Respiratory Anatomy — Alveolus

Alveolus

• Hollow cavity found in the lung parenchyma, Ends of the respiratory tree.
• There are ~300 million alveoli that are engulfed by a rich capillary network
• cumulative Surface Area of all alveoli is ~85 m² with a volume of 4000 mL
• Polyhedral-shaped → 0.33 mm in diameter
• Alveolar walls contain dense mesh of capillaries large enough for a red cell to pass through

• Each alveolar wall facing air is lined by Type I cells (one-cell-thick) → parts of alveolar
  wall consists of interstitial space that contain pulmonary capillaries
• Diffusion barrier that alveolar gas has to travel to pulmonary blood is short (0.3 um only!)
  → consists of (i) Type I cell (epithelium), (ii) Basement membrane of type I cell, (iii)
  Basement membrane of pulmonary capillary endothelium, and (iv) Pulmonary capillary
  endothelium
• In some alveolar walls, there are pores between alveoli called Pores of Kohn
• Alveoli are inherently unstable (Ie. collapse) given their small size and surface tension of
  their liquid-lining → this is offset by surfactant produced by Type II cells (round cells
  with large nuclei and cytoplasmic granules containing surfactant)

Alveolar cells

JC / Bianca 2019

Answer by Bianca, JC