Diffusion is a passive process driven by random movement of molecules.
Diffusion of gas through tissues, in this case, O 2 and CO 2 between alveolus and blood, is described by Fick's Law. Net transfer of gas is proportional to the tissue area, the partial pressure gradient, and the solubility of the gas. It is inversely proportional to the tissue thickness and the square root of the molecular weight. In the lung, resistance to diffusion of oxygen is dependent on the alveolar-capillary interface and the interaction between oxygen and hemoglobin. It is a measure of how well the lung can allow diffusion for any partial pressure gradient.