I am currently working on an animation about air pollution for Aarhus University. In that project, I have modelled the alveoli, which is the place for exchange of gases (oxygen, carbondioxide) between the exterior and the blood. Unoxygenized blood arrives from the heart, pass by the capillary network surrounding the alvoli sacs (the wall of which is only one cell layer thick) where it is oxygenized and releases its carbondioxide. Then it runs back to the heart, from where it is pumped out into the entire body.