Alveolo-Capillary Membrane

The exchange of respiratory gases takes place through the alveolo-capillary membrane (hemo-respiratory barrier). It is a thin layer (0.6–2 μm) that is part of the pulmonary alveoli. The total size of the alveolo-capillary membrane is 60–160 m2.

Layers alveolocapillary membranes
In the thinnest places, the membrane consists of the following 6 layers :
 * 1) A layer of fluid containing a surfactant on the inner surface of the alveoli
 * 2) Alveolar epithelium - bodies of first order pneumocytes
 * 3) Basal membrane of the epithelium
 * 4) Interstitium between the basement membrane of the epithelium and the endothelium (a very thin tissue space that contains collagen and elastin fibers)
 * 5) Basal membrane of the endothelium
 * 6) Capillary endothelium

For respiration to be effective, the diffusion path must be as short as possible. Therefore, sites that are functionally important (sites that do not contain second-order pneumocytes, no cell nuclei, interstitial ligament) have a thickness of 0,6 μm. If we also measure the interalveolar septa, the mean thickness is 2 μm.