The total thickness of respiratory diffusion membrane is

The diffusion membrane is made up of three major layers namely, the thin squamous epithelium of alveoli, the endothelium of alveolar capillaries and the basement substance (composed of a thin  basement membrane supporting the squamous epithelium and the basement membrane surrounding the single layer endothelial cells of capillaries) in between them.  However, its total thickness is much less than a millimetre. Very thin diffusion membrane helps in easy diffusion of gases between alveoli and blood. Overall thickness of the respiratory membrane in some areas is as little as 0.2 -0.6 micrometer.