Alveolus (plural: alveoli, from Latin alveolus, "little cavity") is an anatomical structure that has the form of a hollow cavity.[1] Found in the lung parenchyma, the pulmonary alveoli are the terminal ends of the respiratory tree, which outcrop from either alveolar sacs or alveolar ducts, which are both sites of gas exchange with the blood as well.[2] Alveoli are particular to mammalian lungs. Different structures are involved in gas exchange in other vertebrates.[3] The alveolar membrane is the gas-exchange surface. Carbon dioxide rich blood is pumped from the rest of the body into the alveolar blood vessels where, through diffusion, it releases its carbon dioxide and absorbs oxygen.[4]


The alveoli are located in the respiratory zone of the lungs, at the distal termination of the alveolar ducts and atria. These air sacs are the forming and termination point of the respiratory tract. They provide total surface area of about 100 m2.[5]


The alveoli contain some collagen and elastic fibres. The elastic fibers allow the alveoli to stretch as they are filled with air during inhalation. They then spring back during exhalation in order to expel the carbon dioxide-rich air.

A typical pair of human lungs contain about 700 million alveoli, producing 70m² of surface area.[6] Each alveolus is wrapped in a fine mesh of capillaries covering about 70% of its area. An adult alveolus has an average diameter of 200 micrometres, with an increase in diameter during inhalation.[7]

The alveoli consist of an epithelial layer and extracellular matrix surrounded by capillaries. In some alveolar walls there are pores between alveoli called Pores of Kohn.


  1. Weibel, E. R. (1963). Academic Press. ed. Morphometry of the human lung. p. 151. ISBN 3-540-03073-5.
  2. Hansen, J. E.; Ampaya, E. P.; Bryant, G. H. and Navin, J. J. (1975). "The Branching Pattern of Airways and Air Spaces of a Single Human Terminal Bronchiole". Journal of Applied Physiology 38 (6): 983–989. PMID 1141138.
  3. Daniels, Christopher B. and Orgeig, Sandra (2003). "Pulmonary Surfactant: The Key to the Evolution of Air Breathing". News in Physiological Sciences 18 (4): 151–157. PMID 12869615
  4. C. Michael Hogan. 2011. Respiration. Encyclopedia of Earth. Eds. Mark McGinley & C. J. cleveland. National council for Science and the Environment. Washington DC
  5. "Alveoli: Gas Exchange and Host Defense". Functional Ultrastructure: An Atlas of Tissue Biology and Pathology. Springer Vienna. 2005. pp. 224–225. doi:10.1007/b137527. ISBN 978-3-211-83564-7.
  6. Roberts, M., Reiss, M., Monger, G. (2000) Gaseous exchange. In: Advanced Biology. Surrey, Nelson. P167.
  7. Ochs M., Nyengaard J. R., Jung A., Knudsen L., Voigt M., Wahlers T., Richter J., and Gundersen H. J. G., 2004, “The number of alveoli in the human lung.,” American journal of respiratory and critical care medicine, 169(1), pp. 120-4.