Figure 1: Mechanical and fluid dynamic equilibrium at the level of the air-blood barrier (ABB). A: Transmission electron microscopy image of the ABB to show the extreme thinness of the endothelial and epithelial cells and intervening interstitial basement membrane (EN, EP and BM, respectively). B: the lymphatic pump sets a sub-atmospheric pulmonary interstitial pressure that, in turn, entrains a minimalmicrovascular filtration owing to the low permeability of the capillary endothelium. In black laminar (collagen IV) and fibrillar (collagen I) components of the macromolecular interstitial matrix; in pink the family of proteoglicans link proteins controlling endothelial permeability (perlecan) and mechanical stability of the extracellular matrix (hyaluronan and versican).