AskDefine | Define alveolus

Dictionary Definition



1 a tiny sac for holding air in the lungs; formed by the terminal dilation of tiny air passageways [syn: air sac, air cell]
2 a bony socket in the alveolar ridge that holds a tooth [syn: tooth socket] [also: alveoli (pl)]

User Contributed Dictionary



  1. A small cavity or pit.
  2. an anatomical structure that has the form of a hollow cavity
  3. a small air sac in the lungs, where oxygen and carbon dioxide are exchanged with the blood.


(air sac in lung):


anatomical structure that has the form of a hollow cavity
  • Croatian: alveola
small air sac in the lungs
  • Croatian: alveola
  • Czech: plicní sklípek


(hollow cavity, air sac in lung):

Extensive Definition

An alveolus (plural: alveoli, from Latin alveolus, "little cavity") is an anatomical structure that has the form of a hollow cavity. Mainly found in the lung, the pulmonary alveoli are spherical outcroppings of the respiratory bronchioles and are the primary sites of gas exchange with the blood. Alveoli are particular to mammalian lungs. Different structures are involved in gas exchange in other vertebrates.


The alveoli are found in the respiratory zone of the lungs.


The lungs contain about 300 million alveoli., representing a total surface area of approx. 70-90 square meters (m2). Each alveolus is wrapped in a fine mesh of capillaries covering about 70% of its area.
The alveoli have radii of about 0.05 mm but increase to around 0.1 mm during inhalation.
The alveoli consist of an epithelial layer and extracellular matrix surrounded by capillaries. In some alveolar walls there are pores between alveoli.
There are three major alveolar cell types in the alveolar wall (pneumocytes):
  • Type I cells that form the structure of an alveolar wall
  • Type II cells that secrete pulmonary surfactant to lower the surface tension of water and allows the membrane to separate thereby increasing the capability to exchange gases. Surfactant is continuously released by exocytosis. It forms an underlying aqueous protein-containing hypophase and an overlying phospholipid film composed primarily of dipalmitoyl phosphatidylcholine.
  • Macrophages that destroy foreign material, such as bacteria.
The alveoli have an innate tendency to collapse (atelectasis) because of their spherical shape, small size, and surface tension due to water vapor. Phospholipids, which are called surfactants, and pores help to equalize pressures and prevent collapse.


  • In asthma, the bronchioles, or the "bottle-necks" into the sac are restricted causing the amount of air flow into the lungs to be greatly reduced. It can be triggered by irritants in the air, photochemical smog for example, as well as substances that a person is allergic to.
  • Emphysema is another disease of the lungs, whereby the elastin in the walls of the alveoli is broken down by an imbalance between the production of neutrophil elastase (elevated by cigarette smoke) and alpha-1-antitrypsin (the activity varies due to genetics or reaction of a critical methionine residue with toxins including cigarette smoke). The resulting loss of elasticity in the lungs leads to prolonged times for exhalation, which occurs through passive recoil of the expanded lung. This leads to a smaller volume of gas exchanged per breath.
  • Chronic bronchitis occurs when an abundance of mucus is produced by the lungs. The production of this substance occurs naturally when the lung tissue is exposed to irritants. In chronic bronchitis, the air passages into the alveoli, the broncholiotes, become clogged with mucus. This causes increased coughing in order to remove the mucus, and is often a result of extended periods of exposure to cigarette smoke.
  • Cystic fibrosis is a genetic condition caused by the dysfunction of a transmembrane protein responsible for the transport of chloride ions. This causes huge amounts of mucus to clog the bronchiolites, similar to chronic bronchitis. The result is a persistent cough and reduced lung capacity.
  • Lung cancer is a common form of cancer causing the uncontrolled growth of cells in the lung tissue. Due to the sensitivity of lung tissue, such malignant growth is often hard to treat effectively.
  • Pneumonia is an infection of the alveoli, which can be caused by both viruses and bacteria. Toxins and fluids are released from the virus causing the effective surface area of the lungs to be greatly reduced. If this happens to such a degree that the patient cannot draw enough oxygen from his environment, then the victim may need supplemental oxygen.
  • Cavitary pneumonia is a process in which the alveoli are destroyed and produce a cavity. As the alveoli are destroyed, the surface area for gas exchange to occur becomes reduced. Further changes in blood flow can lead to decline in lung function.

Additional images

Image:Bronchial anatomy.jpg|Bronchial anatomy


External links

alveolus in Catalan: Alvèol
alveolus in Czech: Plicní sklípky
alveolus in German: Lungenbläschen
alveolus in Dhivehi: ކުދި ވައިކޮތަޅު
alveolus in Spanish: Alvéolo pulmonar
alveolus in French: Alvéole (anatomie)
alveolus in Italian: Alveolo polmonare
alveolus in Lithuanian: Plaučių alveolė
alveolus in Macedonian: Алвеола
alveolus in Dutch: Longblaasje
alveolus in Japanese: 肺胞
alveolus in Norwegian: Alveol
alveolus in Polish: Pęcherzyki płucne
alveolus in Portuguese: Alvéolo pulmonar
alveolus in Slovak: Pľúcny mechúrik
alveolus in Finnish: Keuhkorakkula
alveolus in Swedish: Alveol
alveolus in Ukrainian: Альвеола
alveolus in Chinese: 肺泡

Synonyms, Antonyms and Related Words

alveolar ridge, alveolation, antrum, apex, armpit, arytenoid cartilages, back, basin, blade, bowl, cavity, concave, concavity, crater, crypt, cup, dent, depression, dimple, dint, dip, dorsum, fold, follicle, funnel chest, furrow, gouge, hard palate, hole, hollow, hollow shell, honeycomb, impress, impression, imprint, indent, indentation, indention, indenture, lacuna, larynx, lips, nasal cavity, notch, oral cavity, palate, pharyngeal cavity, pharynx, pit, pock, pocket, pockmark, print, punch bowl, scoop, shell, sink, sinus, socket, soft palate, speech organ, sunken part, syrinx, teeth, teeth ridge, tip, tongue, trough, velum, vocal chink, vocal cords, vocal folds, vocal processes, voice box, vug
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