Pulmonary alveolus

"Alveolus" redirects here. For other uses, see Alveolus (disambiguation).
"Alveolar" redirects here. For the consonant class, see Alveolar consonant.
Pulmonary alveolus

The alveoli
Details
Identifiers
Latin alveolus pulmonis
Code TH H3.05.02.0.00026

Anatomical terminology

An 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]

Structure

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 75 m2.[5]

Bronchial anatomy

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. The alveoli contain some collagen and elastic fibres. The elastic fibres 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.

Histology

A histiologic slide of a human alveolar sac

There are three major cell types in the alveolar wall (pneumocytes):

The wall of each alveolus, lined by thin flat cells (Type I cells) and containing numerous capillaries, is the site of gas exchange, which occurs by diffusion. The relatively low solubility (and hence rate of diffusion) of oxygen necessitates the large internal surface area (about 80 square m [96 square yards]) and very thin walls of the alveoli. Weaving between the capillaries and helping to support them is a meshlike fabric of elastic and collagenous fibres. The collagen fibres, being more rigid, give the wall firmness, while the elastic fibres permit expansion and contraction of the walls during breathing.

Among the other cells found in the alveolar walls are a group called granular pneumocytes (Type II cells), which secrete surfactant, a film of fatty substances believed to contribute to the lowering of alveolar surface tension. Without this coating, the alveoli would collapse and very large forces would be required to re-expand them. Another type of cell, known as an alveolar macrophage, resides on the internal surfaces of the air cavities of the alveoli, the alveolar ducts, and the bronchioles. They are mobile scavengers that serve to engulf foreign particles in the lungs, such as dust, bacteria, carbon particles, and blood cells from injuries.[8]

Reinflation of the alveoli following exhalation is made easier by pulmonary surfactant, which is a phospholipid and protein mixture that reduces surface tension in the thin fluid coating within all alveoli. The fluid coating is produced by the body in order to facilitate the transfer of gases between blood and alveolar air. The surfactant is produced by great alveolar cells (granular pneumonocytes, a cuboidal epithelia), which are the most numerous cells in the alveoli, yet do not cover as much surface area as the squamous alveolar cells (a squamous epithelium).

Great alveolar cells also repair the endotheilium of the alveolus when it becomes damaged. Insufficient pulmonary surfactant in the alveoli can contribute to atelectasis (collapse of part or all of the lung). Without pulmonary surfactant, atelectasis is a certainty; however, there are other causes of lung collapse such as trauma (pneumothorax), COPD, and pleuritis.[9]

Clinical significance

Diseases

Main article: Respiratory disease

See also

References

  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. & 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. & 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, D.C.
  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." Advanced Biology. Surrey, Nelson. p. 167.
  7. Ochs M.; Nyengaard J. R.; Jung A.; Knudsen L.; Voigt M.; Wahlers T.; Richter J.; Gundersen H. J. G. (2004). "The number of alveoli in the human lung". American Journal of Respiratory and Critical Care Medicine. 169 (1): 120–4. doi:10.1164/rccm.200308-1107oc. PMID 14512270.
  8. By: the editors of Encyclopædia Britannica
  9. Saladin, Kenneth S. (2007). Anatomy and Physiology: the unity of form and function. New York: McGraw Hill. ISBN 0-07-322804-4.
  10. Britton, the editors Nicki R. Colledge, Brian R. Walker, Stuart H. Ralston ; illustated by Robert (2010). Davidson's principles and practice of medicine. (21st ed.). Edinburgh: Churchill Livingstone/Elsevier. ISBN 978-0-7020-3085-7.
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