THE SENSE OF SMELL The nose is normally mistakenly assumed to be the organ of smell reception. It is not. The primary function of the nose is to regulate the temperature and humidity of inspired air, thereby protecting the delicate linings of the lungs. This is achieved by the breathed air passing through narrow passageways formed by three nasal turbinates in each nostril. The turbinates are covered by spongy vascular cells which can expand or contract to open or close the nasal pathways. The olfactory receptors, innervated by the 1st cranial nerve, are located at the top of the nose. There are about 50 million smell receptors in the human olfactory epithelia, the total size of which, in humans, is about that of a small postage stamp, with half being at the top of the left and half at the top of the right nostril. The receptive surfaces of olfactory cells are ciliated and extend into a covering layer of mucus. There is a constant turnover of olfactory cells. Their average active life has been estimated to be about 28 days.