A mass transport model of olfaction.

A theoretical model of olfaction involving all the major mechanisms in the mass transport of odorant molecules from inspired air to the olfactory receptors is developed. The mechanisms included are: (i) convective bulk flow of odorant molecules to the olfactory region of the nasal cavity by inhaled air, (ii) lateral transport of odorant molecules from the flowing gas stream in the olfactory region onto the olfactory mucus surface, (iii) sorption of odorant molecules into the mucus at the air-mucus interface, (iv) diffusion of odorant molecules through the mucus layer, and (v) interaction of odorant molecules with the olfactory receptor cells. The model is solved to yield the olfactory response as a function of various physical variables such as the inspiratory flow rate, the mass transfer coefficient, the initial concentration of odorant molecules in the inhaled air, the length of the olfactory mucosa, the thickness of the olfactory mucosa, and the air-mucus partitioning (or solubility in the mucus) of odorant molecules. It was determined that the flow rate of the odorant carrier gas, length of the olfactory mucus surface, and the solubility of odorant molecules in the olfactory mucus should play important roles in determining the odor intensity for these odorants. The model predicts that, given adequate mucus surface for sorption, increase in the flow rate results in an increase in perceived odor intensity for the readily sorbed or highly soluble odorants (such as carvone) and a decrease in odor intensity for the poorly sorbed or insoluble odorants (such as octane). With a substantial decrease in the mucus surface for sorption, increase in the flow rate results in a decrease in perceived odor intensity for all odorants. The theoretical results show good agreement with various experimental data obtained from both psychophysical and electrophysiological studies of olfaction using animals and human subjects.