Alveoli Function, Structure, and Lung Disorders That Affect Them

How these tiny sacs in the lungs affect your health

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Alveoli are tiny, balloon-shaped air sacs in your lungs. The function of the alveoli is to move oxygen and carbon dioxide (CO2) molecules into and out of your bloodstream. Alveoli are an important part of your respiratory system, which includes the parts of your body that help you breathe

This article discusses the structure and function of the alveoli. It also describes some of the medical conditions that can affect alveoli.

What Are Alveoli?
Verywell / JR Bee 

What the Alveoli Do

Alveoli are the endpoint of the respiratory system. When you breathe, the air moves through different parts of your respiratory system as follows:

  1. You inhale air into your mouth or nose.
  2. The air travels down the trachea, also called the windpipe.
  3. The air travels through airways called the bronchi and into the lungs.
  4. The air is directed through smaller and smaller passages, called bronchioles.
  5. The air moves through a tiny duct called the alveolar duct and finally enters an individual alveolus, which is a tiny, balloon-shaped structure.
  6. At this point, the oxygen molecules move through a single layer of lung cells in the alveolus, then 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 produce energy. As oxygen moves out of the alveolus, CO2 molecules pass into it. They are then breathed out of the body through the nose or mouth.

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

Alveoli in the lungs.
Alveoli in the lungs.

Dorling Kindersley / Getty Images

Your diaphragm is the muscle that controls your breathing. When you inhale, your diaphragm contracts. This creates negative pressure in your chest, causing the alveoli to expand and pull in air.

When you exhale, your diaphragm relaxes. This causes the alveoli to 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 branches of your respiratory tree, which is the tree-like structure of passageways that brings air into the lungs.

The walls of the alveoli are very thin. This lets oxygen and CO2 pass easily between the alveoli and capillaries, which are very small blood vessels.

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

Cells of the Alveoli

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

  • Type I pneumocytes. These are the cells responsible for the exchange of oxygen and CO2.
  • Type II pneumocytes. These cells perform two important functions. They produce surfactant, the fluid inside the alveoli that helps keep the balloon shape from collapsing. They can also turn into type I cells in order to repair damage.

Alveoli also contain immune cells called alveolar macrophages. Macrophages are like the garbage trucks of the immune system. These cells phagocytize, or eat debris.

Macrophages clean up any particles that are breathed in and make it to the alveoli. They also remove dead cells and bacteria.

What Causes Damage to the Alveoli?

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

Known as alveolar lung diseases, these include:

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

Alveoli function also depends on the sacs being inflated the right amount. Some conditions and injuries can cause this balance to be off by causing either overdistention or surfactant dysfunction.

Overdistention is the overstretching of the alveoli. A healthy connective tissue support system usually prevents this. The use of mechanical ventilation, or a respirator that helps a patient breathe, can cause overdistention.

Surfactant prevents the alveoli from collapsing between breaths. Medical conditions like respiratory distress syndrome in infants can cause surfactant dysfunction.

Certain genetic conditions can also cause this. These problems can cause alveoli to collapse, making it harder for the lungs to work.

Pneumonia

Pneumonia is a lung infection. It can be caused by bacteria, viruses, or fungus. Pneumonia causes inflammation in the alveoli in one or both lungs. The inflamed alveoli fill with pus, which makes breathing difficult.

Emphysema

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

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

Emphysema patients 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

Tuberculosis (TB) is an infectious disease caused by bacteria. The disease causes the growth of nodules (masses) in lung tissue. TB bacteria multiply in the alveoli. The disease can cause the destruction of alveolar cells.

Alveolar Proteinosis

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

PAP usually occurs in adults aged 20 to 50. It may also occur as a congenital condition. Congenital conditions are present at birth.

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 lung cancer. 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, fluids accumulate in the alveoli. This 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. Babies born too early do not have enough 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 caused by excess fluid in the lungs. This fluid collects in the alveoli and can cause respiratory failure. Respiratory failure is when your blood does not get enough oxygen.

Smoking's Effect on Alveoli

Cigarette smoke affects how the alveoli work and causes damage down to the molecular level. It disrupts your body's ability to repair itself after an infection or trauma. If exposure to smoke continues, alveolar damage continues to worsen.

Summary

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

Diseases that affect the alveoli can cause serious health problems. These include chronic lung conditions like emphysema and tuberculosis. Certain cancers can also begin in the alveoli.

Other diseases, like pneumonia, are short-term but still serious. Some conditions that affect the alveoli can lead to respiratory failure.

Smoking is an important risk factor for lung disease. Quitting smoking can lower your risk of developing diseases related to the alveoli.

A Word From Verywell

The alveoli perform one of the body's most important functions. They are the gateway through which oxygen enters the bloodstream. They are also the primary way that the waste product carbon dioxide exits the body.

Diseases that damage the alveoli affect the entire body. Damaged alveoli deliver less oxygen to tissues. This is called hypoxia. Hypoxia can cause damage to every major organ.

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."