The respiratory membrane — defined as the alveolar epithelium (type I simple squamous cells specialized for gas exchange and type II simple cuboidal cells producing surfactant), the fused basement membrane or basal lamina, and the endothelial lining of the pulmonary capillaries — is the diffusion barrier across which external respiration occurs. Its geometry sets two determinants of the exchange rate: membrane thickness, which is inversely proportional to gas exchange because it defines the diffusion distance, and available surface area, which is directly proportional to gas exchange because it defines the area over which diffusion proceeds; normal thickness falls within a narrow range that yields the appropriate exchange rate, and departures in either direction perturb arterial oxygen and carbon dioxide partial pressures. This concept belongs to respiratory physiology within human anatomy and physiology, and together with ventilation-perfusion matching and partial pressure gradients with solubility constitutes the set of factors governing external respiration; it is a physiological application of the general diffusion law relating flux to area and inversely to path length.
The respiratory membrane — defined as the alveolar epithelium (type I simple squamous cells specialized for gas exchange and type II simple cuboidal cells producing surfactant), the fused basement me…