The olfactory sense measures the chemical composition of the environment using a diverse array of olfactory receptors. In vertebrates, the olfactory receptors reside in a mucus layer in the nasal cavity and can thus only detect odorants that are inhaled with the airflow and dissolved in mucus. These physical processes fundamentally affect how many odorant molecules contact the receptors. We hypothesize that the olfactory system works efficiently by optimizing the placement of receptors for maximal information transmission. Using a simplified model, we capture all relevant physical processes and show that odorant concentrations generally exhibit an exponential distribution. Combining this result with information theory, we further show that receptors separated into distinct spatial zones maximize the transmitted information. Our results are consistent with experimentally observed receptors zones and might help to improve artificial smell sensors.