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This equation shows that variables that affect the rate of diffusion, expressed as volume per unit time, are the pressure difference , surface area , diffusion capacity and thickness . The units are mmHg, , and , respectively. The rate of diffusion is expressed in rather than . Known values for the diffusion capacity are used. You can select the pressure difference, surface area and thickness using the sliders.

The pink square is a simplified 2D model for the movement of the particles inside the alveolus. It shows how the oxygen and carbon dioxide molecules move as variables change. The blue molecules (oxygen) move downward, representing oxygen entering the capillary. The red molecules (carbon dioxide) move upward, representing carbon dioxide escaping the lungs.

The 3D alveolus figure models what is happening during these changes. As the surface area changes, the alveolus will be filled with fluid. This constitutes acute respiratory distress syndrome, which occurs when fluid leaks into the lungs. The change in thickness is associated with pulmonary fibrosis, which is the thickening of the alveolar walls, usually caused by scarring.