The respiratory surface (diffusion membrane), across which exchange of gases occurs in the lungs, consists of all the following, except

The diffusion membrane is made up of three major layers:
1. the thin squamous epithelium of alveoli
2. the endothelium of alveolar capillaries
3. the basement substance
The basement substance is 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.