Smelling

Olfaction, also known as olfactics, is the sense of smell. This sense is mediated by specialized sensory cells, called olfactory receptors, in the nasal cavity of vertebrates that are analogous to sensory cells in the antennae of invertebrates. In humans, olfaction – the detection of smells – occurs when odorant molecules bind to specific sites on these receptors. These receptors come together at the glomerulus, a structure that transmits signals to the olfactory bulb (a brain structure located directly above the nasal cavity and below the frontal lobe). Many vertebrates, including most mammals and reptiles, have two distinct olfactory systems—the main olfactory system, and the accessory olfactory system (used mainly to detect pheromones). For air-breathing animals, the main olfactory system detects volatile chemicals, and the accessory olfactory system detects fluid-phase chemicals. Olfaction, along with taste, is a form of chemoreception. The chemicals that activate the olfactory system, generally at very low concentrations, are called odorants. Although taste and smell are separate sensory systems in land animals, water-dwelling organisms often have one chemical sense.

Volatile small molecule odorants, non-volatile proteins, and non-volatile hydrocarbons may all produce olfactory sensations. Some animal species are able to smell carbon dioxide in minute concentrations.

In vertebrates, smells are sensed by olfactory sensory neurons in the olfactory epithelium. The olfactory epithelium is made up of at least six morphologically and biochemically different cell types. The proportion of olfactory epithelium compared to respiratory epithelium (not innervated, or supplied with nerves) gives an indication of the animal's olfactory sensitivity. Humans have about 10 cm² (1.6 sq in) of olfactory epithelium, whereas some dogs have 170 cm² (26 sq in). A dog's olfactory epithelium is also considerably more densely innervated, with a hundred times more receptors per square centimeter.

Molecules of odorants passing through the superior nasal concha of the nasal passages dissolve in the mucus that lines the superior portion of the cavity and are detected by olfactory receptors on the dendrites of the olfactory sensory neurons. This may occur by diffusion or by the binding of the odorant to odorant-binding proteins. The mucus overlying the epithelium contains mucopolysaccharides, salts, enzymes, and antibodies (these are highly important, as the olfactory neurons provide a direct passage for infection to pass to the brain). This mucus acts as a solvent for odor molecules, flows constantly, and is replaced approximately every ten minutes.

...
Wikipedia