Because gas exchange at the lungs occurs by diffusion, any factor that interferes with the rate of diffusion diminishes the ability to exchange gases. The diffusion rate is influenced by (1) surface area, (2) thickness of the alveolar membrane, and (3) distance across which gases must move. Here we will see how many of the diseases that affect the respiratory system do so by slowing the rate of diffusion across the surface of the alveoli in the lungs. As you learn about these respiratory ailments, ask yourself how they change the surface area, thickness, and distance between the alveolar air and the blood.