[en] Weibel's morphologic data has been referred to not only for predicting aerosol deposition in the lungs but also lung physiology. During breathing the volume of air passes all through the mouth, the larynx, the trachea and the conductive airways into the alveolar space. When the airflow is not laminar and disturbed at the bifurcation or the irregular airway surface, eddies and turbulence occur there to result in deposition of aerosol by impaction or sedimentation. At high flow rates, there are more chances for turbulence to occur at these sites. Because the cross-section and volume of the subsequent airways increase, the flow rate decreases. It is worthwhile to remember that the pressure drop and resistance do not necessarily follow Poiseuille's law even in the large airways, and that with the turbulent flow the density of a gas plays an important role. If Poiseuille's law is applied, the resistance becomes sixteenfold when the radius of the airway segment is halved. When we breathe quietly, the flow in the trachea and the intermediate conductive airways is laminar and in very small conductive airways including the terminal bronchioles the airflow becomes so slow in velocity that the axial diffusion becomes more prominent, especially distal to the terminal bronchioles the cross-sectional area increases so much that molecular diffusion becomes more important. For gas transfer to occur, molecules of oxygen should pass through the surfactant layer, the alveolar epithelium, the basement membrane, the endothelium of the capillaries, and the plasma to get to the red blood cell to combine with hemoglobin