Imagine that you can design your own respiratory airways – what would they look like? This is a question I ask my own students, to see if they bring in ideas about Fick’s law, and the factors that increase the rate of diffusion. That’s what ideal airways would do – maximise diffusion, specifically of oxygen from the air to our blood. In this podcast, I discuss the gross structure of the human respiratory system and compare it to that of fish and insects – there are surprising similarities between all of these organsisms, and in each case, diffusion of oxygen remains the key goal.