How the Lungs Work

Also known as Respiratory System
Your lungs are a pair of pyramid-shaped organs inside your chest that allow your body to take in oxygen from the air. They have a spongy texture and are pinkish-gray in color. The lungs bring oxygen into the body when breathing in and send carbon dioxide out of the body when breathing out. Carbon dioxide is a waste gas produced by the cells of the body.

The process of breathing in is called inhalation. The process of breathing out is called exhalation. Breathing is a vital function of life. The lungs add oxygen to the blood and remove carbon dioxide in a process called gas exchange.

In addition to the lungs, your respiratory system includes airways, muscles, blood vessels, and tissues that help make breathing possible. Your brain controls your breathing based on your body’s need for oxygen.

A healthy lifestyle 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 to improve health, and where to find more information.

The Respiratory System - How the Lungs Work

The respiratory system helps you breathe. The main parts of the respiratory system are the lungs, the airways, and the muscles that enable breathing. The circulatory system, which is made up of the heart, veins, arteries, and capillaries, brings blood to and from the lungs and delivers nutrients and oxygen to tissues of the body while removing carbon dioxide and waste products. Other body systems that work with the respiratory system include the nervous system, lymphatic system, and immune system.

An enlarged view of the airways and lungs as well as the trachea; bronchial tubes, or bronchi; and bronchioles.
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 © 2019 Nucleus Medical Media Inc. All rights reserved.

 

Lungs
- How the Lungs Work

Your lungs. Your lungs are two spongy organs in your chest. The left lung is divided into two lobes and the right lung is divided into three lobes. Medical Animation Copyright © 2019 Nucleus Medical Media Inc. All rights reserved.

Your lungs lie on each side of your breastbone and fill the inside of your chest cavity. The right lung is divided into three main sections called lobes, and the left lung has two lobes to allow space for the heart. Your left lung is slightly smaller than your right lung.

Airways
- How the Lungs Work

Breathing air into 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, or bronchi, then branches into smaller bronchioles. The bronchioles end in tiny air sacs, called alveoli. Here the oxygen you inhale 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 © 2019 Nucleus Medical Media Inc. All rights reserved.

The airways are pipes that carry oxygen-rich air to your lungs. They also carry carbon dioxide, a waste gas, out of your lungs. The airways include your:

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

Air first enters your body through your nose or mouth, which wets and warms the air. Cold, dry air can irritate your lungs. The air then travels past your voice box and down your windpipe. The windpipe splits into two bronchial tubes that enter your lungs. A tough tissue called cartilage helps the bronchial tubes stay open.

Within the lungs, your bronchial tubes branch into thousands of smaller, thinner tubes called bronchioles. The muscular walls of the bronchioles are different from the bronchial tubes. The bronchioles do not have cartilage to help them stay open, so the walls can widen or narrow to allow more or less airflow through the tubes.

The thousands of bronchioles end in clusters of tiny round air sacs called alveoli. Your lungs have about 150 million alveoli. Normally, your alveoli are elastic, meaning that their size and shape can change easily. Surfactant coats the inside of the sacs or alveoli and helps the air sacs stay open.

Each of these alveoli is covered in a mesh of tiny blood vessels called capillaries. The space where the alveoli come into contact with the capillaries is called the lung interstitium. The capillaries connect to a network of arteries and veins that move blood through your body.

The pulmonary artery and its branches deliver blood rich in carbon dioxide and lacking in oxygen to the capillaries that surround the air sacs. 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 pleura and diaphragm. Your lungs are encased by pleura, a thin membrane that protects them and helps them slide back and forth as you breathe in and out. Underneath your lungs is the diaphragm, a smooth thin muscle that helps your lungs expand and contract as you breathe. Medical Animation Copyright © 2019 Nucleus Medical Media Inc. All rights reserved.

 

The pleura and the muscles used for breathing
- How the Lungs Work

The lungs are enclosed by the pleura, a membrane that has two layers. The space between these two layers is called the pleural cavity. The membrane’s cells create pleural fluid, which acts as a lubricant to reduce friction during breathing.

The lungs are like sponges; they cannot move on their own. Muscles in your chest and abdomen contract, or tighten, to create space in your lungs for air to flow in. The muscles then relax, causing the space in the chest to get smaller and squeeze the air back out.

These 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.
  • Intercostal muscles, which are located between your ribs. They also play a major role in helping you breathe.
  • Abdominal muscles. They 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 cause sleep apnea.
  • Muscles in the neck and collarbone area. These muscles help you breathe in when other muscles involved in breathing are not working well or when lung disease impairs your breathing.

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.

Role of the nervous system
- How the Lungs Work

Your breathing usually does not require any thought, because it is controlled by the autonomic nervous system, also called the involuntary nervous system. The autonomic nervous system has different effects on your breathing.

  • The parasympathetic nervous system tells the diaphragm and intercostal muscles to tighten and relax more quickly or more slowly to adjust your breathing rate in response to carbon dioxide and oxygen levels in the brain. This system also causes your bronchial tubes to narrow and the pulmonary blood vessels to widen.
  • The sympathetic nervous system increases your breathing rate through what is called the fight-or-flight response. When this system is activated, it releases a chemical signal called norepinephrine that 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. In contrast, your body needs to restrict how much air you breathe if the air contains irritants or toxins. 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 help your respiratory system provide enough oxygen to the body while removing carbon dioxide. The sensors transmit signals to the centers of the brain involved in breathing. These sensors include:

  • Sensors in the airways that 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 in two blood vessels, the aorta and the carotid artery in the neck, that detect carbon dioxide or oxygen levels in your blood and change your breathing rate as needed.
  • Sensors in your joints and muscles that detect the movement of your arms or legs. These sensors may play a role in increasing your breathing rate when you are physically active.

Some health conditions can interfere with brain signals to the airways and chest muscles, causing central sleep apnea. In central sleep apnea, the respiratory centers of the brain do not respond properly to rising carbon dioxide levels and do not properly control the diaphragm and intercostal muscles. As a result, breathing stops for a short period. Central sleep apnea can lead to stroke and other medical conditions.

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- How the Lungs Work

What Breathing Does for the Body - How the Lungs Work

Breathing involves two phases: breathing in and breathing out. Your lungs deliver oxygen and remove carbon dioxide from your blood in a process called gas exchange. Gas exchange happens in the capillaries surrounding the alveoli, where the oxygen that is breathed in enters the circulatory system and carbon dioxide in the blood is released to the lungs and then breathed out. If you have problems breathing, gas exchange may be impaired, increasing the risk of serious health problems.

Breathing in
- How the Lungs Work

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
- How the Lungs Work

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. Here the oxygen you inhaledi 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 © 2019 Nucleus Medical Media Inc. All rights reserved.

Through the very 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 the capillaries into the air sacs. This process of exchanging oxygen for carbon dioxide is called gas exchange. The oxygen in your blood is stored inside your red blood cells by a protein called hemoglobin.

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

As carbon dioxide is released from the cells of the body, it travels in the bloodstream to the heart. The blood rich in carbon dioxide is then pumped from the right side of the heart through the pulmonary artery to the lungs, where gas exchange occurs.

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

Breathing out
- How the Lungs Work

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 the 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
- How the Lungs Work

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.

Keeping Your Lungs Healthy - How the Lungs Work

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

  • Quitting 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).
  • Avoiding 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.
  • Aiming 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.
  • Being 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 recommendations for physical activity. Before starting any exercise program, ask your doctor what level of physical activity is right for you.
  • Limiting exposure to outdoor air pollution by checking the Air Quality Index before taking part in outdoor activities and avoiding heavy traffic when possible.
  • Reducing 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.
  • Taking 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.
  • Testing 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.
  • Using 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?

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- How the Lungs Work

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.

[At NIH Clinical Center] Causes and progression of chronic lung disease

This study aims to develop a better understanding of lung disorders. This study also identifies patients who are eligible to participate in other research studies. To participate in this study, you must be between 7 and 90 years old and have symptoms of lung disease. This study is located in Bethesda, Maryland, at the NIH Clinical Center.

[At 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.

Are you an adult who is healthy or has lung disease?

This study aims to collect blood, urine, and airway samples from healthy volunteers and from people who have lung disease. Researchers will use the samples to study genes and proteins that may be involved in lung disease. To participate in this study, you must be at least 18 years old or older. This study is located in New York, New York.

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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