Role of diffusion screening in pulmonary diseases.

It has been shown recently that the acinus is only partially efficient in normal conditions. This is due to a "screening effect" governed by the relative values of the oxygen diffusivity and the membrane resistance as well as design and size of the acinus. These effects depend on the fraction of the acinus in which gas transport is governed by diffusion, then on the location of the convection-diffusion transition. It is shown that these screening effects gives to respiration at rest a partial protection against several types of pulmonary diseases. This is true for mild emphysema, mild asthma or COPD and mild edema. In contrast, under exercise respiration, the acinus is totally efficient and the protective effects linked to the existence of screening do not exist anymore.