The air blood barrier (ABB) extending over 4 × 108 alveoli supplied by a dense network of capillaries provides a surface as high as ~2000 cm2/g of tissue and a thickness as low as 0.5-1 microns. Thus, the geometrical model of the ABB appears as a redundant and oversized gas exchanger to assure oxygen diffusion-transport in order to meet the metabolic requirements. We briefly trace here the long story that allowed over the centuries to decipher lung diffusion.