Alveoli

The alveoli (singular: alveolus) are tiny hollow air sacs that comprise the basic unit of respiration.

Gross Anatomy

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.

Structure

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. Pores of Kohn are also located within the walls.

Function

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.

Related pathology

• respiratory distress syndrome