What are Alveoli?

What are the Alveoli and What Do They Do in the Lungs?

diagram illustrating the alveoli in the lung
What are the alveoli and what do they do?. A.D.A.M. Medical Encyclopedia

What are the alveoli, what is their function, and what medical conditions affect this part of the respiratory system?


The alveoli are the tiny air sacs in the lungs through which the exchange of oxygen and carbon dioxide takes place. There are millions of alveoli in the human body with a surface area of roughly 70 meters squared. If they were flattened and stretched end-to-end they could cover a tennis court.

Air enters the respiratory system at the mouth or the nose, travels past the trachea into the right or left lung via the right or left bronchus, on through the bronchioles and alveolar duct finally entering a balloon shaped alveolus. The alveoli are lined by a fluid layer known as surfactant, which increases the surface tension of the alveoli, and in doing so, acts to increase the surface area through which exchange of oxygen and carbon dioxide can occur.


The alveoli are the area of the lungs in which air exchange takes place.  Once oxygen reaches the alveoli, it then diffuses through a single cell in an alveolus followed by a single cell in a capillary to enter the bloodstream. At the same time, carbon dioxide produced via cellular processes in the body is released from the capillary to the alveoli and then exhaled.

During inhalation capillaries expand as negative pressure in the chest is created by contraction of the diaphragm.

During exhalation, the alveoli recoil (spring back) as pressure increases in the chest as the diaphragm relaxes.

Medical Conditions Involving the Alveoli 

Many medical conditions affect the alveoli in the lungs (sometimes called alveolar lung disease.) The alveoli can become inflamed and scarred (such as with emphysema) or filled with water (such as with pulmonary edema,) pus (as with pneumonia,) and/or blood.

Some conditions involving the alveoli include:

  • Emphysema – Emphysema is a condition in which inflammation in the lungs results in the dilation and destruction of alveoli. In addition to less surface area due to lost alveoli, the elastic recoil of alveoli present is reduced. This can lead to “air trapping” or difficulty exhaling air from the body. This explains why expiration rather than inspiration is often more difficult in people with emphysema.
  • Pneumonia. 
  • Tuberculosis.
  • Lung cancer - Bronchioloalveolar carcinoma (BAC) is a form of lung cancer that begins in the alveoli. (BAC has been recategorized as a subtype of lung adenocarcinoma.)
  • Adult respiratory distress syndrome (ARDS) - With ARDS fluid from the capillaries in the lungs leaks into the alveoli, preventing the exchange of oxygen and carbon dioxide.
  • Respiratory distress syndrome (RDS) in premature babies – When babies are born prematurely, their bodies have not yet produced enough surfactant to line the inside of the alveoli. Without surfactant present to hold the alveoli open, the surface area of the lungs is greatly decreased, resulting in a decreased ability to absorb oxygen.
  • Pulmonary edema.
  • Pneumoconioses (such as black lung disease.)
  • Rare diseases such as pulmonary alveolar lung disease and others also affect the alveoli.

The Oxygen Journey: Following the Travel of Oxygen and Carbon Dioxide From the Air to the Body and Back

To understand the function of the alveoli, it can help to follow the flow of gases (oxygen and carbon dioxide) from the atmosphere to the body and back.

Inspiration - Oxygen in the air is first inhaled through the mouth or the nose. It then travels down past the trachea and enters either the left bronchus or right bronchus to travel to the respective lung. From the bronchi, air containing oxygen travels to the bronchioles and then to the respiratory bronchioles (the smallest bronchioles.) It reaches the alveolar duct, and then one of the alveoli clustered around the alveolar duct like grapes.

Oxygen Exchange - Once oxygen is present in an alveolus it diffuses first into the fluid of the alveolus (surfactant) and through the single-celled wall of the alveolus, then through the single-celled wall of the capillary and into the plasma. It does so by diffusion since the oxygen concentration is lower in the blood of the capillary than it is in the alveolus. Within the blood of the capillary oxygen then binds to hemoglobin floating in the plasma. Each molecule of hemoglobin carries 4 molecules of oxygen. 

Flow of Oxygen and Cellular Respiration – Oxygen is carried on hemoglobin in the blood on from the capillary through arterioles and arteries and back to capillaries, to all of the tissues and organs in the body. From tissue capillaries, this oxygen then diffuses into cells due to a lower concentration of oxygen being in the cells than in the plasma. Oxygen is then used by the body in the process of cellular respiration which results in the production of carbon dioxide.  

Flow of Carbon Dioxide - Carbon dioxide produced in the cells then diffuses from the cell into capillaries, into veins (from small to large,) to the right side of the heart, and back to the lungs via the pulmonary arteries. The pulmonary arteries then branch into smaller arteries and arterioles until they reach the capillaries near the alveoli in the lungs.

Carbon Dioxide Exchange  - From the plasma in capillaries, carbon dioxide diffuses into the alveoli, as the carbon dioxide concentration in the alveoli is lower than that in the capillaries.

Expiration - Carbon dioxide then travels in a reverse direction of the oxygen through the respiratory tract, traveling from alveoli, through the alveolar ducts, bronchioles, bronchi, and trachea, to exit through the mouth or nose.


Hamid, Q., Shannon, J., and J. Martin. The Physiologic Basis of Respiratory Disease. PMPH-USA. 2005.

Sharafkhaneh, A., Hanania, N., and V. Kim. Pathogenesis of Emphysema. Proceedings of the American Thoracic Society. 2008. 5:475-77.

U.S. National Library of Medicine. Medline Plus. Gas exchange. Updated 10/27/14. https://medlineplus.gov/ency/anatomyvideos/000059.htm

U.S. National Library of Medicine. PubMed Health. Alveoli. Accessed 09/20/15. http://www.ncbi.nlm.nih.gov/pubmedhealth/PMHT0022134/

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