The blood-air barrier of the alveolus (400X)

Focus on the lung alveoli and try to comprehend how thin the blood-air barrier actually is at its thinnest, well under 1 micrometre.

The blood-air barrier is formed by the type I pneumocytes of the alveolar wall, the endothelial cells of the capillaries and the basement membrane in between.

Type II pneumocytes are easily spotted too, as the cytoplasmic granules containing surfactant (a phospholipid) that turn brown in this section.

Club cells are found in the ciliated simple epithelium. These cells may secrete glycosaminoglycans to protect the bronchiole lining.