Table of Contents

What Are the Lungs?

The Respiratory System

What Happens When You Breathe?

What Controls Your Breathing?

Lung Diseases and Conditions

Clinical Trials

Links to Other Information About the Lungs

What Are the Lungs?

Your lungs are organs in your chest that allow your body to take in oxygen from the air. They also help remove carbon dioxide (a waste gas that can be toxic) from your body.

The lungs' intake of oxygen and removal of carbon dioxide is called gas exchange. Gas exchange is part of breathing. Breathing is a vital function of life; it helps your body work properly.

Other organs and tissues also help make breathing possible. (For more information, go to "The Respiratory System" section of this article.)

The Respiratory System

The respiratory system is made up of organs and tissues that help you breathe. The main parts of this system are the airways, the lungs and linked blood vessels, and the muscles that enable breathing.

The Respiratory System

Figure A shows the location of the respiratory structures in the body. Figure B is an enlarged view of the airways, alveoli (air sacs), and capillaries (tiny blood vessels). Figure C is a closeup view of gas exchange between the capillaries and alveoli. CO2 is carbon dioxide, and O2 is oxygen.

Airways

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:

- Nose and linked air passages (called nasal cavities)

- Mouth

- Larynx (LAR-ingks), or voice box

- Trachea (TRA-ke-ah), or windpipe

- Tubes called bronchial tubes or bronchi, and their branches

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 through your voice box and down your windpipe. The windpipe splits into two bronchial tubes that enter your lungs.

A thin flap of tissue called the epiglottis (ep-ih-GLOT-is) covers your windpipe when you swallow. This prevents food and drink from entering the air passages that lead to your lungs.

Except for the mouth and some parts of the nose, all of the airways have special hairs called cilia (SIL-e-ah) that are coated with sticky mucus. The cilia trap germs and other foreign particles that enter your airways when you breathe in air.

These fine hairs then sweep the particles up to the nose or mouth. From there, they're swallowed, coughed, or sneezed out of the body. Nose hairs and mouth saliva also trap particles and germs.

Lungs and Blood Vessels

Your lungs and linked blood vessels deliver oxygen to your body and remove carbon dioxide from your body. Your lungs lie on either side of your breastbone and fill the inside of your chest cavity. Your left lung is slightly smaller than your right lung to allow room for your heart.

Within the lungs, your bronchi branch into thousands of smaller, thinner tubes called bronchioles. These tubes end in bunches of tiny round air sacs called alveoli (al-VEE-uhl-eye).

Each of these air sacs is covered in a mesh of tiny blood vessels called capillaries. The capillaries connect to a network of arteries and veins that move blood through your body.

The pulmonary (PULL-mun-ary) artery and its branches deliver blood rich in carbon dioxide (and lacking in oxygen) to the capillaries that surround the air sacs. Inside the air sacs, carbon dioxide moves from the blood into the air. At the same time, oxygen moves from the air into the blood in the capillaries.

The oxygen-rich blood then travels to the heart through the pulmonary vein and its branches. The heart pumps the oxygen-rich blood out to the body. (For more information about blood flow, go to the Health Topics How the Heart Works article.)

The lungs are divided into five main sections called lobes. Some people need to have a diseased lung lobe removed. However, they can still breathe well using the rest of their lung lobes.

Muscles Used for Breathing

Muscles near the lungs help expand and contract (tighten) the lungs to allow breathing. These muscles include the:

- Diaphragm (DI-ah-fram)

- Intercostal muscles

- Abdominal muscles

- Muscles in the neck and collarbone area

The diaphragm is a dome-shaped muscle located below your lungs. It separates the chest cavity from the abdominal cavity. The diaphragm is the main muscle used for breathing.

The intercostal muscles are located between your ribs. They also play a major role in helping you breathe.

Beneath your diaphragm are abdominal muscles. They help you breathe out when you're breathing fast (for example, during physical activity).

Muscles in your neck and collarbone area help you breathe in when other muscles involved in breathing don't work well, or when lung disease impairs your breathing.

What Happens When You Breathe?

Breathing In (Inhalation)

When you breathe in, or inhale, your diaphragm contracts (tightens) and moves downward. This increases the space in your chest cavity, into which your lungs expand. The intercostal 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 finally reaches and enters the alveoli (air sacs).

Through the very thin walls of the alveoli, oxygen from the air passes to the surrounding capillaries (blood vessels). A red blood cell protein called hemoglobin (HEE-muh-glow-bin) helps move oxygen from the air sacs to the blood.

At the same time, carbon dioxide moves from the capillaries into the air sacs. The gas has traveled in the bloodstream from the right side of the heart through the pulmonary artery.

Oxygen-rich blood from the lungs is carried through a network of capillaries to the pulmonary vein. This vein delivers the oxygen-rich blood to the left side of the heart. The left side of the heart pumps the blood to the rest of the body. There, the oxygen in the blood moves from blood vessels into surrounding tissues.

(For more information on blood flow, go to the Health Topics How the Heart Works article.)

Breathing Out (Exhalation)

When you breathe out, or exhale, your diaphragm relaxes and moves upward into the chest cavity. The intercostal muscles between the ribs also relax to reduce the space in the chest cavity.

As the space in the chest cavity gets smaller, air rich in carbon dioxide is forced out of your lungs and 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're 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.

Click here to view an online animation of how the lungs work.

What Controls Your Breathing?

A respiratory control center at the base of your brain controls your breathing. This center sends ongoing signals down your spine and to the muscles involved in breathing.

These signals ensure your breathing muscles contract (tighten) and relax regularly. This allows your breathing to happen automatically, without you being aware of it.

To a limited degree, you can change your breathing rate, such as by breathing faster or holding your breath. Your emotions also can change your breathing. For example, being scared or angry can affect your breathing pattern.

Your breathing will change 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.

To adjust your breathing to changing needs, your body has many sensors in your brain, blood vessels, muscles, and lungs.

Sensors in the brain and in two major blood vessels (the carotid (ka-ROT-id) artery and the aorta) detect carbon dioxide or oxygen levels in your blood and change your breathing rate as needed.

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 alveoli (air sacs) can detect fluid buildup in the lung tissues. These sensors are thought to trigger rapid, shallow breathing.

Sensors in your joints and muscles detect movement of your arms or legs. These sensors may play a role in increasing your breathing rate when you're physically active.

Lung Diseases and Conditions

Breathing is a complex process. If injury, disease, or other factors affect any part of the process, you may have trouble breathing.

For example, the fine hairs (cilia) that line your upper airways may not trap all of the germs you breathe in. These germs can cause an infection in your bronchial tubes (bronchitis) or deep in your lungs (pneumonia). These infections cause a buildup of mucus or fluid that narrows the airways and limits airflow in and out of your lungs.

If you have asthma, breathing in certain substances that you're sensitive to can trigger your airways to narrow. This makes it hard for air to flow in and out of your lungs.

Over a long period, breathing in cigarette smoke or air pollutants can damage the airways and air sacs. This can lead to a disease called COPD (chronic obstructive pulmonary disease). COPD prevents proper airflow in and out of your lungs and can hinder gas exchange in the air sacs.

An important step to breathing is the movement of your diaphragm and other muscles in your chest, neck, and abdomen. This movement lets you inhale and exhale. Nerves that run from your brain to these muscles control their movement. Damage to these nerves in your upper spinal cord can cause breathing to stop, unless a machine is used to help you breathe. (This machine is called a ventilator or a respirator.)

A steady flow of blood in the small blood vessels that surround your air sacs is vital for gas exchange. Long periods of inactivity or surgery can cause a blood clot called a pulmonary embolism (PE) to block a lung artery. A PE can reduce or block the flow of blood in the small blood vessels and hinder gas exchange.

Clinical Trials

The National Heart, Lung, and Blood Institute (NHLBI) is strongly committed to supporting research aimed at preventing and treating heart, lung, and blood diseases and conditions and sleep disorders.

NHLBI-supported research has led to many advances in medical knowledge and care. Often, these advances depend on the willingness of volunteers to take part in clinical trials.

Clinical trials test new ways to prevent, diagnose, or treat various diseases and conditions. For example, new treatments for a disease or condition (such as medicines, medical devices, surgeries, or procedures) are tested in volunteers who have the illness. Testing shows whether a treatment is safe and effective in humans before it is made available for widespread use.

By taking part in a clinical trial, you can gain access to new treatments before they're widely available. You also will have the support of a team of health care providers, who will likely monitor your health closely. Even if you don't directly benefit from the results of a clinical trial, the information gathered can help others and add to scientific knowledge.

If you volunteer for a clinical trial, the research will be explained to you in detail. You'll learn about treatments and tests you may receive, and the benefits and risks they may pose. You'll also be given a chance to ask questions about the research. This process is called informed consent.

If you agree to take part in the trial, you'll be asked to sign an informed consent form. This form is not a contract. You have the right to withdraw from a study at any time, for any reason. Also, you have the right to learn about new risks or findings that emerge during the trial.

For more information about clinical trials related to lung diseases and conditions, talk with your doctor. You also can visit the following Web sites to learn more about clinical research and to search for clinical trials:

- http://clinicalresearch.nih.gov

- www.clinicaltrials.gov

- www.nhlbi.nih.gov/studies/index.htm

- www.researchmatch.org

For more information about clinical trials for children, visit the NHLBI's Children and Clinical Studies Web page.

Links to Other Information About the Lungs

NHLBI Resources

- Lungs (Health Topics)—online version of this eBook

- Asthma (Health Topics)

- Bronchitis (Health Topics)

- COPD (Health Topics)

- Cough (Health Topics)

- Pneumonia (Health Topics)

- Pulmonary Embolism (Health Topics)

Clinical Trials

- Children and Clinical Studies

- Clinical Trials (Health Topics)

- NHLBI Clinical Trials

- NIH Clinical Research Trials and You (National Institutes of Health)

- ResearchMatch (funded by the National Institutes of Health)

The National Heart, Lung, and Blood Institute (NHLBI) of the National Institutes of Health (NIH) has developed Health Topics articles for patients, caregivers, and the public. Health Topics articles provide science-based, plain-language information related to heart, lung, and blood diseases and conditions and sleep disorders. The Health Topics website contains articles on diseases, conditions, tests, procedures, and other relevant topics, which you can email, print, and share.