All around us, things like coffee or gasoline emit tiny molecules that can enter our olfactory system in two ways: either through our nostrils or the back of the throat (mostly everything emits molecules, from perfume to bread). We’re mainly familiar with smelling through our nostrils, although eating food which releases molecules into the back of the throat can also cause us to smell.
Once inside your nostrils, these air molecules land on the olfactory epithelium — a tissue covered in mucus that lines the nasal cavity. The epithelium contains millions of olfactory receptors, or neurons that are capable of binding with specific odor molecules. These are the “locks and keys” of the olfactory system, which help identify certain smells. An odor molecule from a cup of coffee floating up into your nose will find and bind to an olfactory receptor that’s specifically designed to identify that molecule. This notion was uncovered by Richard Axel and Linda Buck, who won the 2004 Nobel Prize in Physiology or Medicine for their discovery.
Once the olfactory receptors bind with a specific odor, they send their electrical impulses to a certain microregion, also known as the glomerulus (of which there are some 2,000 in the olfactory bulb), which then passes it along to other parts of the brain. The “odorant patterns” that are released from the glomerulus are interpreted in the brain as smell.
For a while, it was assumed that the human nose was capable of only smelling 10,000 different scents. It was only recently that scientists overturned that notion with a new theory: that the nose was capable of much, much more, smelling up to one trillion scents. This is because odor molecules have a myriad of different shapes that can fit into several receptors at once, making it possible for the nose to identify more smells than the number of receptors available.
The nose is also considered the “guardian of your lungs,” acting as a filter that retains tiny particles, and humidifies and warms the air you breathe to keep the bronchial tubes moist.