Thin section (alveolar-capillary complex)

The thin section of the alveolar-capillary complex is the site at which most gaseous exchange occurs within the lungs. It occupies the vast majority of the alveolar surface area. It consists of, from superficial alveolar surface inwards to capillary:

• surfactant layer
• type I pneumocyte cell; adapted to be very thin
• a common basal lamina; the result of fusion of the basal lamina of both the type I pneumocyte and the capillary cells
• endothelium of capillary cell

The overall diffusion distance is as low as 0.3-0.5 micrometres. The capillary at the centre of the wall is part of a ramifying network of capillaries throughout the wall of the alveolus. It is supported by a fine network of elastin and reticular fibres. The arrangement described above exists on both sides of a given capillary as a result of the close proximity of neighbouring alveoli.