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The alveoli (singular: alveolus) are tiny hollow air sacs that comprise the basic unit of respiration.
Alveoli are found within the lung parenchyma and are found at the terminal ends of the respiratory tree, clustered around alveolar sacs and alveolar ducts. Each alveolus is approximately 0.2 mm in diameter. There are around 300 million to 1 billion alveoli in the human lungs, covering an area of 70 square meters 1.
The alveolar walls are comprised of collagen and elastic fibers which facilitate expansion during inspiration and return to the original shape during expiration 2. There are numerous capillaries within the alveolar walls where gas exchange occurs. Collateral pathways connect adjacent alveoli through the pores of Kohn, located in alveolar walls. Alveoli are also connected to nearby bronchioles through the canals of Lambert.
Alveoli contain two major types of epithelial cells 3. The most abundant, type 1 pneumocytes (95%) are squamous cells in which gas exchange occurs. The remaining 5%, type 2 pneumocytes, are granular cells which secrete surfactant. Surfactant is a lipoprotein with a high phospholipid content which reduces surface tension. This increases pulmonary compliance, prevents atelectasis and aids recruitment of collapsed airways.
Alveolar macrophages are also located in the alveoli. They protect the alveoli from foreign material by engulfing it, including bacteria, dust and carbon particles.