What Are Alveoli?

What these tiny sacs in the lungs do and conditions that can affect them

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Alveoli are tiny, balloon-shaped air sacs located at the end of the bronchioles, the branch-like tubes in the lungs. The alveoli move oxygen and carbon dioxide (CO2) molecules into and out of your bloodstream.

This article discusses the structure and function of the alveoli. It also describes some of the medical conditions that can affect alveoli and how you can keep your lungs healthy. 

What Are Alveoli?
Verywell / JR Bee 

What the Alveoli Do

Alveoli are the endpoint of the respiratory system. When you breathe, air moves through your respiratory system in the following order:

  1. You inhale air into your mouth or nose.
  2. The air travels down the trachea (windpipe).
  3. The air travels through the airways (bronchi) into your lungs.
  4. The air is directed through smaller and smaller passages (bronchioles).
  5. The air moves through a tiny duct (alveolar duct) and finally enters an individual alveolus (the singular of alveoli).
  6. At this point, the oxygen molecules move through a single layer of lung cells in the alveolus. From there, they travel through a single cell layer in a capillary to enter the bloodstream.
  7. CO2 is a byproduct of the process in cells that uses oxygen to make energy. As oxygen moves out of the alveolus, CO2 molecules pass into it. Then, they are breathed out of your body through your nose or mouth.

Alveoli are lined by a fluid (surfactant) that maintains the shape of each air sac and helps keep it open so oxygen and CO2 can pass through.

Your diaphragm is the muscle that controls your breathing. When you breathe in, your diaphragm contracts and creates negative pressure in your chest. When this happens, the alveoli expand and pull in air.

When you breathe out, your diaphragm relaxes. The alveoli then recoil or spring back, pushing out air.

Structure of the Alveoli

Alveoli are the smallest structures in the respiratory system. They are arranged in clusters throughout the lungs at the ends of the passageways that bring air into the lungs (respiratory tree).

The walls of the alveoli are very thin, which makes it easier for oxygen and CO2 to pass between the alveoli and very small blood vessels (capillaries).

Oxygen can pass from the alveoli to the capillaries because the concentration of oxygen is lower in the capillaries than it is in the alveoli. Similarly, CO2 moves the other way because the concentration of carbon dioxide is lower in the alveoli than it is in the capillaries.

Cells of the Alveoli

The alveoli are made up of two different types of cells. Each type has different functions:

  • Type 1 pneumocytes are the cells responsible for the exchange of oxygen and CO2.
  • Type 2 pneumocytes are cells that have two important functions. They make the fluid that helps keep alveoli from collapsing and turn into type 1 cells to repair damage.

Alveoli also have immune cells in them (alveolar macrophages). These are like the garbage trucks of the immune system because they “eat” (phagocytize) debris. This includes any particles that are breathed in and make it to the alveoli, as well as dead cells and bacteria.

What Causes Damage to the Alveoli?

The alveoli only work if the tissue is healthy. Certain medical conditions can negatively affect the alveoli by causing inflammation, scarring, infection, and fluid (water, pus, or blood) build-up.

Alveolar lung diseases include:

  • Pneumonia
  • Emphysema
  • Tuberculosis
  • Alveolar proteinosis
  • Bronchioloalveolar carcinoma
  • Acute respiratory distress syndrome
  • Respiratory distress syndrome
  • Pulmonary edema

The function of the alveoli also depends on the sacs being inflated the right amount. Some conditions and injuries can cause overdistention or surfactant dysfunction, which cause the alveoli to collapse. This makes it harder for the lungs to work.

  • Overdistention is overstretching of the alveoli. A healthy connective tissue support system usually prevents this, but the use of a respirator to help a patient breathe (mechanical ventilation) can push tissue past healthy limits.
  • Surfactant dysfunction: Medical conditions like respiratory distress syndrome in infants and some genetic conditions can negatively impact the fluid that prevents the alveoli from collapsing.

Diseases that hurt the alveoli can affect more than just your respiratory health. Damaged alveoli deliver less oxygen to tissues (hypoxia), which can damage every major organ.


Pneumonia is a lung infection that can be caused by bacteria, viruses, or fungi. Pneumonia causes inflammation in the alveoli in one or both lungs.

The inflamed alveoli fill with pus, which makes it hard to breathe.


Emphysema is a chronic, or long-term lung disease. It usually develops in people with a long history of smoking. People with emphysema have inflammation in their lungs that causes the destruction of the alveoli.

The alveoli that remain do not work as well. They lose their ability to stretch or spring back when a person exhales. This leads to a condition called air trapping, which means air remains in the lungs even after exhaling.

People with emphysema usually have more trouble exhaling than inhaling. The inability to expel air from the lungs leads to more stretching of the alveoli. This increases the loss of function.


Tuberculosis (TB) is an infectious disease caused by bacteria. The disease causes the growth of masses (nodules) in lung tissue.

TB bacteria multiply in the alveoli, so the disease can cause the destruction of alveolar cells.

Alveolar Proteinosis

Pulmonary alveolar proteinosis (PAP) is a rare disease in which proteins accumulate in the alveoli. It is most often an autoimmune condition, meaning the immune system attacks healthy cells.

PAP usually occurs in adults ages 20 to 50. It can also be a condition that is present at birth (congenital condition).

Bronchioloalveolar Carcinoma

Bronchioloalveolar carcinoma (BAC) is a type of lung cancer. It is a subtype of lung adenocarcinoma, one of the most common types of the disease.

BAC begins in the alveoli and is often found in one or both lungs.

Acute Respiratory Distress Syndrome

Acute respiratory distress syndrome (ARDS) is a life-threatening lung condition. In ARDS, fluid builds up in the alveoli and prevents oxygen from getting to the lungs.

ARDS is common in critically ill patients.

Respiratory Distress Syndrome

Respiratory distress syndrome (RDS) is seen in premature babies. Infants born too early have an insufficient amount of surfactant lining the alveoli.

This means there is less surface area available for the exchange of oxygen and CO2.

Pulmonary Edema

Pulmonary edema is a condition that is caused by extra fluid in the lungs. The fluid collects in the alveoli and can cause respiratory failure.

Respiratory failure is when your blood does not get enough oxygen.

How to Heal Lungs After Alveoli Damage

There are some ways you can help your lungs do their job more effectively and keep your alveoli healthy, such as:

  • Quitting smoking and substances that harm your lungs: Cigarette smoke affects how the alveoli work and causes damage all the way down to the molecular level. It disrupts your body's ability to repair itself after an infection or trauma, too. The longer you’re exposed to smoke, the worse the alveolar damage will get.
  • Work on better breathing: Doing deep breathing exercises and learning how to perform diaphragmatic breathing can help support your lungs and make them stronger. 
  • Using herbal and alternative remedies: Echinacea, ginseng, licorice root, astragalus root, and ginger may help with lung health.

Also speak with your healthcare provider about whether exercise training is right for you.


The alveoli are an important part of your respiratory system. They are responsible for moving oxygen into, and CO2 out of, the bloodstream.

Health problems like emphysema and tuberculosis can affect how the alveoli function. Certain cancers can also start in the alveoli. Even short-term lung diseases like pneumonia can still be very serious. Some conditions that affect the alveoli can even lead to respiratory failure.

Frequently Asked Questions

  • How many alveoli are in the human body?

    One cubic millimeter of lung tissue contains around 170 alveoli. Human lungs have a surface area of roughly 70 square meters. Though the total number varies from person to person, this means there are millions of alveoli in a person's lungs.

  • What is surfactant?

    Pulmonary surfactant is a fluid made of phospholipids and proteins that lines the alveoli in the lungs. It helps air sacs maintain their shape and allows oxygen and carbon dioxide to pass through.

  • Where does gas exchange occur?

    Gas exchange occurs in the alveoli, which are tiny, balloon-shaped structures in the lungs. It is when oxygen is absorbed into the bloodstream and carbon dioxide is released.

4 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Reed JC. Chest Radiology (Seventh Edition). Elsevier; 2019.

  3. Ravimohan S, Kornfeld H, Weissman D, Bisson GP. Tuberculosis and lung damage: from epidemiology to pathophysiology. Eur Respir Rev. 2018;27(147). doi:10.1183/16000617.0077-2017

  4. Bernhard W. Lung surfactant: Function and composition in the context of development and respiratory physiology. Ann Anat. 2016 Nov;208:146-150. doi:10.1016/j.aanat.2016.08.003

Additional Reading

By Lynne Eldridge, MD
 Lynne Eldrige, MD, is a lung cancer physician, patient advocate, and award-winning author of "Avoiding Cancer One Day at a Time."