How the Lungs Work

Also known as Respiratory System

Your lungs are a pair of organs in your chest. They have a spongy texture and are pinkish-gray in color.

When you inhale (breathe in), air enters your lungs and oxygen from the air moves from your lungs to your blood. At the same time, carbon dioxide, a waste gas, moves from your blood to the lungs and is exhaled (breathe out). This process is called gas exchange and is essential to life.

In addition to the lungs, your respiratory system includes the trachea (windpipe), muscles of the chest wall and diaphragm, blood vessels, and tissues that make breathing and gas exchange possible. Your brain controls your breathing rate (how fast or slow you breathe), by sensing your body’s need for oxygen and its need to get rid of carbon dioxide.

Healthy lifestyle habits, such as physical activity and not smoking, can help prevent lung injury and disease.

Explore this Health Topic to learn more about how the lungs work, our role in research and clinical trials, and where to find more information.

The Respiratory System - How the Lungs Work

Your lungs lie on each side of your heart inside your chest cavity. The right lung is divided into three lobes (sections), and the left lung is divided into two lobes. Your left lung is slightly smaller than your right lung, since your heart takes up some space on the left side. When you breathe in, air enters your airways and travels down into the alveoli (air sacs) in your lungs. This is where gas exchange takes place.

An NHLBI animation walks through the structures of the lungs. Medical Animation Copyright © 2019 Nucleus Medical Media Inc. All rights reserved.

The circulatory system, which is made up of the heart and blood vessels, supports the respiratory system by bringing blood to and from the lungs. The circulatory system helps  to deliver nutrients and oxygen from the lungs to tissues and organs throughout the body and removes carbon dioxide and waste products. Other body systems that work with the respiratory system include the nervous system, lymphatic system, and immune system.

The Respiratory System
The Respiratory System. The image shows an enlarged view of the airways and lungs, as well as the trachea; bronchial tubes, or bronchi; and bronchioles. The image also shows a close-up view of gas exchange at the alveoli. The blue arrows show the oxygen in the air you inhale passing into the bloodstream, and the green arrows show the carbon dioxide from your body passing out of the bloodstream. Medical Illustration Copyright © 2020 Nucleus Medical Media Inc. All rights reserved.


The airways are pipes that carry oxygen-rich air to the alveoli in your lungs. They also carry the waste gas carbon dioxide out of your lungs. The airways include these parts of your body:

  • Mouth
  • Nose and linked air passages called the nasal cavity and sinus
  • Larynx (voice box)
  • Trachea (windpipe)
  • Tubes called bronchial tubes, or bronchi, and their branches
  • Small tubes called bronchioles that branch off of the bronchial tubes

An NHLBI video provides an animation about what happens in your lungs when you breathe in and breathe out. Medical Animation Copyright © 2020 Nucleus Medical Media Inc. All rights reserved.


Air comes into your body

Air first enters your body through your nose or mouth, which moistens and warms the air since cold, dry air can irritate your lungs. The air then travels past your voice box and down your windpipe. Rings of tough tissue, called cartilage, acts as a support to keep the bronchial tubes open.

Inside your lungs, the bronchial tubes branch into thousands of thinner tubes called bronchioles. The bronchioles end in clusters of tiny air sacs called alveoli.

Air fills your lung’s air sacs

Your lungs have about 150 million alveoli. Normally, your alveoli are elastic, meaning that their size and shape can change easily. Alveoli are able to easily expand and contract, because their insides are coated with a substance called surfactant. Surfactant reduces the work it takes to breathe by helping the lungs inflate more easily when you breathe in and preventing the lungs from collapsing when you breath out.

Each of these alveoli is made up of a mesh of tiny blood vessels called capillaries. The capillaries connect to a network of arteries and veins that move blood through your body.

Blood low in oxygen flows through the lungs

The pulmonary artery and its branches deliver blood to the capillaries that surround the alveoli. This blood is rich in carbon dioxide and low in oxygen.

Oxygen flows into your blood

Carbon dioxide moves from the blood into the air inside the alveoli. At the same time, oxygen moves from the air into the blood in the capillaries.

How does my body protect the airways from food or bacteria?

How Your Body Controls Breathing - How the Lungs Work

The body’s muscles and nervous system help control your breathing.

The muscles used for breathing

The lungs are like sponges; they cannot expand (get bigger) on their own. Muscles in your chest and abdomen contract (tighten) to create a slight vacuum around your lungs. This causes air to flow in. When you exhale, the muscles relax and the lungs deflate on their own, much like an elastic balloon will deflate if left open to the air.

The breathing muscles include the:

  • Diaphragm, which is a dome-shaped muscle below your lungs. It separates the chest cavity from the abdominal cavity. The diaphragm is the main muscle used for breathing.
  • The muscles between your ribs, called intercostal muscles, play a role in breathing during physical activity.
  • Abdominal muscles help you breathe out when you are breathing fast, such as during physical activity.
  • Muscles of the face, mouth, and pharynx. The pharynx is the part of the throat right behind the mouth. These muscles control the lips, tongue, soft palate, and other structures to help with breathing. Problems with these muscles can narrow the airway, make it more difficult to breathe, and contribute to sleep apnea.
  • Muscles in the neck and collarbone area help you breathe in.
Cross-section of lungs to show the diaphragm.
Cross-section of lungs to show the diaphragm. The main image shows the location of the lungs, pleura, and diaphragm.

Damage to the nerves in the upper spinal cord can interfere with the movement of your diaphragm and other muscles in your chest, neck, and abdomen. This can happen due to a spinal cord injury, a stroke, or a degenerative disease such as muscular dystrophy. The damage can cause respiratory failure. Ventilator support or oxygen therapy may be necessary to maintain oxygen levels in the body and protect the organs from damage.

The nervous system

Your breathing usually does not require any thought, because it is controlled by the autonomic nervous system, also called the involuntary nervous system.

  • The parasympathetic system slows your breathing rate. It causes your bronchial tubes to narrow and the pulmonary blood vessels to widen.
  • The sympathetic system increases your breathing rate. It makes your bronchial tubes widen and the pulmonary blood vessels narrow.

Your breathing changes depending on how active you are and the condition of the air around you. For example, you need to breathe more often when you do physical activity. At times, you can control your breathing pattern, such as when you hold your breath or sing.

To help adjust your breathing to changing needs, your body has sensors that send signals to the breathing centers in the brain.

  • Sensors in the airways detect lung irritants. The sensors can trigger sneezing or coughing. In people who have asthma, the sensors may cause the muscles around the airways in the lungs to contract. This makes the airways smaller.
  • Sensors in the brain and near blood vessels detect carbon dioxide and oxygen levels in your blood.
  • Sensors in your joints and muscles detect the movement of your arms or legs. These sensors may play a role in increasing your breathing rate when you are physically active.

In central sleep apnea, the brain temporarily stops sending signals to the muscles needed to breathe. Learn more at our Sleep Apnea Health Topic.

What Breathing Does for the Body - How the Lungs Work

Breathing involves two phases: breathing in and breathing out. If you have problems breathing, gas exchange may be impaired, which can be a serious health problem.

Breathing in

When you breathe in, or inhale, your diaphragm contracts and moves downward. This increases the space in your chest cavity, and your lungs expand into it. The muscles between your ribs also help enlarge the chest cavity. They contract to pull your rib cage both upward and outward when you inhale.

As your lungs expand, air is sucked in through your nose or mouth. The air travels down your windpipe and into your lungs. After passing through your bronchial tubes, the air travels to the alveoli, or air sacs.

Gas exchange

Gas exchange in your lungs. When you breathe in, air enters your nose or mouth, and passes into your windpipe, also called the trachea. At the bottom, the windpipe divides into two bronchial tubes, then branches into smaller bronchioles. The brochioles end in tiny air sacs, called alveoli. In the alveoli, the oxygen you inhaled passes into the bloodstream, and carbon dioxide from your body passes out of the bloodstream. The carbon dioxide is expelled from your body when you exhale. Medical Animation Copyright © 2020 Nucleus Medical Media Inc. All rights reserved.



Through the thin walls of the alveoli, oxygen from the air passes into your blood in the surrounding capillaries. At the same time, carbon dioxide moves from your blood into the air sacs. The oxygen in your blood is carried inside your red blood cells by a protein called hemoglobin.

The oxygen-rich blood from your lungs is carried to the left side of the heart through the pulmonary veins. The heart pumps the blood to the rest of the body, where oxygen in the red blood cells moves from blood vessels into your cells.

Your cells use oxygen to make energy so your body can work. During this process, your cells also make a waste gas called carbon dioxide. Carbon dioxide needs to be breathed out or it can damage your cells.

Carbon dioxide moves from the cells into the bloodstream, where it travels to the right side of your heart. The blood rich in carbon dioxide is then pumped from the heart through the pulmonary artery to the lungs, where it is breathed out.

For more information on blood flow, visit our How the Heart Works Health Topic.

Breathing out

When you breathe out, or exhale, your diaphragm and rib muscles relax, reducing the space in the chest cavity. As the chest cavity gets smaller, your lungs deflate, similar to releasing of air from a balloon. At the same time, carbon dioxide-rich air flows out of your lungs through the windpipe and then out of your nose or mouth.

Breathing out requires no effort from your body unless you have a lung disease or are doing physical activity. When you are physically active, your abdominal muscles contract and push your diaphragm against your lungs even more than usual. This rapidly pushes air out of your lungs.

Conditions that affect the respiratory system

Damage, infection, or inflammation in the lungs or airways or both, can lead to the following conditions.

Exposure to cigarette smoke, air pollutants, or other substances can damage the airways, causing disease of the airways or making a disease more severe.

How to Keep Your Lungs Healthy - How the Lungs Work

You can take these steps to help protect your lungs from injury or disease:

  • Quit smoking, or not starting if you do not smoke. Visit Smoking and Your Heart and the National Heart, Lung, and Blood Institute’s Your Guide to a Healthy Heart. Although these resources focus on heart health, they include basic information about how to quit smoking. For free help and support to quit smoking, you may call the National Cancer Institute’s Smoking Quitline at 1-877-44U-QUIT (1-877-448-7848).
  • Avoid secondhand tobacco smoke by staying away from places where smoking is allowed. Ask friends and family members who smoke not to do it in the house or car.
  • Aim for a healthy weight. Unhealthy eating patterns and lack of physical activity can lead to overweight and obesity, which can result in sleep apnea. Research has shown that losing weight can reduce sleep apnea in people who have also been diagnosed with obesity.
  • Be physically active. By being physically active, you can help strengthen your heart and lungs so they work more efficiently. Physical activity may also reduce your risk of lung injury or disease. Our Move More Fact Sheet includes basic information about physical activity. Before starting any exercise program, ask your doctor what level of physical activity is right for you.
  • Limit exposure to outdoor air pollution by checking the Air Quality Index before taking part in outdoor activities and avoiding heavy traffic when possible.
  • Reduce indoor air pollution by making sure that the places where you live and work are well ventilated and cleaned regularly to prevent the buildup of allergens, dust, and mold. You can also remove products that create fumes, such as strong cleaning products and aerosols. Avoid burning solid fuels such as wood for heating and cooking.
  • Take precautions against seasonal flu and pneumonia. Get a flu shot every year. You may also want to ask your doctor or healthcare provider about the pneumonia vaccine.
  • Test your home for radon gas. Radon is a colorless, tasteless, and odorless gas that forms naturally. It can enter buildings through cracks in the wall and can cause lung cancer. Inexpensive testing kits are available from many hardware stores. If radon levels are hazardous, take recommended steps to reduce radon levels in your home, or alert the property owners so they can fix the building.
  • Use protective gear if you work in an industry that involves exposure to dust, silica, allergens, chemical fumes, or other indoor or outdoor air pollution.

How do your lungs change as you get older?

Research for Your Health

The NHLBI is part of the U.S. Department of Health and Human Services’ National Institutes of Health (NIH)—the Nation’s biomedical research agency that makes important scientific discoveries to improve health and save lives. We are committed to advancing science and translating discoveries into clinical practice to promote the prevention and treatment of heart, lung, blood, and sleep disorders. Learn about current and future NHLBI efforts to improve health through research and scientific discovery.

Improving health with current research
- How the Lungs Work

Learn about the following ways the NHLBI continues to translate current research to prevent and treat lung problems. Research on this topic is part of the NHLBI’s broader commitment to advancing lung disease scientific discovery.

  • Increasing and Sustaining Research to Reduce the Burden of COPD. The NHLBI, with input from federal and nonfederal partners, developed a COPD National Action Plan to guide stakeholders nationwide in their efforts to reduce the burden of COPD. In addition, the NHLBI’s Learn More Breathe Better® program seeks to increase awareness about lung diseases and conditions, including COPD, and understanding of how to manage and treat them.
  • Collaborating to Improve Asthma Awareness. The National Asthma Education and Prevention Program (NAEPP) raises awareness about asthma as a major public health problem. Working with medical associations, voluntary health organizations, and community programs, NAEPP helps to educate patients, healthcare professionals, and the public about asthma. The Learn More Breathe Better® program also aims to raise awareness about asthma as well as educate people about how to treat it.

Learn about some of the pioneering research contributions we have made over the years that have improved clinical care.

Advancing research for improved health
- How the Lungs Work

In support of our mission, we are committed to advancing lung research in part through the following ways.

  • We perform research. Our Division of Intramural Research and its Pulmonary Branch study diseases that affect the lungs. Specific projects aim to answer clinically relevant questions using methods ranging from molecular-level studies to clinical studies of diagnostics, therapeutics, and interventions.
  • We fund research. The research we fund today will help improve our future health. Our Division of Lung Diseases oversees much of the research on the lungs and other parts of the respiratory system, helping us to understand, prevent, and manage health conditions affecting these organs and tissues. The Airway Biology and Disease Branch supports research and research training in chronic obstructive pulmonary disease, asthma, cystic fibrosis, bronchiolitis, lung imaging, and airway function in health and disease. The Lung Biology and Disease Branch supports research and research training in pulmonary vascular biology and pulmonary hypertension, lung development and repair, acute lung injury and critical care, pulmonary fibrosis and other interstitial lung diseases, rare lung diseases, lung transplantation, and lung responses to HIV/AIDS and other infections. Search the NIH RePORTer to learn about research the NHLBI is funding on the lungs.
  • We stimulate high-impact research. Our Trans-Omics for Precision Medicine (TOPMed) program includes participants who have lung conditions, which may help us understand how genes contribute to differences in disease severity and how patients respond to treatment. The NHLBI Strategic Vision highlights ways we may support research over the next decade.

Learn about exciting research areas the NHLBI is exploring about the lungs.

Participate in NHLBI Clinical Trials

We lead or sponsor studies on the lungs. See if you or someone you know is eligible to participate in a clinical trials.

Trials at the NIH Clinical Center

Genes involved in development of lung diseases

This study will investigate the genes involved in the breathing process and in the development of lung diseases such as asthma or sarcoidosis to improve understanding of the role they play. To participate in this study, you or your child must be between 2 and 90 years old and have been diagnosed and living with a lung disease. This study is located in Bethesda, Maryland, at the NIH Clinical Center.

To learn more about clinical trials at the NIH Clinical Center or to talk to someone about a study that might fit your needs, call the Office of Patient Recruitment 800-411-1222.

More Information

After reading our How the Lungs Work Health Topic, you may be interested in additional information found in the following resources.

Non-NHLBI resources
- How the Lungs Work

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