out, so you can breathe it away.
This is general information, not medical advice. Talk to your doctor or pharmacist about your own situation.
Your lungs sit on each side of your heart, inside your chest. They're the main organs of your respiratory system. According to the National Heart, Lung, and Blood Institute (NHLBI, part of the National Institutes of Health):
The right lung is divided into three lobes, or sections, and the left lung into two.
The left lung is slightly smaller, because your heart takes up some space on the left side.
The lungs are surrounded by the pleura, a membrane with two layers. A slippery fluid between the layers reduces friction as you breathe.
Cleveland Clinic adds that a typical adult lung weighs about 2.2 pounds (1 kilogram). It sums up the lungs' main function as bringing oxygen to your blood and removing carbon dioxide from your body.
Nose or mouth. Air comes in through your nose or mouth, which moisten and warm it. NHLBI explains that cold, dry air can irritate your lungs.
Voice box and windpipe. The air travels past your voice box (larynx) and down your windpipe (trachea).
Bronchi. Two tubes called bronchi branch off the windpipe and carry air into the lungs. Rings of tough tissue called cartilage help keep them open.
Bronchioles. Inside your lungs, the bronchi branch into thousands of thinner tubes called bronchioles.
Alveoli. The bronchioles end in clusters of tiny air sacs called alveoli. Your lungs have about 150 million of them. They're stretchy, and a coating called surfactant helps them inflate easily and keeps them from collapsing when you breathe out.
Capillaries. Each air sac is made up of a mesh of tiny blood vessels called capillaries. This is where air and blood meet.
Gas exchange is the whole point of breathing. NHLBI describes it like this:
The pulmonary artery brings blood that's low in oxygen and rich in carbon dioxide to the capillaries around the air sacs.
Oxygen from the air you inhaled passes through the thin walls of the alveoli into the capillaries. Red blood cells pick it up using a protein called hemoglobin.
At the same time, carbon dioxide moves the other way, from the blood into the air sacs.
Oxygen-rich blood travels through the pulmonary veins to the left side of your heart, which pumps it out to the rest of your body.
Your cells use oxygen to make energy, which creates carbon dioxide as waste. That carbon dioxide travels back to the right side of the heart, then to the lungs, and you breathe it out.
Your heart and lungs work as a team. Our guide to how the heart works follows the same blood on the rest of its trip.
NHLBI compares your lungs to sponges: they can't get bigger on their own. Muscles in your chest and abdomen do the work by creating a slight vacuum around the lungs.
Your diaphragm, a dome-shaped muscle below your lungs, contracts and moves down. That makes more room in your chest, and your lungs expand into it. The muscles between your ribs contract too, pulling your rib cage upward and outward. As your lungs expand, air is drawn in through your nose or mouth.
Your diaphragm and rib muscles relax, your chest gets smaller, and your lungs deflate, much like air leaving a balloon. Breathing out takes no effort unless you have a lung disease or you're physically active. During activity, your abdominal muscles push your diaphragm up to move air out faster.
NHLBI lists these breathing muscles:
Diaphragm: the main muscle used for breathing
Intercostal muscles: the muscles between your ribs, which play a role during physical activity
Abdominal muscles: help you breathe out when you're breathing fast
Muscles of the face, mouth and throat: control your lips, tongue and soft palate to help with breathing
Neck and collarbone muscles: help you breathe in
You don't have to think about breathing, because your autonomic, or involuntary, nervous system handles it. One part, the parasympathetic system, slows your breathing. Another part, the sympathetic system, speeds it up. You can take control for a while, for example when you hold your breath or sing.
Sensors keep the system tuned, according to NHLBI:
In your airways: sensors detect irritants and can trigger a sneeze or a cough.
In your brain and near blood vessels: sensors track the carbon dioxide and oxygen levels in your blood.
In your joints and muscles: sensors detect movement and may help raise your breathing rate when you're active.
Cleveland Clinic says the cycle of taking in oxygen and breathing out carbon dioxide happens 12 to 20 times per minute. You breathe more often during physical activity.
NHLBI notes that damage, infection or inflammation in the lungs or airways can lead to conditions such as asthma, bronchitis, COPD and pneumonia. It also says exposure to cigarette smoke, air pollutants or other substances can damage the airways and make a condition you already have more serious. Steering clear of smoke and polluted air is one of the simplest ways to look after them.
Cleveland Clinic advises talking to a healthcare provider if you have symptoms that affect your breathing. The NHS gives clear guidance on shortness of breath:
Get emergency help right away (call 911 in the US or 999 in the UK) if you have severe difficulty breathing, such as gasping, choking or not being able to get words out, your chest feels tight or heavy, pain spreads to your arms, back, neck or jaw, your lips or skin are turning very pale, blue or gray, or you suddenly feel confused.
Get urgent medical advice if, for example, you're more short of breath than usual, you're coughing up blood, you have pain or swelling in one leg, or you have heart palpitations.
See your doctor if your shortness of breath gets worse with your normal activities or when you lie down, you feel short of breath and have swollen ankles, or you've been coughing for 3 weeks or more.
The diaphragm, a dome-shaped muscle below your lungs. It separates your chest from your abdomen.
About 150 million, according to NHLBI. Each one is made of a mesh of tiny blood vessels where gas exchange happens.
Your heart takes up some space on the left side of your chest. The left lung has two lobes, and the right lung has three.
Cleveland Clinic puts the breathing cycle at 12 to 20 times per minute. You breathe faster when you're physically active.
Body Unpacked: Health Atlas is a free iPhone app for learning how food and the body connect. Its Physiology Lab steps through three teaching models, one of them on breathing and circulation, and Assemble the Body lets you place organs with hints and learn a fact about each one. It's built for learning, not diagnosis, with no account and no ads, and all of its content works without a connection.
Get Body Unpacked free on the App Store
How the Lungs Work: What Breathing Does for the Body (NHLBI, NIH)
How the Lungs Work: How Your Body Controls Breathing (NHLBI, NIH)
Lungs: Location, Anatomy, Function and Complications (Cleveland Clinic)