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.

You have about 480 million alveoli, 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 to expel carbon dioxide.

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 all together form a very large surface area to do the work of your breathing, both when you’re at rest and when you are exercising. The alveoli cover a surface that measures more than 1,076.4 square feet (100 square meters).

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 1.3 to 2.1 gallons (5 to 8 liters) of air per minute. When you’re at rest, the alveoli send 10.1 ounces (0.3 liters) of oxygen to your blood 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 normal 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 lung 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.”

BioDigital Human

Alveoli in cross-section

The alveoli are organized into bunches, each bunch grouped is what’s called the alveolar sac.

The alveoli touch each other, like grapes in a tight bunch. The number of alveoli and alveolar sacs are what give your lungs a spongy consistency. Each alveolus (singular of alveoli) is about 0.2 millimeters in diameter (about 0.008 inches).

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, which allows the gas exchange of respiration to take place rapidly. The wall of an alveolus and the wall of a capillary are each about 0.00004 inches (0.0001 centimeters).

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 percent of the alveolar surface and constitute the air-blood barrier.

Type 2 alveoli cells are smaller and responsible for producing the surfactant that coats the inside surface of the alveolus and helps reduce surface tension. The surfactant helps keep the shape of each alveolus when you breathe in and out.

The type 2 alveoli cells can also turn into stem cells. If necessary for 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
  • normal aging
  • smoking and air pollution

Smoking

According to the U.S. Centers for Disease Control, tobacco smoke injures your lungs and 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.

Pollution

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.

Disease

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.

Aging

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

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

Read more about growing older and your lung health.

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’re exposing yourself to extra dust, mold, or allergens.

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

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 to not smoke.

If you’re interested in ways to quit, there are new methods to try, 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 doing in general.
  • Maintain a strong immune system. This can include keeping up-to-date with vaccinations and flu shots.
  • Eat a healthy diet, with a variety of fruits, vegetables, grains, and protein sources.
  • Get regular exercise. Exercise helps keep your lungs in good shape by making them work harder.

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

As you learn more about your lungs, or if you have a problem with your lungs, 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.