Hello physicsforums, I was wondering in the case of co2 exchange in the capillary and alveolar system, if there is less co2 in the alveolar, I understand that co2 will move in the direction to the alveolar from the capillary through diffusion. My question is, if we are looking at brownian motion of co2, why don't we also look at the other particles that are around co2. To further clarify with examples, in the alveolar, there is also a lot of oxygen as well as co2, so why would co2 go into the direction of lower pressure despite the fact that there are other particles it might have to rub against? Why do we only look at co2 by itself in this case of diffusion? I hope I made sense. Thank you.