Alveolar epithelium, fused basement membranes, capillary endothelium
Mnemonic
Three layers, totalling only about 0.5 micrometres:
- Type I pneumocyte and its surfactant lining
- Fused basement membranes of the alveolus and capillary
- Capillary endothelium
“Thin enough to diffuse across, large enough to cover a tennis court”: about 70 square metres of surface area.
Fick’s law explains why both dimensions matter: diffusion is proportional to area and the partial pressure gradient, and inversely proportional to thickness.
Each term names a disease: thickness rises in pulmonary fibrosis and oedema; area falls in emphysema and after lobectomy; the gradient falls at altitude.
Carbon dioxide is about 20 times more soluble than oxygen, which is why diffusion impairment causes hypoxia long before hypercapnia, and why type 1 respiratory failure precedes type 2 in parenchymal disease.
Expansion
Passing from air to blood:
- Surfactant and the alveolar fluid lining layer
- Type I pneumocyte cytoplasm
- Its basement membrane, fused with
- The capillary endothelial basement membrane
- Capillary endothelial cell cytoplasm
Fick’s law explains why the anatomy is arranged this way: diffusion is proportional to surface area and inversely proportional to thickness. The lung maximises area (around 70 square metres) and minimises thickness.
Anything that thickens the barrier impairs gas transfer, interstitial fibrosis, oedema fluid, or cellular infiltrate. Because carbon dioxide is around 20 times more soluble than oxygen, hypoxaemia appears before hypercapnia in diffusion impairment.