page_title: Pulmonary alveolus
section_title: Structure
hierarchical_section_title: Pulmonary alveolus / Structure
caption_attribution_description: English: Bronchial anatomy detail of alveoli and lung circulation. Français&#160;: Anatomie pulmonaire: détail des alvéoles et de la circulation pulmonaires .
context_page_description: A pulmonary alveolus is a hollow cup-shaped cavity found in the lung parenchyma where gas exchange takes place. Lung alveoli are found in the acini at the beginning of the respiratory zone. They are located sparsely in the respiratory bronchioles, line the walls of the alveolar ducts, and are more numerous in the blind-ended alveolar sacs. The acini are the basic units of respiration, with gas exchange taking place in all the alveoli present. The alveolar membrane is the gas exchange surface, surrounded by a network of capillaries. Across the membrane oxygen is diffused into the capillaries and carbon dioxide released from the capillaries into the alveoli to be breathed out.
  Alveoli are particular to mammalian lungs. Different structures are involved in gas exchange in other vertebrates.
context_section_description: The alveoli are located in the alveolar sacs of the lungs in the pulmonary lobules of the respiratory zone, representing the smallest functional units in the respiratory tract. They are also present in the respiratory bronchioles as scattered outpockets, extending from their lumens. The respiratory bronchioles lead into alveolar ducts which are deeply lined with alveoli. Each respiratory bronchiole gives rise to between two and eleven alveolar ducts. Each duct opens into five or six alveolar sacs into which clusters of alveoli open. New alveoli continue to form until the age of eight years. 
  A typical pair of human lungs contain about 300 million alveoli, producing 70 m² (750 sq ft) of surface area. Each alveolus is wrapped in a fine mesh of capillaries covering about 70% of its area. The diameter of an alveolus is between 200 and 500 μm.
caption: Alveolar sacs and capillaries.
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