Alveoli are tiny air sacs in your lungs that take up the oxygen you breathe in and keep your body going. Although they’re microscopic, alveoli are the workhorses of your respiratory system.

People have an average of 480 million alveoli in their lungs, located at the end of bronchial tubes. When you breathe in, the alveoli expand to take in oxygen. When you breathe out, the alveoli shrink from expelling carbon dioxide.

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Illustration by Paul Lawrence

There are three overall processes involved in your breathing:

  • moving air in and out of your lungs (ventilation)
  • oxygen-carbon dioxide exchange (diffusion)
  • pumping blood through your lungs (perfusion)

Although tiny, the alveoli are the center of your respiratory system’s gas exchange. The alveoli pick up the incoming energy (oxygen) you breathe in and release the outgoing waste product (carbon dioxide) you exhale.

As it moves through blood vessels (capillaries) in the alveoli walls, your blood takes the oxygen from the alveoli and gives off carbon dioxide to the alveoli.

These tiny alveoli structures, taken together, form a very large surface area to do the work of your breathing when you’re resting and exercising. The alveoli cover a surface of more than 1,399 feet (ft) or 130 square meters (m2).

This large surface area is necessary to process the huge amounts of air involved in breathing and getting oxygen to your lungs. Your lungs take in about 1.5 gallons (gl) or 6 liters (L) of air per minute.

To push the air in and out, your diaphragm and other muscles help create pressure inside your chest. When you breathe in, your muscles create a negative pressure — less than the atmospheric pressure that helps suck air in. When you breathe out, the lungs recoil and return to their typical size.

Picture your lungs as two well-branched tree limbs, one on each side of your chest. The right lung has three sections (lobes), and the left has two sections (above the heart). The larger branches in each lobe are called bronchi.

The bronchi divide into smaller branches called bronchioles. And at the end of each bronchiole is a small duct (alveolar duct) that connects to a cluster of thousands of microscopic bubble-like structures, the alveoli.

The word alveolus comes from the Latin word for “little cavity.”

Alveoli in cross-section

The alveoli are organized into bunches, and each bunch is grouped in the alveolar sac.

The alveoli touch each other like grapes in a tight bunch. The number of alveoli and alveolar sacs is what gives your lungs a spongy consistency. Each alveolus (singular of alveoli) is about 200 micrometers (µm) or 0.007 centimeters (cm) in diameter.

Each alveolus is cup-shaped with very thin walls. It’s surrounded by networks of blood vessels called capillaries that also have thin walls.

The oxygen you breathe in diffuses through the alveoli and the capillaries into the blood. The carbon dioxide you breathe out is diffused from the capillaries to the alveoli, up the bronchial tree, and out your mouth.

The alveoli are just one cell in thickness, allowing the gas exchange of respiration to occur rapidly.

About alveoli cells

The outside layer of alveoli, the epithelium, is composed of two types of cells: type 1 and type 2.

Type 1 alveoli cells cover 95% of the alveolar surface and constitute the air-blood barrier.

Type 2 alveoli cells are smaller and responsible for producing the substance (a “surfactant”) that coats the inside surface of the alveolus and helps reduce surface tension. The surfactant helps keep the alveolus’s shape when breathing in and out.

The type 2 alveoli cells can also turn into stem cells. If necessary for the repair of injured alveoli, alveoli stem cells can become new alveoli cells.

This seemingly perfect machine for breathing can break down or become less efficient because of:

  • disease
  • aging
  • smoking and air pollution


According to the U.S. Centers for Disease Control and Prevention (CDC), tobacco smoke injures your lungs. It leads to lung diseases like chronic obstructive pulmonary disease (COPD), emphysema, and chronic bronchitis.

Tobacco smoke irritates your bronchioles and alveoli and damages the lining of your lungs.

Tobacco damage is cumulative. Years of exposure to cigarette smoke can scar your lung tissue so that your lungs can’t efficiently process oxygen and carbon dioxide. The damage from smoking isn’t reversible.


Indoor pollution from secondhand smoke, mold, dust, household chemicals, radon, or asbestos can damage your lungs and worsen existing lung disease.

Outdoor pollution, such as car or industrial emissions, is also harmful to your lungs.


Chronic smoking is a known cause of lung disease. Other causes include genetics, infections, or compromised immune systems. Chemotherapy and radiation treatments for cancer can also contribute to lung disease. Sometimes the cause of lung disease is unknown.

Lung disease has many types, all of which affect your breathing. Here are some common lung diseases:

  • chronic obstructive pulmonary disease (COPD): airway obstruction from damaged alveoli walls
  • asthma: inflammation narrows your airways and blocks them
  • COPD: damage to the alveoli causes them to break down, reducing the surface area available for gas exchange
  • idiopathic pulmonary fibrosis: the walls surrounding the alveoli become scarred and thickened
  • lung cancer: cancer can start in your alveoli
  • pneumonia: the alveoli fill with fluid, limiting oxygen intake


The aging process can slow down your respiratory system. You may notice that your lung capacity is lessened or your chest muscles are weaker.

Older people also tend to be more at risk for bacterial and viral pneumonia.

Read more about growing older and your lung health.

Read on to learn how you can keep your lungs healthy.

Limit your exposure to pollutants

Use an air cleaner or purifier at work or at home to reduce indoor dust and fumes. You can also wear a mask if you expose yourself to extra dust, mold, or allergens.

Be aware of days when outdoor air pollution is high. You can find forecasts online for the following:

On days when the air quality index (AQI) is at an unhealthy range, keep your exposure minimal by keeping doors and windows closed and circulating air inside.

Decrease how often you smoke

Number one on the list for keeping your lungs healthy is not smoking.

If you’re interested in quitting, there are new methods, such as nicotine replacement therapy. You can also check out blogs for people trying to quit. Or join a support group, such as Quit Now: Freedom From Smoking, sponsored by the American Lung Association.

Take care of your health

  • Get regular health checkups to know how your physical health is generally doing.
  • Maintain a strong immune system. This can include keeping up to date with vaccinations and flu shots.
  • Eat a healthy diet with various fruits, vegetables, grains, and protein sources.
  • Get regular exercise. Exercise helps keep your lungs in good shape by making them work harder.

A respiratory system is a complex machine with millions of alveoli. But most of the time, we don’t even think about it. We breathe in and out in the typical course of our day.

As you learn more about your lungs, or if you have a lung problem, you may want to do some “maintenance” work to help your lungs function well. Breathing exercises to increase lung capacity may be a good place to start.