A dog can smell week-old human fingerprints and detect scents up to 40 feet underground, just two examples of his incredible olfactory abilities. A person's sense of smell is weak compared to a dog's because the anatomy of a dog's nose -- and the way it functions -- is different from a human nose, and a larger portion of the dog's brain is devoted to interpreting smells.
A person breathes in and out through the same airways inside her nose. As she exhales, incoming scents are pushed out with the spent air. To get a good smell of something, a person has to inhale repeatedly while trying not to exhale. A dog's nose is more sophisticated in the way it functions. His nostrils, unlike a human's, can move independently, helping him to locate the direction scents are coming from. The dog's nostrils quiver as he sniffs, so that air already in the nasal passageways is pushed deeper into his nose where it doesn't compete with incoming odors. The exhaled air leaves through slits in the side of his nose, and the swirl of wind created by the exhalation helps send more new scents into the dog's nose.
Inside the nose, air passes over the turbinates -- complex folds of bones, covered in nasal membranes and scent-detecting receptor cells and nerves. This is a small area on the roof of a person's nasal cavity, and the incoming air, along with the exhaled air, passes through it. A dog has a shelf just inside his nose that the odor-laden air passes over into a recessed area. In this recessed area the incoming air is able to filter through the turbinates unhindered by expelled air.
Smell Receptor Cells
The average dog has around 220 million smell receptor cells. A dog with a long, wide nose has a larger area covered in receptors, and more cells, than a dog with a flat, short face. A dachshund for example has about 125 million cells, a beagle and a German shepherd around 225 million cells each and a bloodhound comes out on top with about 300 million cells. A human has only 5 million scent receptor cells, just 2 percent of the cells found in a beagle's nose. A dog not only has more cells than a person, he has more types of cells. This enables a dog to detect a greater variety of smells. As air reaches the receptor cells, the odor molecules in the air are dissolved in mucus -- a person produces a pint of nasal mucus each day, a dog proportionately more -- which helps the molecules stick to the microscopic hairs of the receptor cells, where the chemical smell signals are converted to electrical signals and sent via the nerves to the brain. The part of the brain where these scent signals are interpreted is 40 times larger, proportionately, in a dog than a human.
The Vomeronasal Organ
A dog has another organ for smelling, absent in a human, called the vomeronasal organ; it's a sac covered with receptor cells situated above the roof of the dog's mouth, with ducts that open into his mouth and nose. This organ has its own nerves that go to a part of the dog's brain dedicated to interpreting signals sent by the vomeronasal organ. It's thought to detect pheromones -- hormone-like substances released by animals and picked up by others of the same species -- that provide sex-related information such as readiness to mate.