Necessary for breathing and the production of sound, this upper section of the respiratory tract also comprises our organ of smell. The air we breathe in brings with it odorous substances in a gaseous state and these are detected thanks to a layer of tissue located on the roof of the nasal cavity, which is relatively inaccessible to the air passing by the nose. When we sniff, more air is able to come into contact with this tissue, thereby improving our ability to recognise smells. This tissue contains millions of olfactory cells that play the role of smell receptors with olfactory cilia at the end, which increase their surface area. Odours can be composed of hundreds of chemical substances and we humans are capable of identifying between 10,000 and 20,000 different odours.
Our sense of smell is less efficient in dry weather and may seem to disappear altogether when we have a cold. Appetising smells increase the production of saliva and stimulate the digestive system, while unpleasant odours can provoke a defensive reflex such as sneezing.